ImmunoglobulinSuperfamilie

Immunoglobulin, or antibody, is a large glycoprotein produced mainly by plasma cells that recognizes and binds to specific antigens. Each immunoglobulin molecule is a Y-shaped structure made of two identical heavy chains and two identical light chains, linked by disulfide bonds. The molecule has variable regions that form the antigen-binding site and constant regions that determine its effector properties. The antigen-binding fragment (Fab) binds antigen, while the crystallizable fragment (Fc) interacts with cell receptors and complement proteins.

Immunoglobulins are divided into five major isotypes: IgA, IgD, IgE, IgG, and IgM. IgM is typically the first antibody produced in an immune response and is often present as a pentamer in the bloodstream. IgG is the most abundant in serum and provides long-term protection; it has four subclasses (IgG1–IgG4) with differences in function. IgA is abundant in mucosal secretions and exists as dimers in secretions like saliva and breast milk. IgD primarily serves as a B-cell receptor on naive B cells. IgE is involved in defense against parasites and in allergic reactions.

Antibodies are produced by B cells and diversify through V(D)J recombination and somatic hypermutation. Class-switch recombination alters the isotype without changing antigen specificity, enabling different effector responses. Functions include neutralization of pathogens and toxins, opsonization to enhance phagocytosis, activation of the complement system, and antibody-dependent cellular cytotoxicity.

Clinically, immunoglobulin levels aid in diagnosing immunodeficiencies or dysgammaglobulinemias. Therapeutic immunoglobulins, such as intravenous immunoglobulin (IVIG) or subcutaneous immunoglobulin (SCIG), provide passive immunity for selected disorders and autoimmune diseases and are derived from pooled plasma or produced as monoclonal antibodies.