Biological activities of IgA

Immunoglobulin A (IgA) is the most heterogeneous of immunoglobulin isotypes, as it occurs in a variety of molecular forms as well as subclasses and allotypes. However, the patterns of heterogeneity vary significantly between different species of mammals and birds. In humans, chimpanzees, gorillas, and gibbons, there are two unique subclasses (IgA1 and IgA2), whereas most other animals that have been investigated have only one, with the remarkable exception of the lagomorphs (rabbits and their allies), which have 13 IgA subclasses. Two, possibly more, allotypes of human IgA2 appear to represent different combinations of constant-region domains of the α-heavy chains. In humans and other primates, the predominant molecular form of circulating (serum) IgA is monomeric, in contrast to the pIgA that is produced in mucosal tissues and transported into the secretions as S-IgA. Further levels of heterogeneity arise from the variable number and composition of the oligosaccharide side-chains present on α-heavy chains. S-IgA antibodies to various viruses can effectively neutralize them. While inhibition of viral binding to cellular receptors is a plausible mechanism in many cases, inhibition of viral replication may occur by other means, depending upon the epitope specificity, isotype, and concentration of antibody as well as the virus and cells involved.