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Immunology Terms

Terms

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Mature dendritic cells
Dendritic cells in secondary lymphoid tissues that express co-stimulatory molecules and other cell-surface molecules that enable them to present antigen to naive T cells and activate them.
Hypervariable regions (HV regions)
Small regions of high amino acid sequence diversity within the variable regions of immunoglobulin and T-cell receptors. They correspond to the complementarity-determining regions.
Lytic granules
Intracellular storage granules of cytotoxic T cells and NK cells that contain perforin and granzymes.
Humoral immunity
The type of adaptive immunity that is mediated by antibodies. Humoral immunity is the principal defense mechanism against extracellular microbes and their targets. Humoral immunity can be transferred to a non-immune recipient by serum.
T cells (T lymphocytes)
Lymphocytes that develop in the thymus and are responsible for cell-mediated immunity. Their cell-surface antigen receptor is called the T-cell receptor.
Antigen receptor
For a B cell, the antigen receptor is its cell-surface immunoglobulin; for a T cell the antigen receptor is a rather similar molecule called the T-cell receptor. Each individual lymphocyte bears receptors of a single antigen specificity.
Mucus
Slimy protective secretion produced by many internal epithelia.
Mucosa-associated lymphoid tissue (MALT)
Aggregations of lymphoid cells in mucosal epithelia and in the lamina propria beneath. The main mucosa-associated lymphoid tissues are the gut-associated lymphoid tissues (GALT) and the bronchial-associated lymphoid tissues (BALT).
Mannose-binding protein pathway
See lectin-mediated pathway (complement activation)
B lymphocytes
See B cells
Edema
Abnormal accumulation of fluid in connective tissue, leading to swelling.
Protective immunity
The specific immunological resistance to a pathogen that follows either from specific vaccination or recovery from an infection with the pathogen.
CD8 T cells
The subset of T cells that express the CD8 co-receptor and recognize peptide antigens presented by MHC class I molecules. See cytotoxic T cells.
Myeloid progenitors
Stem cells in the bone marrow that give rise to granulocytes, monocytes, and macrophages.
IgE
The class of immunoglobulin having ɛ heavy chains. It is involved in allergic reactions.
Membrane-attack complex
The complex of terminal complement components that forms a pore in the membrane of the target cell, damaging the membrane and leading to cell lysis.
Lysosome
A membrane-bound, acidic organelle abundant in phagocytic cells, which contains proteolytic enzymes that degrade proteins derived mainly from the extracellular environment.
Phagolysosome
Intracellular vesicle formed by fusion of a phagosome with a lysosome, in which the phagocytosed material is broken down by degradative lysosomal enzymes.
Linear epitope
Epitope of a protein recognized by antibody that consists of a linear sequence of amino acids within the protein's primary structure.
Hinge region
The region of immunoglobulin heavy chains that can assume multiple comformations, thereby imparting a flexibility in the orientation of the 2 antigen-binding sites. Because of the hinge region the Ab can simultaneously bind 2 epitopes that are anywhere within reach of one another.
HLA type
The combination of HLA class I and class II allotypes that a person expresses.
Antigen-binding site
The site on an immunoglobulin or T-cell receptor molecule that binds specific antigen.
Macrophage activation
Stimulation of macrophages, which increases their phagocytic, antigen-presenting, and bacterial killing functions. It occurs in the course of infection.
Chemotaxis
Movement of a cell directed by a chemical concentration gradient. The movement of lymphocytes, PMNs, monocytes, and other leukocytes into various tissues is often directed by gradients of chemokines.
T-cell activation
The stimulation of mature naive T cells by antigen presented to them by professional antigen-presenting cells. It leads to their proliferation and differentiation into effector T cells.
Self antigens
A term used to describe all the normal constituents of the body to which the immune system would respond were it not for the mechanisms of tolerance that destroy or inactivate self-reactive B and T cells.
Carrier
Foreign protein to which small non-immunogenic antigens, or haptens, can be coupled to render the hapten immunogenic. In vivo, self proteins can also serve as carriers if they are suitably modified by the hapten; this is important in allergy to drugs.
CD4
A cell-surface glycoprotein on some T cells that recognize antigens presented by MHC class II molecules. CD4 binds to MHC class II molecules on the antigen-presenting cell and acts as a co-receptor to augment the T cell's response to antigen.
Secondary lymphoid tissues
The lymph nodes, spleen and mucosa-associated lymphoid tissues. These are the tissues in which immune responses are initiated. The more highly organized tissues such as lymph nodes and spleen are also often known as secondary lymphoid organs.
Cell-mediated immunity (cellular immunity)
Any adaptive immune response in which antigen-specific effector T cells dominate. It is defined operationally as all adaptive immunity that cannot be transferred to a naive recipient with serum antibody. Cell-mediated immune responses include CD4+ T cell-mediated activation of macrophages that have phagocytosed microbes and CD8+ cytolytic T lymphocyte killing of infected cells.
