Butyrophilin Subfamily 1 Member A1 (BTN1A1) is the major protein associated with fat droplets in the milk. It belongs the immunoglobulin superfamily. BTN1A1 acts as a specific membrane-associated receptor for the association of cytoplasmic droplets with the apical plasma membrane. It is localized to the major histocompatibility complex (MHC) class I region of 6p. It may have arisen relatively recently in evolution by the shuffling of exons between 2 ancestral gene families. It is shown that BTN1A1 inhibits the proliferation of CD4 and CD8 T-cells activated by anti-CD3 antibodies, T-cell metabolism and IL2 and IFNG secretion.