Jawetz

Adhesion molecules: 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.

Alleles: Variants of a single genetic locus.

Anaphylatoxins: Fragments of complement proteins released during activation. Result in increased vascular

permeability and attract leukocytes.

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.

Antigen (Ag): A substance that can react with an antibody. Not all antigens can induce antibody production;

those that can are also called immunogens.

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 in nonavian species. B cells are the precursors of plasma cells that produce

antibody.

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).

Chemokines: Low-molecular-weight proteins that stimulate leukocyte movement.

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

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

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.

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

Endotoxins: Bacterial toxins released from damaged cells.

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

Hapten: A molecule that is not immunogenic by itself but can react with specific antibody.

Histocompatible: Sharing transplantation antigens.

Humoral immunity: Pertaining to immunity in a body fluid and used to denote immunity mediated by antibody

and complement.

Hypersensitivity reactions:

(1) 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.

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).

Type III. Immune complex: Antigen-antibody immune complexes are deposited in tissues,

complement is activated, and polymorphonuclear cells are attracted to the site, causing tissue damage.

(2) 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.

Immune response: Development of resistance (immunity) to a foreign substance (eg, infectious agent). It can

be antibody-mediated (humoral), cell-mediated (cellular), or both.

Immunity:

(1) Innate immunity: Nonspecific resistance not acquired through contact with an antigen. It includes skin

and mucous membrane barriers to infectious agents and a variety of nonspecific immunologic factors, and it

may vary with age and hormonal or metabolic activity.

(2) 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).

Immunoglobulin: A glycoprotein, composed of H and L chains, that functions as antibody. All antibodies are

immunoglobulins, but not all immunoglobulins have antibody function.

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.

Inflammation: Local accumulation of fluid and cells after injury or infection.

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.

Leukocyte: General term for a white cell.

Lymphocyte: A mononuclear cell 712 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).

Major histocompatibility complex (MHC): A cluster of genes located in close proximity, eg, on human

chromosome 6, that encode the histocompatibility antigens (MHC molecules).

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.

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.

Monocyte: A circulating phagocytic blood cell that develops into tissue macrophages.

Natural killer (NK) cells: Large granular lymphoid cells with no known antigen-specific receptors. They are

able to recognize and kill certain abnormal cells, eg, tumor cells, and also activate the innate response.

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

opsonins.

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.

Plasma cell: A terminally differentiated B cell that secretes antibody.

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.

T cell (also T lymphocyte): A thymus-derived cell that participates in a variety of cell-mediated immune

reactions.

Thymocytes: Developing T cells found in the thymus.

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