Light chain (L chain)
The smaller of the two types of polypeptide chain that make up immunoglobulins. It consists of one variable and one constant domain, and in the immunoglobulin molecule it is disulfide-bonded to a heavy chain. There are two classes of light chain, known as κ and λ.
Granulocytes
White blood cells with irregularly shaped, multilobed nuclei and cytoplasmic granules. There are three types of granulocyte: neutrophils, eosinophils, and basophils. They are also known as polymorphonuclear leukocytes.
IgD
The class of immunoglobulin having δ heavy chains. It appears as surface immunoglobulin on mature naive B cells but its function is unknown.
Pattern recognition receptors
Receptors of the innate immune system that recognize frequently encountered structures called "molecular patterns" produced by microorganisms and that facilitate innate immune responses against the microorganism. Examples of pattern recognition receptors of phagocytes include Toll-like receptors.
Pluripotent hematopoietic stem cell
Stem cell in bone marrow that gives rise to all the cellular elements of the blood.
Target cell
Any cell that is acted on directly by effector T cells, effector cells or molecules. For example, virus-infected cells are the targets of cytotoxic T cells, which kill them, and naive B cells are the targets of effector CD4 T cells, which help to stimulate them to produce antibodies.
Mutagen
Any agent, such as a chemical or radiation, that can cause a mutation.
HLA class II molecules
The name for the human version of the MHC class II molecules.
Complement receptors (CR)
Cell-surface proteins on various cell types that recognize and bind complement proteins bound to antigens (chiefly C3b) Complement receptors on phagocytes facilitate the phagocytic engulfment of pathogens coated with complement. Complement receptors include CR1, CR2, CR3, CR4, and the receptor for C1q.
Allergy
A state of hypersensitivity to a normally innocuous environmental antigen. It results from the interaction between the antigen and antibodies or T cells produced by earlier exposure to the same antigen.
Efferent lymphatic vessel
Vessel in which lymph and lymphocytes leave a lymph node en route to the blood.
Lymphatic system
A system of vessels throughout the body that collects tissue fluid (lymph) originally derived from the blood, and returns it, via the thoracic duct to the circulation. Lymph nodes are interspersed along these vessels to trap and retain antigens present in the lymph.
Antigen presentation
The display of antigen as peptide fragments bound to MHC molecules on the surface of cells. This is the form in which antigen is recognized by most T cells.
Histamine
A vasoactive amine stored in mast cell granules. Histamine is released when antigen binds to IgE molecules on mast cells and causes dilation of local blood vessels and contraction of smooth muscle, producing some of the symptoms of immediate hypersensitivity reactions. Anti-histamines are drugs that counter histamine action.
Fc fragment
A fragment of an antibody, resulting from proteolytic cleavage, that consists of the carboxy-terminal halves of the two heavy chains disulfide-bonded to each other. It is called Fc because it was the Fragment that was most readily crystallized in early studies of IgG antibody structure. In an intact antibody the part corresponding to the Fc fragment is called Fc, Fc region, or Fc piece.
Tolerance
When the immune system of a person does not or cannot respond to an antigen. In such cases the individual is said to be tolerant of the antigen.
Cytotoxic T cells
T cells that can kill other cells. Almost all cytotoxic T cells are CD8 T cells. Cytotoxic T cells are important in host defense against viruses and other cytosolic pathogens, because they recognize and kill the infected cells.
Antigen
Originally defined as any molecule that binds specifically to an antibody, the term now also refers to any molecule that can bind specifically to an antibody or T-cell receptor. Antigens that bind to antibodies include all classes of molecules. TCRs only bind peptide fragments of proteins complexed with MHC molecules. See also epitope.
Effector mechanisms
The physiological and cellular processes used by the immune system to destroy pathogens and remove them from the body.
Cytotoxic CD8 T cells
The subset of T cells that express the CD8 coreceptor and recognize peptide antigen presented by MHC class I molecules.
Mannose-binding lectin (MBL)
An acute-phase protein in the blood that binds to mannose residues on pathogen surfaces and when bound activates the complement system. It is also known as mannose-binding protein (MBP) and mannan-binding lectin.
IgM
The class of immunoglobulin having μ heavy chains. It is the first immunoglobulin to appear on the surface of B cells and the first antibody secreted during an immune response. It is secreted in pentameric form.
Major histocompatibility complex (MHC)
A cluster of genes on the short arm of human chromosome 6 that encodes a set of polymorphic membrane glycoproteins called the MHC molecules, which are involved in presenting peptide antigens to T cells.
Fc region
See Fc fragment
Peyer's patches
Gut-associated lymphoid tissue present in the wall of the small intestine, especially the ileum.
