It commonly refers to the immunoglobulin heavy chain. The heavy (H) chain is the larger of the two types of chains that comprise a normal immunoglobulin or antibody molecule. The heavy chain portion of an antibody contains 2 regions; the Fab (Antigen-binding portion) and Fc (Constant region; confers biological activity such as phagocytosis of microorganisms, lysis, and clumping of organisms together). Amino acid sequence determines the type of heavy chains, and heavy chains define the isotype of Ig. Immunoglobulin G has γ gamma heavy chains, IgA has α alpha heavy chains, IgM has μ mu heavy chains, IgD has δ delta heavy chains, and IgE has ε epsilon heavy chains. In contrast, all light chains are either κ kappa or λ lambda light chains, either of which may be found on any Ig molecule, regardless of isotype. Each Ig unit is made up of 2 heavy chains, 2 light chains, and has 2 antigen-binding sites. Heavy chain is joined with the light chain with the help of disulphide bond.
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