Immunology Glossary Terms

Monocyte

A circulating phagocytic blood cell that develops into tissue macrophages.

Major histocompatibility complex (MHC):

A cluster of genes located in close proximity, eg, on human chromosome 6, that encode the histocompatibility antigens (MHC molecules)

Immunoglobulin

A glycoprotein, composed of H and L chains, that functions as antibody. All antibodies are immunoglobulins, but not all immunoglobulins have antibody function.

Hapten

A molecule that is not immunogenic by itself but can react with specific antibody after being joined to a suitable carrier molecule.

Lymphocyte

A mononuclear cell 7-12 m in diameter containing a nucleus with densely packed chromatin and a small rim of cytoplasm. Lymphocytes include the T cells and B cells, which have primary roles in immunity.

Macrophage

A phagocytic mononuclear cell derived from bone marrow monocytes and found in tissues and at the site of inflammation. Macrophages serve accessory roles in immunity, particularly as antigen-presenting cells (APCs).

Chemotaxis

A process whereby phagocytic cells are attracted to the vicinity of invading pathogens.

Antibody (Ab)

A protein produced as a result of interaction with an antigen. The protein has the ability to combine with the antigen that stimulated its production.

Complement

A set of plasma proteins that is the primary mediator of antigen-antibody reactions.

Immunoglobulin class:

A subdivision of immunoglobulin molecules based on structural (amino acid sequence) differences. In humans there are five immunoglobulin classes: IgG, IgM, IgA, IgE, and IgD.

Immunoglobulin subclass:

A subdivision of the classes of immunoglobulins based on structural differences in the H chains. For human IgG there are four subclasses: IgG1, IgG2, IgG3, and IgG4.

Opsonin

A substance capable of enhancing phagocytosis. Antibodies and complement are the two main opsonins.

Antigen (Ag)

A substance that can react with an antibody. Not all antigens can induce antibody production; those that can are also called immunogens.

Plasma cell:

A terminally differentiated B cell that secretes antibody.

T cell (also T lymphocyte)

A thymus-derived cell that participates in a variety of cell-mediated immune reactions.

Polymorphonuclear cell (PMN)

Also known as a neutrophil or granulocyte, a PMN is characterized by a multilobed nucleus. PMNs migrate from the circulation to a site of inflammation by chemotaxis and are phagocytic for bacteria and other particles.

Antibody-mediated hypersensitivity Type III. Immune complex

Antigen-antibody immune complexes are deposited in tissues, complement is activated, and poly-morphonuclear cells are attracted to the site, causing tissue damage.

Antibody-mediated hypersensitivity Type II.

Antigens on a cell surface combine with antibody, which leads to complement-mediated lysis (eg, transfusion or Rh reactions) or other cytotoxic membrane damage (eg, autoimmune hemolytic anemia).

Endotoxins

Bacterial toxins released from damaged cells.

Thymocytes

Developing T cells found in the thymus.

Immune response

Development of resistance (immunity) to a foreign substance (eg, infectious agent). It can be antibody-mediated (humoral), cell-mediated (cellular), or both.

Monoclonal antibodies

Each B lymphocyte produces antibody of a single specificity. However, normal B cells do not grow indefinitely. If B cells are fused to a myeloma cell by somatic cell hybridization and fused cells that secrete the desired antibody specificity are selected, an immortalized antibody-producing cell line, known as a hybridoma, is obtained, and these hybrid cells produce monoclonal antibodies.

Cell Adhesion Molecules (CAMs):

For example, the integrins and selectins. These are molecules that mediate the binding of cells to other cells or to extracellular matrix molecules such as fibronectin.

Anaphylatoxins

Fragments of complement proteins released during activation. Result in increased vascular permeability and attract leukocytes.

Leukocyte

General term for a white blood cell.

Antibody-mediated hypersensitivity: Type I. Immediate

IgE antibody is induced by allergen and binds via its Fc receptor to mast cells and eosinophils. After encountering the antigen again, the fixed IgE becomes cross-linked, inducing degranulation and release of mediators, especially histamine.

Cell-mediated (cellular) immunity

Immunity in which the participation of lymphocytes and macrophages is predominant. Cell-mediated immunity is a term generally applied to the type IV hypersensitivity reaction (see below).

Vaccination

Induction of immunity by injecting a dead or attenuated form of a pathogen.

Natural killer (NK) cells

Large granular lymphoid cells with no known antigen-specific receptors. They are able to recognize and kill certain virally infected cells, and also activate the innate response.

Inflammation

Local accumulation of fluid and cells after injury or infection.

Chemokines

Low-molecular-weight proteins that stimulate leukocyte movement.

Innate immunity

Nonspecific host defense not acquired through contact with an antigen. It includes skin and mucous membrane barriers to infectious agents and a variety of nonspecific immunologic factors.

Interferon

One of a heterogeneous group of low-molecular-weight proteins elaborated by infected host cells that protect noninfected cells from viral infection. Interferons, which are cytokines, also have immunomodulating functions.

Humoral immunity

Pertaining to immunity in a body fluid and used to denote immunity mediated by antibody and complement.

Adaptive immunity

Protection acquired by deliberate introduction of an antigen into a responsive host. Active immunity is specific and is mediated by either antibody or lymphoid cells (or both).

Histocompatible

Sharing major histocompatibility complex (transplantation) antigens.

Epitope

Site on an antigen recognized by an antibody. Also known as an antigenic determinant.

B cell (also B lymphocyte)

Strictly, a bursa-derived cell in avian species and, by analogy, a cell derived from the equivalent of the bursa (bone marrow) in nonavian species. B cells are the precursors of plasma cells that produce antibody.

Cytotoxic T cell

T cells that can kill other cells, eg, cells infected with intracellular pathogens.

Cell-mediated hypersensitivity: Type IV. Delayed

T lymphocytes, sensitized by an antigen, release cytokines upon second contact with the same antigen. The cytokines induce inflammation and activate macrophages.

Opsonization

The coating of an antigen or particle (eg, infectious agent) by substances, such as antibodies, complement components, fibronectin, and so forth, that facilitate uptake of the foreign particle into a phagocytic cell.

Membrane attack complex

The end product of activation of the complement cascade, which contains C5, C6, C7, and C8 (and C9). The membrane attack complex makes holes in the membranes of gram-negative bacteria, killing them and, in red blood or other cells, resulting in lysis.

Cytolysis

The lysis of bacteria or of cells such as tumor or red blood cells by insertion of the membrane attack complex derived from complement activation

allele

one of two alternate forms of a gene that can have the same locus on homologous chromosomes and are responsible for alternative traits