Epitope
The specific portion of a macromolecular antigen to which an antibody binds. In the case of a protein antigen recognized by a T cell, an epitope is the peptide portion that binds to a major histocompatibility complex molecule for recognition by the T cell receptor. Also called an antigenic determinant.
Mannose receptor
A carbohydrate binding receptor (lectin) expressed by macrophages that binds mannose and fucose residues on microbial cell walls and mediates phagocytosis of the organisms.
Anaphylactic shock
IgE-mediated allergic reaction to systemically administered antigen that causes circulatory collapse and suffocation due to tracheal swelling. Also called systemic anaphylaxis.
Lectin pathway of complement activation
One of the three pathways of complement activation. It is activated by the binding of a mannose-binding lectin present in blood plasma to mannose-containing peptidoglycans on bacterial surfaces. See also alternative pathway of complement activation; classical pathway of complement activation.
Present
Cells carrying cell-surface complexes of peptide antigens and MHC molecules are said to present these antigens to T lymphocytes.
Endothelium
The cell layer that lines blood vessels.
Endocytic vesicle
Membrane vesicle that is pinched off from the plasma membrane and takes extracellular material into cells.
Interleukin (IL)
A generic term used for many of the cytokines produced by leukocytes. See also IL-1, IL-2, IL-3, etc.
Allergen
An antigen that elicits hypersensitivity or allergic reactions. Allergens are usually innocuous proteins that do not inherently threaten the integrity of the body.
Naive T cell
A mature T cell that has left the thymus but not yet encountered its specific antigen.
Leukocyte
A general term for a white blood cell. Lymphocytes, granulocytes, and monocytes are all leukocytes.
Passive immunization
The injection of specific antibodies to provide protection against a pathogen or toxin. The administered antibodies may derive from human blood donors, immunized animals or hybridoma cell lines.
Endotoxin
A component of the outer leaflet of gram negative bacteria, also called lipopolysaccharide, which is released from dying bacteria, which stimulates many innate immune responses including the secretion of cytokines and induction of microbicidal activities in macrophages and the expression of adhesion molecules for leukocytes on endothelium.
Afferent lymphatic vessels
Vessels that bring lymph draining from connective tissue into a lymph node en route to the blood.
Plasma cells
Terminally differentiated B lymphocytes that secrete antibody.
Constant domains (C domains)
The constituent domains of the constant regions of immunoglobulin and T-cell receptor polypeptides.
IgA
The class of immunoglobulin having α heavy chains, IgA antibodies in dimeric form are the antibodies present in mucosal secretions. IgA in monomeric form is present in the blood.
Isotype switching
The process by which a B cell changes the class of immunoglobulin made while preserving the antigenic specificity of the immunoglobulin. Isotype switching involves somatic recombination that attaches a different heavy-chain constant-region gene to the variable-region exon.
Spleen
Organ situated adjacent to the cardiac end of the stomach. One function of the spleen is to remove old or damaged red blood cells from the circulation; the other is as a secondary lymphoid organ that responds to blood-borne pathogens and antigens.
Self-tolerance
The normal situation whereby a person's immune system does not respond to constituents of the person's body.
Defensins
Peptides produced in epithelia and phagocyte granules, which act as broad-spectrum antibiotics that kill a wide variety of bacteria and fungi
Immune complex
The protein complex formed from the binding of antibodies to soluble antigens. The size of the immune complexes formed depends on the relative concentrations of antigen and antibody. Large immune complexes are cleared by phagocytes bearing Fc and complement receptors. Small soluble immune complexes tend to be deposited on the walls of small blood vessels, where they can activate complement and cause damage.
Diapedesis
The movement of white blood cells from the blood across blood vessel walls into tissues.
Adjuvant
Substance used to enhance the adaptive immune response to any antigen. To exert its effect an adjuvant must be mixed with the antigen before injection or vaccination.
Sepsis
The toxic effects of infection of the bloodstream. Usually caused by Gram-negative bacteria. See also septic shock.
Monoclonal antibodies
Antibodies produced by a single clone of B lymphocytes and that are therefore identical in structure and antigen specificity.
B cells
Lymphocytes that are dedicated to making immunoglobulins and antibodies.
Lymphatic vessels (lymphatics)
Thin-walled vessels that carry lymph from tissues to secondary lymphoid tissues (with the exception of the spleen) and from secondary lymphoid tissues to the thoracic duct.
CD8
A cell-surface glycoprotein on some T cells that recognize antigens presented by MHC class I molecules. CD8 binds to MHC class I molecules on the antigen-presenting cell and acts as a co-receptor to augment the T-cell's response to antigen.
Germinal center
Area in secondary lymphoid tissue that is a site of intense B-cell proliferation, selection, maturation, and death. Germinal centers form around follicular dendritic cell networks when activated B cells migrate into lymphoid follicles.
