The underlying theme in numerous studies on coated vesicles is that these very distinctive and ubiquitous organelles are intimately involved in the selective uptake and transport of exogenous proteins by cells. Yet, even though many recent studies have helped elucidate the structure of these vesicles, it is still not clear what their precise transport functions are in most of the tissues where they have been identified.

Many of the best examples of coated vesicles being implicated in selective transport come from studies on tissues that transfer maternal immunoglobulins to the mammalian young. The tissues involved, depending on the particular species, include the yolk sac and chorioallantoic placenta of the foetus, and the small intestine of the newborn. In each tissue, absorptive cells are present which contain prominent populations of coated vesicles. Furthermore, considerable evidence has established that immunoglobulin transmission across each tissue is selective and, by implication, involves membrane carriers or receptors (Brambell, Halliday & Morris, 1958; Brambell, 1970). However, the exact site of the receptors and the degree of selection within cells are both ambiguous. Much of this uncertainty lies in the fact that all of the tissues serve a second, equally important function for the young, this being the relative non-selective uptake and intracellular digestion of proteins for nutrition. This review considers the physiology of these two functions and the possible roles of coated vesicles in each. It examines the evidence for the presence and locations of specific receptors for immunoglobulins and other proteins, particularly within coated vesicles.