Transport of proteins and other macromolecules by transcytotic pathways involves endocytic uptake of substances at the basal membrane, formation and maturation of endosomes, and sorting to lysosomes for degradation or to the apical recycling compartment for exocytosis at the apical membrane. The best studied molecule in this regard is immunoglobulin A (IgA). IgA is synthesized by plasma cells in the interstitial spaces of the mammary gland or elsewhere in the body and binds to receptors on the basal surface of the mammary alveolar cell; the entire IgA-receptor complex is endocytosed and transferred to the apical membrane, where the extracellular portion of the receptor is cleaved and secreted together with the IgA. It is thought that many other proteins, hormones, and growth factors that find their way into milk from the plasma are secreted by a similar mechanism.
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