Serology
The study of the blood (serum) antibodies and their reactions with antigen. The term serology is often used to refer to the diagnosis of infectious disease by detection of microbe-specific antibodies in the serum of the patient.
MHC class II molecules
The class of MHC molecules that present peptides generated in intracellular vesicles to CD4 T cells. They consist of a heterodimer of class II α and β chains.
Autoimmune response
An adaptive immune response directed at an antigenic component of the individual's own body.
Immunological memory
The capacity of the immune system to make quicker and stronger adaptive immune responses to successive encounters with an antigen. Immunological memory is specific for a particular antigen and is long-lived.
Myeloid lineage
A subset of bone-marrow derived cells comprising granulocytes, monocytes, and macrophages.
Professional antigen-presenting cells
Cells that can present antigen to naive T cells and activate them. A professional antigen-presenting cell not only displays peptide antigens bound to appropriate MHC molecules but also has co-stimulatory molecules on its surface that are needed to activate the T cell. Only dendritic cells, macrophages, and B cells can be professional antigen-presenting cells.
Helper CD4 T cells
CD4 T cells are sometimes generally known as helper cells because their function is to help other cell types to perform their functions. The term helper T cell sometimes refers to TH2 cells only, the cells that help B cells to produce antibody.
Tumor necrosis factor-α (TNF-α)
A cytokine produced by macrophages and T cells that has several functions in the immune response and is the prototype of the TNF family of cytokines. These cytokines function as cell-associated or secreted proteins that interact with receptors of the tumor necrosis factor receptor (TNFR) family.
Mutation
An alteration in the DNA sequence of a gene.
Suppressor CD4 T cells
See regulatory CD4 T cells.
Self peptides
Peptides produced from the body's own proteins. In the absence of infection these peptides occupy the peptide-binding sites of MHC molecules on cell surfaces. In a normal functioning immune system, self peptides would not elicit an immune response.
Effector cells
The cells that perform effector functions during an immune response, such as secreting cytokines (e.g. helper T cells), killing microbes (e.g., macrophages, neutrophils, and eosinophils), killing microbe-infected host cells (e.g., CTL's), or secreting antibodies (e.g., differentiated B cells). B cell effector cells would be memory cells or plasma cells; T cell effector cells would be T helper cells or cytotoxic T cells.
T-cell priming
The activation of mature naive T cells by antigen presented to them by professional antigen-presenting cells.
Fab fragment
A proteolytic fragment of IgG that consists of the light chain and the amino-terminal half of the heavy chain held together by an interchain disulfide bond. It is called Fab because it is the fragment with antigen binding specificity. In the intact IgG molecule the parts corresponding to the Fab fragment are often called Fab or Fab arms.
Memory B cells
Long-lived antigen-specific B cells that are produced from activated naive B cells during the primary immune response to an antigen. On subsequent exposure to their specific antigen they are reactivated to differentiate into plasma cells as part of the secondary and subsequent immune responses.
Antigen processing
The intracellular degradation of proteins into peptides that bind to MHC molecules for presentation to T cells.
Primary immune response or primary response
The adaptive immune response that follows a person's first exposure to an antigen.
Variable region (V region)
The extracellular amino-terminal region of an immunoglobulin heavy or light chain or a T-cell receptor that contains variable amino acid sequences that are different between every clone of a lymphocyte and that are responsible for specificity for antigen. The antigen-binding variable sequences are localized to hypervariable segments.
Fever
A rise of body temperature above the normal. It is caused by cytokines produced in response to infection.
Affinity maturation
The increase in affinity of the antigen-binding sites of antibodies for the antigen that occurs during the course of an adaptive immune response.
Diversity
The existence of a large number of lymphocytes with different antigenic specificities in any individual (i.e. the lymphocyte repertoire is large and diverse).
Natural killer cells (NK cells)
Large, granular, cytotoxic lymphocytes that circulate in the blood. NK cells are important in innate immunity to viruses and other intracellular pathogens and also kill certain tumor cells. They are the cytotoxic cells in antibody-dependent cell-mediated cytotoxicity (ADCC).
Immunoglobulins (Ig)
The antigen-binding molecules of B cells.
HLA class I molecules
The name for the human version of the MHC class I molecules.
Endocytosis
The uptake of extracellular material into cells by endocytic vesicles that form by pinching off pieces of plasma membrane.
Complement
A set of plasma proteins that act in a cascade of reactions to attack extracellular forms of pathogens. As a result of complement activation, pathogens become coated with complement components, which can either kill the pathogen directly or cause its engulfment and destruction by phagocytes. There are 3 pathways of complement activation that differ in how they are initiated. The classical pathway is activated by antigen-antibody complexes, the alternative pathway by microbial surfaces, and the lectin pathway by plasma lectins that bind to microbes. Each complement pathway consists of a cascade of proteolytic enzymes that generate inflammatory mediators and opsonins, and leads to the formation of a lytic complex that inserts into cell membranes.
