Differences between IgA and IgE as secretory proteins.

IgA is the prototype of a secretory immunoglobulin and usually is present in external fluids in proportions greater than in serum. Previous studies have suggested that IgE also is a secretory immunoglobulin. We measured IgE, IgA and albumin levels in nasal washings (NW), parotid fluids (PF) and serum in seventeen individuals. In secretions, all of these proteins were measured by radioimmunoassay (RIA). In serum, IgE was measured by RIA, IgA by a turbidimetric method, and albumin by radial immunodiffusion. IgA was detected in NW and PF in all individuals and IgA/total protein ratios in both of these secretions exceeded the corresponding ratios in serum (P less than 0-01). In contrast, IgE was detected only in NW from atopic subjects and IgE/total protein ratios were less than those in serum (P less than 0-02). IgE was not detected in the PF in any of the seventeen subjects. When the proportion (protein/total protein) of IgA, IgE, or albumin in NW was expressed as a percentage of the proportion in serum, IgA and IgE were increased when compared to albumin (P less than 0-01 and P less than 0-03, respectively). These results support the view that IgE is a secretory immunoglobulin in that it is present in NW in proportions greater than expected if it diffused from serum. However, IgE differs from secretory IgA in that ratios of IgE to total protein are greater in serum than in NW. This difference is discussed in terms of local production of IgE and passive diffusion into serum and nasal secretions. Finally, the observation that IgE is present in one external secretion while absent from another indicates that external secretions may be subdivided into two classes on the basis of their content of IgE.