Prolactin is a peptide hormone of pituitary origin and is well known for the stimulation, initiation, and maintenance of lactation. Prolactin is also known to regulate mammary gland development and immune system, and has an essential role in metabolism and behavioral modification (Bernichtein et al.). Prolactin actions are mediated by the prolactin receptor (PRLR), which is a single-chain transmembrane protein composed of an extracellular, transmembrane, and intracellular domain. It has also been indicated that high to normal circulating levels of prolactin increase breast cancer risk, and dopamine agonists have been suggested to be effective for treatment (Bernichtein et al.). Prolactin levels have been reported to be higher in multiple sclerosis patients, which promotes B cell autoreactivity (Correale et al.). It has been shown that administration of recombinant prolactin after bone marrow transplantation in mice promotes immune and myeloid reconstitution (Sun et al.).