Vaccination
The deliberate induction of protective immunity to a pathogen by the administration of killed or non-pathogenic forms of the pathogen, or its antigens, to induce an immune response.
Tonsils
Large aggregates of lymphoid cells lying on each side of the pharynx.
Isotypes
Classes of immunoglobulin—IgM, IgG, IgD, IgA, and IgE—each of which has a distinct heavy-chain constant region encoded by a different constant-region gene. The heavy-chain constant region determines the effector properties of each antibody class.
B-cell receptor
The antigen receptor on B cells. Each B cell is programmed to make a single type of immunoglobulin. The cell-surface form of this immunoglobulin serves as the B-cell receptor for specific antigen. Associated in the membrane with the immunoglobulin are the signal transduction molecules Igα and Igβ.
Serotypes
Antigenically different strains of a bacterium or other pathogen that can be distinguished by immunological means, for example by antibody-based detection tests. Also used to describe human alloantigens such as HLA and blood group antigens.
Perforin
One of the proteins released by cytotoxic T cells on contact with their target cells. It forms pores in the target cell membrane that contribute to cell killing.
Antigen-presenting cells
Cells that express either MHC class I and/or MHC class II molecules and thus display complexes of MHC molecule and peptide antigen on their surfaces. See professional antigen presenting cell.
Draining lymph node
The lymph node to which extracellular fluid collected at a site of infection first travels.
Lymphoid lineage
All types of lymphocyte, and the bone marrow cells that give rise to them.
Macrophages
Large mononuclear phagocytic cells resident in most tissues. They are derived from blood monocytes and contribute to innate immunity and early nonadaptive phases of host defense. They function as professional antigen-presenting cells and as effector cells in humoral and cell-mediated immunity.
T-cell receptor
The highly variable antigen receptor of T lymphocytes. On most T cells it is composed of a variable α chain and β chain and is known as the α:β T-cell receptor. On a minority of T cells, the variable chains are γ and δ chains, and this receptor is known as the γ:δ T-cell receptor. Both types of receptor are present at the cell surface in association with the complex of invariant CD3 chains and ζ chains, which have a signaling function.
Mucosa
The mucus-secreting epithelia that line the respiratory, intestinal, and urogenital tracts. The conjunctiva of the eye and the mammary glands are also in this category.
Memory T cells
Long-lived antigen-specific T cells that are activated in secondary and subsequent immune responses to an antigen.
M cells
Specialized cells in intestinal epithelium through which antigens and pathogens enter gut-associated lymphoid tissue from the intestines. Short for microfold cells.
Immunodeficiency diseases
A group of inherited or acquired disorders in which some part or parts of host defense are either absent or defective.
Polymorphonuclear leukocytes
See granulocytes
Mast cells
Large bone marrow derived cells found resident in connective tissues throughout the body. They contain large granules that store a variety of chemical mediators including histamine. Mast cells have high-affinity Fcɛ receptors (FcɛRI) that bind free IgE. Antigen binding to mast cell associated IgE triggers mast cell activation and degranulation, producing a local or systemic immediate hypersensitivity reaction. Mast cells have a crucial role in allergic reactions.
CD molecules
Cell surface molecules expressed on various cell types in the immune system that are designated by the "cluster of differentiation" or CD nomenclature.
Peripheral lymphoid tissues
See secondary lymphoid tissues
Clonal deletion
The elimination of immature lymphocytes that bind to self antigens. Clonal deletion is the main mechanism that produces self-tolerance.
Interferons
Cytokines that help cells to resist viral infection. Interferon-α (IFN-α) and interferon-β (IFN-β) are produced by leukocytes and fibroblasts respectively, as well as by other cells, whereas interferon-γ (IFN-γ) is a product of CD4 TH1 cells, CD8 T cells, and NK cells. IFN-γ acts principally to activate macrophages.
Septicemia
Bacterial invasion of the blood.
Opsonins
A macromolecule that becomes attached to the surface of a microbe that can be recognized by surface receptors of neutrophils and macrophages and that increases the efficiency of phagocytosis of the microbe. Opsonins include, antibodies and complement components that bind to pathogens and facilitate their phagocytosis by neutrophils or macrophages.
Framework regions
Relatively invariant regions within the variable domains of immunoglobulins and T-cell receptors that provide a protein scaffold for the hypervariable regions.
Necrosis
The death of cells by lysis that results from chemical or physical injury. It leaves extensive cellular debris that must be removed by phagocytes. Neighboring tissue is also damaged by the molecules released from necrotic cells.
