A protein purified from bovine colostral whey and isolated nucleotide sequences encoding the protein is identified. The isolated bovine protein is termed Bovine Lactation Associated Immunotropic (Bo-LAIT) protein. The human homologue of Bo-LAIT protein, Hu-LAIT protein, is also described. A method of activating B cells, and particularly of activating B cells in a mammal, such as a human, in need of such activation by administering LAIT protein is described. LAIT protein can be incorporated into infant formula. LAIT protein can be administered to an infant, as by feeding to the infant such formula. LAIT protein can be incorporated as part of a vaccination, LAIT protein can be administered to a patient having a T cell immune deficiency, for example, a particular T cell dysfunction in which gp39 (CD40L) is under expressed on or totally absent from the cell surface of patient T cells. Preparation of medicaments including LAIT protein for activating B cells in a mammal in need of such activation is described. Natural or recombinant LAIT protein can be used.