The main function of
secretory IgA is to exert immune exclusion; that is, by intimate cooperation with innate non-specific defence mechanisms, it dampens down penetration of soluble
antigens and inhibits epithelial colonisation of bacteria and viruses.
Secretory IgM may exert a similar protective function in the gut as its local synthesis sometimes is markedly increased, especially in selective
IgA deficiency.
IgG should not be considered a secretory
immunoglobulin because its external translocation depends on passive intercellular diffusion. By activating
complement,
antibodies of this isotype may cause increased mucosal permeability and tissue damage.
IgG may thus contribute to persistent immunopathology in mucosal lesions. The same is true for
IgE antibodies which, in atopic individuals, may be carried into the gut mucosa by mast cells and cause their degranulation with histamine release.
Secretory IgA and
secretory IgM are the products of two cell types: plasma cells synthesise
IgA dimers and
IgM pentamers which, by non-covalent association, become complexed with the
secretory component (SC) which is synthesized by serous-type glandular cells. The adsorption of the Ig
polymers to the SC-expressing epithelial cells depends on J chain-determined binding sites. This fact gives
biological significance to the striking J chain expression shown by mucosal immunocytes regardless of the Ig class they produce. The immunocytes populating the gut mucosa apparently belong to relatively early memory B cell clones. The obvious functional goal of J chain expression at this stage of clonal differentiation is local generation of SC-binding
IgA and
IgM polymers. In various gut diseases, altered immune regulation results in a disproportionately increased number of J chain-negative
IgG-producing cells in the mucosa. Such altered immunological homeostasis may contribute to perpetuation of
inflammatory bowel diseases.