Heavy chain (H chain)
The larger of the two component polypeptides of an immunoglobulin molecule. Each heavy chain has a variable domain and constant domains. The different antibody isotypes, Including IgM, IgG, IgD, IgA, and IgE, are distinguished by structural differences in their heavy chain constant regions. The heavy chain constant regions also mediate the distinctive effector functions on the antibody molecule.
Inflammation
A general term for the local accumulation of fluid, plasma proteins, and white blood cells that is initiated by physical injury, infection, or a local immune response. This is also known as an inflammatory response. The cells that invade tissues undergoing inflammatory responses are often called inflammatory cells or an inflammatory infiltrate.
Self-renewal
The ability of a population of cells to renew itself.
Autoimmunity
Adaptive immunity specific for an antigenic component of the individual's own body.
TH1 cells
A subset of CD4 T cells that are characterized by the cytokines they produce. They are involved mainly in activating macrophages.
Alternative pathway of complement activation
One of three pathways of complement activation. It is triggered by the presence of infection but does not involve antibody. See also classical pathway of complement activation; lectin pathway of complement activation.
Neutralization
The mechanism by which antibodies binding to sites on pathogens prevent growth of the pathogen and/or its entry into cells. The toxicity of bacterial toxins can similarly be neutralized by bound antibody.
Recirculation
Of lymphocytes, their continual movement from blood to secondary lymphoid tissues to lymph and back to the blood. An exception to this pattern is traffic to the spleen; lymphocytes both enter and leave the spleen in the blood.
Cytokines
Proteins made by cells that affect the behavior of other cells. Cytokines made by lymphocytes are often called lymphokines or interleukins (abbreviated IL). Cytokines bind to specific receptors on their target cells.
Variable domain (V domain)
The amino-terminal domain of immunoglobulin and T-cell receptor polypeptide chains. Paired variable domains make up the antigen-binding site.
Dendritic cells
Professional antigen-presenting cells with a branched, dendritic morphology. They are the most potent stimulators of T-cell responses. Also known as interdigitating reticular cells, they are derived from the bone marrow and are distinct from the follicular dendritic cell that presents antigen to B cells. Immature dendritic cells take up and process antigens but cannot yet stimulate T cells. Mature or activated dendritic cells are present in secondary lymphoid tissues and are able to stimulate T cells.
Vaccine
Any preparation made from a pathogen that is used for vaccination and provides protective immunity against infection with the pathogen.
Fc receptors
Cell-surface receptors for the Fc portion of some immunoglobulin isotypes. They include the Fcγ and Fcɛ receptors.
Lymph
Mixture of extracellular fluid and cells that is carried by the lymphatic system.
Lymph nodes
A type of secondary lymphoid tissue found at many sites in the body where lymphatic vessels converge. Antigens are delivered by the lymph and presented to lymphocytes within the lymph node where adaptive immune responses are initiated.
Bone marrow
The tissue in the center of certain bones that is the major site of generation of all the cellular elements of blood (hematopoiesis).
Hives
Itchy red swellings in the skin caused by IgE-mediated reactions. Also called urticaria or nettle rash.
Self-MHC
A person's own MHC molecules.
Specificity
The property of antibodies and other antigen-binding molecules for selective interaction with only one or a few types of molecule or cell.
Gene segments
Multiple short DNA sequences in the immunoglobulin and T-cell receptor genes. These can be rearranged in many different combinations to produce the vast diversity of immunoglobulin or T-cell receptor polypeptide chains. See also D gene segments; I gene segments; V gene segments.
Secondary immune response or secondary response
The adaptive immune response provoked by a second exposure to an antigen. It differs from the primary response by starting sooner and building more quickly.
Monocytes
White blood cells with a bean-shaped nucleus. They are the precursors of tissue macrophages.
IgG
The class of immunoglobulin having γ heavy chains. It is the most abundant class of immunoglobulin in plasma.
Common lymphoid progenitor
Stem cell that gives rise to all lymphocytes and is derived from a pluripotent hematopoietic stem cell.
Extravasation
The movement of cells or fluid from within blood vessels to the surrounding tissues.
Thymocytes
A precursor of a mature T lymphocyte present in the thymus.
T-cell areas
Parts of secondary lymphoid tissues where the lymphocytes are predominantly T cells.
Acute-phase response
A response of innate immunity that occurs soon after the start of an infection and involves the synthesis of acute-phase proteins by the liver and their secretion into the blood.
MHC class I molecules
The class of MHC molecules that present peptides generated in the cytosol to CD8 T cells. They consist of a heterodimer of a class I heavy chain associated with β2-microglobulin.
Nitric oxide
A biologic effector molecule with a broad range of activities that, in macrophages, function as a potent microbicidal agent that kills ingested organisms.
Primary lymphoid tissues
Anatomical sites of lymphocyte development. The bone marrow and the thymus gland.
