Precise immunohistochemical information about the mucosal distribution of diffusible immunoglobulin (Ig) components and the local occurrence of Ig-containing cells can be obtained by studying in parallel directly fixed and saline-extracted biopsy specimens. This combined approach is useful for evaluating systemic and local contributions to the mucosal Ig supply in patients with immunodeficiency. Their mucosal populations of Ig-bearing cells can also be demonstrated immunohistochemically.
A new model for the secretory Ig system is based on experience with this technique applied to normal mucosal specimens from various levels of the human respiratory and gastrointestinal tract. The serous-type secretory epithelial cell produces secretory component (SC) and is also responsible for the selective external transfer and molecular completion of secretory IgA and secretory IgM. Cell surface-associated SC most likely mediates the epithelial affinity for dimeric IgA and 19S IgM, and Ig—SC complexes are probably formed and mobilized in the cell membrane; they may then reach the cytoplasm outside the Golgi apparatus by pinocytosis or facilitated diffusion. The composite molecules finally appear to be extruded into the gland lumen along a general secretory pathway.