Considerable evidence supports the concept that the immune defense against infectious agents that attack primarily the cells of the mucosal linings lies in secreted antibodies belonging to the IgA class of immunoglobulins. The problem that thus arises, in connection with the possible use of adjuvants in active immunization against these infectious agents, is that of the relationships between antibodies in the serum and those in the secretions. We have performed experiments that show that passive transfer of IgA antibodies to the secretions takes place much more readily if the IgA molecules are already conjugated with an additional polypeptide chain that is normally attached to them in the secreting glands, and that has been called the “transport” piece. Our data provide direct evidence for an effective role of the said piece in the transport of IgA antibodies across the secreting glands. It appears likely that, under normal conditions, a high local concentration of IgA antibodies, that is the consequence of the presence of IgA plasma cells in the immediate proximity of the secreting epithelial cells, facilitates the conjugation of the IgA molecules with the “transport” piece; in fact we could show, by immunofluorescence, a prevalence of IgA plasma cells at the level of the mucosal linings of the respiratory tract. It would appear that factors (so far poorly understood) that increment the production of IgA antibodies at the mucosal and glandular levels might be of more practical importance than those that only raise the serum antibody levels with regard to the active immunity against the above mentioned infectious agents.