Conformational epitopes
Epitopes on a protein antigen that are formed from several separate regions in the primary sequence of a protein brought together by protein folding. Antibodies that bind conformational epitopes bind only to native folded proteins. Also called discontinuous epitopes.
Toll-like receptors
See Pattern recognition receptors
Autoimmune disease
Disease in which the pathology is caused by an immune response to normal components of healthy tissue.
Adaptive immune response
The response of antigen-specific B and T lymphocytes to antigen, including the development of immunological memory.
Thymus
A lymphoepithelial organ in the upper part of the middle of the chest, just behind the breastbone. It is the site of T-cell development.
Clonal selection
The central principle of adaptive immunity. It is the mechanism by which adaptive immune responses derive only from individual antigen-specific lymphocytes, which are stimulated by the antigen to proliferate and differentiate into antigen-specific effector cells.
Innate immunity
The host defense mechanisms that act from the start of an infection and do not adapt to a particular pathogen. Also called the innate immune response.
Phagocytosis
Cellular internalization of particulate matter, such as bacteria, by means of endocytosis. Cells specialized in phagocytosis are known as phagocytes.
Phagocyte
A cell specialized to perform phagocytosis. The principal phagocytic cells in mammals are neutrophils and macrophages.
Phagosome
Intracellular vesicle containing material taken up by phagocytosis.
Seroconversion
The phase of an infection when antibodies against the infecting agent are first detectable in the blood.
Lymphoid progenitor
Stem cell in bone marrow cell that gives rise to all lymphocytes.
Constant region (C region)
The portion of immunoglobulin (Ig) or T cell receptor (TCR) polypeptide chains that does not vary in sequence among different clones of B and T cells and is not involved in Ag binding. For immunoglobulins there are several versions of constant regions, but within each isotype (class of antibody) the sequence will be the same.
Type I interferon
Cytokines (interferons-α and -β) produced by virus-infected cells that interfere with viral replication by the infected cell, and also signal neighboring uninfected cells to prepare for infection.
Eosinophil
White blood cell that is one of the three types of granulocyte. It contains granules that stain with eosin and whose contents are secreted when the cell is stimulated. Eosinophils contribute chiefly to defense against parasitic infections.
Immunity
The ability to resist infection.
Stem cell
The undifferentiated cell that divides continuously and gives rise to additional stem cells and to cells of multiple different lineages. For example, all blood cells arise from a common hematopoetic stem cell in the bone marrow.
Human leukocyte antigen (HLA) complex
The human major histocompatibility complex (MHC).
Opsonization
The coating of the surface of a pathogen or other particle with any molecule that makes it more readily ingested by phagocytes. Antibody and complement opsonize extracellular bacteria for phagocytosis by neutrophils and macrophages because the phagocytic cells carry receptors for these molecules.
Immature dendritic cells
Dendritic cells present in tissues, which take up antigen but do not express co-stimulatory molecules and cannot yet act as professional antigen-presenting cells to naive T cells.
Regulatory CD4 T cells (TR)
Antigen-specific CD4 T cells whose actions can suppress immune responses.
HLA
The acronym for Human Leukocyte Antigen. It is the genetic designation for the human MHC. Individual loci are designated by capital letters, as in HLA-A, and alleles are designated by numbers, as in HLA-A*0201.
Classical pathway of complement activation
One of three pathways of complement activation. It is activated by antibody bound to antigen, and involves complement components C1, C4, and C2 in the generation of the C3 and C5 convertases. See also alternative pathway of complement activation; lectin pathway of complement activation.
Immune
Resistant to infection.
T lymphocyte
See T cells
Affinity
A measure of the strength with which one molecule binds to another via a single binding site.
Acute-phase proteins
Plasma proteins made by the liver whose synthesis is rapidly increased in response to infection. They include mannose-binding lectin (MBL), C-reactive protein (CRP), and fibrinogen.
Lymphocytes
A class of white blood cells that consist of small and large lymphocytes. The small lymphocytes bear variable cell-surface receptors for antigen and are responsible for adaptive immune responses. There are two main classes of small lymphocyte—B lymphocytes (B cells) and T lymphocytes (T cells). Large granular lymphocytes are natural killer (NK) cells, lymphocytes of innate immunity.
Lymphoid
Containing lymphocytes, or pertaining to lymphocytes.
Variable (V) gene segment
DNA sequence in the immunoglobulin or T-cell receptor genes that encodes the first 95 or so amino acids of the V domain. There are multiple different V gene segments in the germline genome. To produce a complete exon encoding a V domain, one V gene segment must be rearranged to join up with a J or a rearranged DJ gene segment.
