Prolactin (PRL) is a peptide hormone synthesized and secreted by lactotroph cells in the adenohypophysis (anterior pituitary gland) that is necessary for the proliferation and differentiation of the mammary glands. PRL plays a role in a number of processes including cell growth, reproduction, and immune function, with its primary function being associated with lactation. 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. Anti-Prolactin reacts with lactotroph cells, and is useful in classification of pituitary tumors and the study of pituitary disease.