Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force
Figure 2
The basic structure of immunoglobulins. The basic structural unit of Igs comprises 2 identical light chains (shown in red and blue) and 2 identical heavy chains (shown in red and blue). The polypeptide chains of Ig light and heavy chains consist of repeating units of approximately 110 amino acids, so-called “Ig-like” domains with a characteristic 3-dimensional structure. Ig light chains comprise an N-terminal variable (VL) domain followed by a constant (CL) domain of either the κ or λ type. Ig heavy chains comprise an N-terminal variable domain (VH) followed by 3-4 constant domains ( or 4). The sequence of Ig heavy chain constant regions (designated by Greek letters such as α, γ, or μ) defines the Ig class or isotype. The structure shown here is characteristic of mammalian IgA, IgD, and IgG, in which the and domains of the heavy chains are connected by a short cysteine- and proline-rich hinge region. The structure of all Igs in lower vertebrates (as well as IgM and IgE in mammals) differs in that the domain replaces the hinge region and the Fc region comprises the and domains. Pairing of VH and VL domains on adjacent Ig heavy and light chains forms the antigen-binding site, while the Fc regions of the Ig heavy chains (comprising the two most C-terminal constant domains) confer isotype-specific immune effector functions.
