Immunoglobulin

Immunoglobulins, also known as antibodies, are large glycoproteins produced primarily by plasma cells derived from activated B lymphocytes. They circulate in blood and lymph and are secreted into mucosal secretions. Immunoglobulins recognize and bind specific molecular structures, or antigens, on pathogens or abnormal cells, helping to control infection and shape the immune response.

An immunoglobulin molecule is Y-shaped, composed of two identical heavy chains and two identical light chains. Each chain contains variable (V) and constant (C) regions. The variable regions form the antigen-binding sites, enabling diversity through V(D)J recombination. The constant regions determine the antibody class (isotype) and interact with other components of the immune system. Light chains are of two types, kappa or lambda.

There are five main human isotypes: IgG, IgA, IgM, IgD, and IgE. IgG is the most abundant in serum and provides long-term protection. IgA predominates in mucosal secretions. IgM is typically produced early in the response and forms pentamers that efficiently activate complement. IgD serves mainly as a B-cell receptor, and IgE is involved in defense against parasites and in allergic responses.

Antibodies mediate protection through multiple mechanisms: direct binding and neutralization of pathogens or toxins; agglutination of antigens; opsonization promoting phagocytosis; activation of the classical complement pathway; and, for some isotypes, antibody-dependent cellular cytotoxicity via Fc receptors on immune cells. Secreted immunoglobulins can form immune complexes that modulate inflammation and clearance.

Clinically, immunoglobulin levels are assessed in diagnosis of immunodeficiency or hypergammaglobulinemia. Immunoglobulin replacement therapy (IVIG or subcutaneous immunoglobulin) is used to treat certain primary and secondary immunodeficiencies. Monoclonal antibodies, engineered immunoglobulins, are deployed as targeted therapies in various diseases.