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By Levi Clancy for Student Reader on

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Antibodies (aka immunoglobulin or Ab) are produced by B cells and specifically bind to antigens (aka Ag) in solution (as opposed to TCRs, which bind antigens on cell surfaces).

An antigen is any substance that binds specifically to a T cell receptor or B cell receptor. This antibody-antigen binding can: cause the antigen to be engulfed by macrophages (opsonization); can neutralize a virus and prevent it from entering cells; and can induce the complement cascade (causes bacterial lysis).

All antibodies exist in both secreted and membrane-bound (mIg) forms, differing in their carboxy terminal sequence. Secreted antibodies have a hydrophilic terminus, while membrane immunoglobulins have a hydrophobic sequence which inserts into the plasma membrane and a short cytoplasmic sequence.

Antibodies have various functions, described below:
AgglutininsCause clumping-together and destruction of foreign cells. For example, agglutinin activity will clump transfused blood cells of a foreign blood type.
AntitoxinsNeutralize toxins from foreign microbes.
OpsoninsFacilitate engulfment of foreign microbes.
PrecipitinsForm flocculate (cloudy, lumpy) precipitates of cell-free supernatant of foreign microbes.
Antibodies consist of heavy chains and light chains. The different istotypes of antibody have structurally different heavy chains, leading to functional differences as well.

IgG is the most common immunoglobulin isotype, and its relatively simple structure (consisting of two γ heavy chains, and two light chains) is shown to the right. There are five different types of heavy chain to differentiate the five antibody isotypes (IgA, IgD, IgE, IgG and IgM).

Antibody chains contain constant domains (same among antibodies) and variable domains (different among antibodies).

Heavy chains have four or five constant domains at one end, a variable region at the other and also at least one carbohydrate moiety attached. There are many types of heavy chain variable regions. Light chains have a constant region at one end and a variable region at the other end. Light chain variable regions are either kappa (κ) isotype or lambda (λ) isotype. The two light chain isotypes have no known functional differences. Free light chains are known as Bence-Jones proteins.

Variable regions contain relatively conserved regions and hypervariable regions (aka complementarity determining regions).

Complementarity determining regions (CDRs) of heavy and light chain variable regions group together to form a complex that directly interacts with antigens. X-ray crystallography has shown that CDRs are loops which stick out for accessibility, and that relatively conserved regions are merely a scaffold to make sure these CDR loops stay in place.

Next StepsStudy the different immunoglobulin isotypes.