isotypen

Isotypen, in immunology, refer to variants of a protein that share the same antigen-binding specificity but differ in their constant regions. The term is most often applied to antibody isotypes, which define the class of an immunoglobulin and determine its distribution and effector functions.

Immunoglobulins are Y-shaped molecules composed of two heavy chains and two light chains. The variable regions of the chains determine antigen recognition, while the heavy-chain constant region defines the isotype. In humans, the primary antibody isotypes are IgM, IgD, IgG, IgA, and IgE. B cells can switch from one isotype to another through class switch recombination, a genetic rearrangement that alters the heavy-chain constant region without changing the antigen specificity.

Each isotype has distinct roles and properties. IgM is typically the first antibody produced in an immune response and is efficient at agglutination and activating complement. IgG is the most abundant serum isotype and mediates neutralization, opsonization, and, for IgG1 and IgG3, strong complement activation; some subclasses cross the placenta to protect the fetus. IgA predominates in mucosal secretions, providing local immunity. IgE is involved in defense against parasites and mediates allergic reactions through high-affinity Fc receptors. IgD’s function is less well defined and is mainly co-expressed with IgM on naïve B cells.

In addition to the five main human isotypes, many species exhibit further subclass distinctions, such as IgG1–IgG4 and IgA1–IgA2, reflecting functional specialization. Isotype analysis supports diagnostics, informs vaccine responses, and underpins therapeutic antibody design, where a chosen isotype influences efficacy and half-life.