EVOLUTION OF IMMUNOLOGICAL FUNCTIONS OF THE MAMMARY-GLAND AND THE POSTNATAL-DEVELOPMENT OF IMMUNITY

Physiologic delays in production of immune factors occur in mammals in cluding Homo sapiens. This finding is counter to a basic tenet of biol ogic evolution, because such delays increase the risk of infections. T he disadvantage is, however, offset by defense factors in milk of the species in whom the developmental delay occurs. Reciprocal relationshi ps between the production of immune factors by the lactating mammary g land and the production of those defense agents during early infancy a re found in all investigated mammalian species. Thus, the evolution of these processes is closely related. Certain immunologic components of milk are highly conserved, whereas others vary according to the speci es. The variations most likely evolved by genetic mutations and natura l selection. In addition, the immune composition of mammalian milks is associated with developmental delays in the same immunologic agents. Furthermore, most closely related mammals, such as humans and chimpanz ees, are most similar in the defense agents in their milks and the cor responding developmental delays in their immune systems. Defense facto rs in human milk include antimicrobial agents (secretory IgA, lactofer rin, lysozyme, glycoconjugates, oligosaccharides, and digestive produc ts of milk lipids), antiinflammatory factors (antioxidants, epithelial growth factors, cellular protective agents, and enzymes that degrade mediators of inflammation), immunomodulators (nucleotides, cytokines, and antiidiotypic antibodies), and leukocytes (neutrophils, macrophage s, and lymphocytes). Because of a lack of geographic/ethnic variation in the immunologic composition of human milk and corresponding immunol ogic delays in infants, these evolutionary processes seem stable. This is supported by investigations of diverse populations that indicate t hat this evolutionary outcome is highly beneficial to human infants.