BTN1A1, also known as BTN, is a member of the immunoglobulin superfamily and the major protein linked to milk fat droplets. It may function as a cell surface receptor. Its B30.2 domain binds to xanthine oxidoreductase (XOR), stabilizing XOR's association with the milk fat globule membrane and playing a key role in milk fat globule secretion. This binding is conserved among BTN1A1 orthologs but isn't shared by BTN2A1 or BTN3A1. The B30.2 domain in butyrophilins can sense changes in intracellular phospho-antigen (pAg) levels, and its binding to pAg triggers gamma δ T cell activation. Moreover, BTN1A1 regulates milk lipid content and droplet size, and it can inhibit the proliferation and metabolism of CD4 and CD8 T-cells activated by anti-CD3 antibodies, as well as their secretion of IL2 and IFNG.
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