Ig

Ig is the abbreviation commonly used for immunoglobulin, a type of glycoprotein antibody produced mainly by plasma cells derived from B lymphocytes. Immunoglobulins are key components of the adaptive immune system and can function as soluble antibodies or as membrane-bound receptors on B cells.

Immunoglobulins are Y-shaped molecules composed of two identical heavy chains and two identical light chains linked by disulfide bonds. Each antibody has variable regions at the tips of the arms that form the antigen-binding sites, enabling specific recognition of antigens, and constant regions that determine the class and effector functions. In humans there are five major isotypes: IgA, IgD, IgE, IgG, and IgM. IgM is typically produced first in an immune response; IgG is the most abundant in serum and provides long-term protection; IgA is predominant in mucosal secretions; IgE mediates allergic reactions and defense against parasites; IgD has a less clearly defined role.

Functionally, antibodies neutralize pathogens, promote agglutination or precipitation of antigens, and mark targets for attack by other immune mechanisms through opsonization and activation of the complement system (notably IgM and IgG). IgG can mediate antibody-dependent cellular cytotoxicity, while secretory IgA protects mucosal surfaces. Sustained immunity is supported by memory B cells and long-lived plasma cells that continue to produce antibodies.

Genetically, immunoglobulin genes rearrange during B-cell development via V(D)J recombination, and class switch recombination changes the heavy-chain constant region to produce a different isotype without altering antigen specificity. Abnormal immunoglobulin levels or antibodies underlie various conditions, including immunodeficiencies, autoimmune diseases, and monoclonal gammopathies. Therapeutic antibodies are widely used in medicine and are often based on IgG.