Prolactin is a peptide hormone secreted by the anterior pituitary that is necessary for the proliferation and differentiation of the mammary glands. Prolactin also acts in a cytokine-like manner and as an important regulator of the immune system. Prolactin has important cell cycle related functions as a growth, differentiating and anti-apoptotic factor. Prolactin is secreted by lactotrophs in the anterior pituitary. Prolactin producing cells make up approximately 20 percent of the pituitary. Elevated counts of these cells have been observed in pregnant women, newborns and in multiparous women. An antibody to prolactin is useful for the identification of pituitary tumors.