constantregion

The constant region, in immunology, refers to the portion of an antibody molecule that is relatively conserved within a given class and forms the base of the antibody's structure. In typical antibodies, the heavy chain contains constant domains (CH1, CH2, CH3) in addition to a variable domain, while the light chain has a constant region (CL) adjacent to its variable domain. The constant region, together with a hinge region, determines the antibody class (isotype) such as IgM, IgG, IgA, IgD, or IgE and influences how the antibody interacts with the immune system.

Genetic organization: The variable regions of the heavy and light chains are generated by V(D)J recombination, yielding antigen specificity. The constant region is encoded by downstream constant-region gene segments. Class switch recombination can replace the constant region of the heavy chain with a different C segment, changing the antibody isotype without altering the antigen-binding site.

Function: The constant region mediates effector functions by binding to Fc receptors on immune cells and to complement proteins, enabling opsonization, phagocytosis, antibody-dependent cellular cytotoxicity, and complement activation. The hinge between CH1 and CH2 provides flexibility for antigen binding. The constant region also influences the antibody's half-life in circulation through interaction with the neonatal Fc receptor (FcRn).

Applications and relevance: In medicine, antibodies are engineered by modifying constant regions to adjust half-life, effector function, or immunogenicity. Therapeutic antibodies often retain human constant regions to minimize immune reactions and may incorporate Fc modifications to enhance or suppress Fc-mediated activities. Understanding constant regions is also important in vaccine design and in studying immune responses.