Structure and Function of Mucosal Antibodies

Justification for considering the secretory system as more or less distinct from that responsible for the formation of circulating antibody (systemic sys­ tem) stems from a number of lines of evidence. First, studies in the older medical and veterinary literature [for reviews see Tomasi and Pierce (2) I suggest that the titers of specific antibodies present in external secretions vary independently from serum titers, indicating that secretory antibodies are not derived from serum, at least by simple transudation. Sec­ ond, in certain cases, titers of antibodies in external secretions are better cor­ related with resistance to and recovery from infection than are those of se­ rum. Third, more recent observations indicate that many external secretions contain antibodies primarily of the IgA type, whereas this immunoglobulin is a minor constituent of serum. The finding of a predominance of IgA-type plasma cells locally in secretory organs, and the observation that antibodies of the IgA type may be produced by local immunization, support the concept of local synthesis of secretory antibodies. Although most experimental work has thus far focused on IgA, which is present in highest concentrations in external fluids, other immunoglobulins including IgG and IgM are often detectable in significant amounts, particu­ larly when inflammation is present. Very little work has been done with these immunoglobulins. However, evidence is accumulating that both IgG and IgM and, more recently, IgE may be produced locally in small amounts. In this review, emphasis will be placed on secretory IgA which is the predomin­ ant species of antibody. It should be pointed out that the role of these other immunoglobulins, as well as immunoglobulin fragments, may be of consider­ able importance and deserves more attention in future studies.