May function in the secretion of milk-fat droplets. May act as a specific membrane-associated receptor for the association of cytoplasmic droplets with the apical plasma membrane (By similarity). Inhibits the proliferation of CD4 and CD8 T-cells activated by anti-CD3 antibodies, T-cell metabolism and IL2 and IFNG secretion. Seems to associate with xanthine dehydrogenase/oxidase. Secreted in association with the milk-fat-globule membrane during lactation. Strongly expressed in lactating mammary tissue (at protein level). About 100-fold lower levels in virgin mammary tissue. Also detected in spleen and thymus at 10-20 times lower levels compared to those detected in virgin mammary gland. Very low levels in several other tissues, including brain, heart, kidney, lymph node, lung and small intestine. In the thymus, detected in the stroma, in epithelial cells (at protein level). Most prominent in medullary areas of the thymus and at the corticomedullary junction (at protein level). Expression increases during the last half of pregnancy and is maximal during lactation. N-glycosylated. Belongs to the immunoglobulin superfamily. BTN/MOG family.