ABSTRACT
The human immunoglobulins are a family of proteins that confer humoral immunity and perform vital roles in promoting cellular immunity. Five distinct classes or isotypes of immunoglobulins (IgG, IgA, IgM, IgD, and IgE) have been identifi ed in human serum on the basis of their structural, biological, and antigenic differences.1-4 IgG and IgA have been further subdivided into subclasses IgG1, IgG2, IgG3, and IgG4 or subclasses IgA1 and IgA2 on the basis of unique antigenic determinants.5,6 Multiple allotypic determinants in the constant region domains of human IgG and IgA molecules as well as kappa (κ) light chains indicate inherited genetic markers. Finally, there are several immunoglobulin-associated polypeptides such as secretory component (SC) and J chain that have no structural homology with the immunoglobulins, but serve important functions in immunoglobulin polymerization and transport across membranes into a variety of secretions (e.g., saliva, sweat, nasal secretions, breast milk, and colostrum). This diversity of the immunoglobulin components of the humoral immune system provides a complex network of protective and surveillance functions (e.g., see the role of immunoglobulins in the gastrointestinal tract in Chapter 13 by Guandalini).
