Definitions

Antibodies and Antigens

Antibodies are proteins composed of two heavy chains and two light chains.

from Mike Clark's Immunoglobulin Structure Homepage

Antibodies recognize epitopes displayed on antigens and facilitate clearance of the antigen. Antigens are any substance that binds specifically to an antibody or a T-cell receptor. Antigen-presenting cells (APCs) are cells that can process and present antigenic peptides that are displayed in association with class II MHC molecules and deliver a co-stimulatory signal necessary for T-cell activation. Macrophages, dendritic cells, and B cells constitute the professional APCs. Nonprofessional APCs, which function in antigen presentation only for short periods, include thymic epithelial cells and vascular endothelial cells.

Cell Types

B cells are lymphocytes that mature in the bone marrow and express membrane-bound antibody. Following interaction with antigen, B cells differentiate into antibody-secreting plasma cells and memory cells. T cells are lymphocytes that matures in the thymus and express a T-cell receptor, CD3, and CD4 or CD8. Several distinct T-cell subpopulations are recognized. Cytotoxic T lymphocytes (CTLs) are effector T cells (usually CD8+) that can mediate the lysis of target cells bearing antigenic peptides associated with an MHC molecule. CTLs usually arise from antigen-activated TC cells. Macrophages are large, leukocytes derived from monocytes that functions in phagocytosis, antigen processing and presentation, secretion of cytokines, and antibody-dependent cell-mediated cytotoxicity. Natural killer (NK) cells are large, granular lymphocytes (null cell) that have cytotoxic ability but do not express antigen-binding receptors. NK cells exhibit antibody-independent killing of tumor cells and can also participate in antibody-dependent cell-mediated cytotoxicity.

CD4 and CD8

CD4 is a monomeric membrane molecule, usually present on TH cells, that acts as a coreceptor. It binds to the b2 domain of class II MHC molecules and functions in signal transduction during T-cell activation. CD8 is a heterodimeric membrane molecule, usually present on TC cells, that acts as a coreceptor. It binds to the a3 domain of class I MHC molecules and functions in signal transduction during T-cell activation.

MHC Molecules

Class I MHC molecules are heterodimeric membrane proteins that consist of an a chain encoded in the MHC associated noncovalently with b2 -microglobulin. They are expressed by nearly all nucleated cells and function in antigen presentation to CD8+ T cells. The classical class I molecules are H-2, K, D, and L in mice and HLA-A, -B, and -C in humans. Class II MHC molecules are heterodimeric membrane proteins that consist of noncovalently associated a and b chains, both encoded in the MHC. They are expressed by antigen-presenting cells and function in antigen presentation to CD4+ T cells. The classical class II molecules are H-2 IA and IE in mice and HLA-DP, -DQ, and -DR in humans.

Adapted from: Kuby, Janis. Immunology. W. H. Freeman and Company, USA, 1997, pp. 597-609.

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