Abstract

According to the proposed transport model, the J (‘joining’) chain and epithelial SC (‘secretory component’) represent ‘the lock and key’ in the selective external translocation of dimeric IgA and pentameric IgM through serous-type secretory epithelial cells. Incorporation of J chains into these two immunoglobulin isotypes during their production in gland-associated immunocytes apparently induces a configurational fit (binding site) allowing them to combine with SC in the plasma membrane of the epithelial cell. This complexing on the basolateral surface of the cell seems to stimulate pinocytosis (adsorptive endocytosis); the completed secretory IgA and secretory IgM molecules with bound SC are then transported in cytoplasmic vesicles to the gland lumen along with an excess of free SC. The following observations that support the proposed transport model will be discussed in some detail: 1. Immunoglobulin-binding properties of SC; 2. Characteristics of the immunoglobulin products of gland-associated immunocytes; 3. Localisation of SC and immunoglobulins in normal and neoplastic secretory epithelium.

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