MHC molecules
Major histocompatibility complex molecules. Highly polymorphic glycoproteins encoded by the major histocompatibility complex (MHC). They form complexes with peptides and present peptide antigens to T cells. There are two classes—MHC class I and MHC class II molecules—with different roles in the immune response. They are also known as major histocompatibility antigens because they are the main alloantigens involved in the rejection of transplanted tissues.
Inflammatory mediators
A variety of substances released by various cell types that contribute to the production of inflammation at a site of infection or trauma.
Neutrophils
Phagocytic white blood cells that enter infected tissues in large numbers. They contain granules that stain with neutral dyes. After their entry into infected tissues, neutrophils engulf and kill extracellular pathogens in large numbers. They are also known as neutrophilic polymorphonuclear leukocytes (PMNs) and are a type of granulocyte. The most abundant white blood cell.
CD4 T cells
The subset of T cells that express the CD4 co-receptor and recognize peptide antigens presented by MHC class II molecules. See helper CD4 T cells.
Cytotoxins
Proteins made by cytotoxic T cells that participate in the destruction of target cells. Perforins, granzymes or fragmentins, and granulysin are examples of cytotoxins.
Memory cells
General term for lymphocytes that are responsible for the phenomenon of immunological memory and protective immunity.
Gut-associated lymphoid tissue (GALT)
Lymphoid tissues closely associated with the gastrointestinal tract, including the palatine tonsils, Peyer's patches in the intestine, and layers of intraepithelial lymphocytes.
Immune system
The tissues, cells, and molecules involved in host defense mechanisms, primarily against infectious agents.
Antibody
A type of glycoprotein molecule, also called immunoglobulin (Ig), produced by B lymphocytes, that binds antigens, often with a high degree of specificity and high affinity. The basic structural unit of an antibody is composed of 2 identical heavy chains and 2 identical light chains. Amino-terminal variable regions of the heavy and light chains form the antigen binding sites, whereas the carboxy-terminal constant regions of the heavy chains functionally interact with other molecules in the immune system. In an individual there are millions of different antibodies, each with a unique antigen-binding site. Secreted antibodies perform the various effector functions, including neutralizing antigens, activating complement, and promoting phagocytosis and destruction of microbes. Membrane-bound immunoglobulin can be found on naïve B lymphocytes.
Naive B cell
A mature B cell that has left the bone marrow but has not yet encountered its specific antigen.
Complement activation
The initiation by pathogens of a series of reactions involving the complement components of plasma, leading to the death and elimination of the pathogen. See also alternative pathway of complement activation; classical pathway of complement activation; lectin pathway of complement activation.
Erythrocyte
Red blood cell.
Septic shock
Shock syndrome that is frequently fatal, caused by the systemic release of the cytokine TNF-α after bacterial infection of the bloodstream, usually with Gram-negative bacteria.
Hematopoiesis
The generation of the cellular elements of blood, including the red blood cells, white blood cells, and platelets. These cells all originate from pluripotent hematopoietic stem cells whose differentiated progeny divide under the influence of various hematopoietic growth factors.
Agglutination
The clumping together of particles, usually caused by antibody or some other multivalent binding molecule interacting with antigens on the surfaces of adjacent particles. Such particles are said to be agglutinated. When the particles are red blood cells, the phenomenon is called hemagglutination. When they are white blood cells it is called leukoagglutination.
Hypersensitivity reactions
Immune responses to innocuous antigens that lead to symptomatic reactions on reexposure. These can cause hypersensitivity diseases if they occur repetitively. This state of heightened reactivity to an antigen is called hypersensitivity. Hypersensitivity reactions are classified by mechanism: type I hypersensitivity reactions involve the triggering of mast cells by IgE antibodies; type II hypersensitivity reactions involve IgG antibodies against cell-surface or matrix antigens; type III hypersensitivity reactions involve antigen:antibody complexes; and type IV hypersensitivity reactions are mediated by effector T cells.
Immunization
The deliberate provocation of an adaptive immune response by introducing antigen into the body.
Adaptive immunity
The state of resistance to infection that is produced by the adaptive immune response.
TH2 cells
A subset of CD4 T cells that are characterized by the cytokines they produce. They are involved mainly in stimulating B cells to produce antibody.
Chemokines
Large group of small proteins involved in guiding white blood cells to sites where their functions are needed. They have a central role in inflammatory responses.
Serum
The cell-free fluid that remains when blood or plasma forms a clot. Blood antibodies are found in the serum fraction.
Lymphoid organs (lymphoid tissues)
Organized tissues that contain very large numbers of lymphocytes held in a non-lymphoid stroma. The primary lymphoid organs, where lymphocytes are generated, are the thymus and bone marrow. The main secondary lymphoid tissues, in which adaptive immune responses are initiated, are the lymph nodes, spleen, and mucosa-associated lymphoid tissues such as tonsils, Peyer's patches, and the appendix.
Complement system
See complement

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