Apelin (APLN) is an endogenous ligand for APJ, an alternative coreceptor with CD4 for HIV-1 infection. It inhibits HIV-1 entry in cells coexpressing CD4 and APJ. It is widely expressed in various organs. Apelin-36 has a greater inhibitory activity on HIV infection than other synthetic apelin derivatives. The oral intake in the colostrum and the milk could have a role in the modulation of the immune responses in neonates. May also have a role in the central control of body fluid homeostasis by influencing AVP release and drinking behavior. Recently, 46 different apelin peptides ranging from apelin 55 (proapelin) to apelin 12 have been identified in bovine colostrum.