Birth thrusts the mammalian neonate into an environment rich in antigenic stimulation. Because of the lack of exposure of the infant’s immune system to environmental antigens, the possibility exists that untoward responses to food and microbial antigens results in the development of hypersensitivity reactions. It is also possible that the ingestion of maternal lymphocytes could result in graft-versus-host reactivity. The presence of immunoregulatory factors in colostrum would modulate such unfavorable reactions. Further clarification of the nature of humoral factors, colostral cells, their immune reactivity and their effect on the immune response of other lymphocytes would give insight into the immunological consequences associated with breast feeding.