The origin of salivary IgA.

Abstract The IgA of human saliva may originate as a result of selective transport from serum or by local synthesis. Studies in man with radioiodinated IgA, albumin, and IgG were conducted to decide between these possibilities. Following intravenous administration of labeled proteins to patients, very small quantities of protein-bound activity did indeed appear in the saliva. To determine the quantitative significance of this passage, the specific activity of IgA and other proteins was determined in the serum and saliva for 8 days following intravenous injection of labeled protein. The albumin specific activity in serum and saliva was equal, whereas the salivary IgG specific activity was approximately 45 per cent of that in serum, indicating that all of the salivary albumin and about half of the salivary IgG originates from the plasma. In contrast, the specific activity of the IgA in the saliva was only 1 to 8 per cent of that of the serum at the corresponding time. The lower specific activity of IgA in the saliva can be explained by dilution of serum IgA by unlabeled locally synthesized IgA. Therefore, at least 96 per cent of salivary IgA is synthesized at local sites. To test for the presence of selective IgA transport, the clearance of IgA into the saliva relative to that of albumin, was studied by the simultaneous administration of 125 I-IgA and 131 I-albumin, and then measurement of the ratio of IgA and albumin counts in both serum and saliva. A clearance ratio of IgA to albumin in the saliva over 1.0 would indicate preferential IgA transport. In fact, the ratio was only 0.78 to 0.98, indicating that, if anything, relatively less IgA than albumin entered the saliva from the serum. These studies strongly support the view that, while small amounts of IgA do pass from serum to saliva, this accounts for no more than a small percentage of the total IgA in saliva. In addition, there is no significant specific preferential transport of IgA from the serum to saliva, and the IgA found in the saliva probably enters by nonspecific transudation common to all serum proteins.