Human Prolactin (PRL) is a peptide hormone belonging to the prolactin/growth hormone family, which also includes growth hormone (GH) and placental lactogen (CSH) that regulate growth, metabolism, and reproductive physiology. Prolactin is produced primarily by lactotroph cells in the anterior pituitary gland and signals through the prolactin receptor (PRLR), activating intracellular pathways such as JAK2/STAT5, MAPK, and PI3K, which regulate gene expression and cellular responses. The hormone plays a central role in lactation, stimulating mammary gland development and milk production following childbirth. In addition to its role in reproduction, prolactin influences immune regulation, metabolism, reproductive function, and behavior, and can be produced by several extrapituitary tissues including immune cells, the uterus, and mammary tissue. Dysregulation of prolactin levels is associated with disorders such as hyperprolactinemia, infertility, and prolactin-secreting pituitary adenomas (prolactinomas). Because prolactin participates in both endocrine and immune signaling, human PRL is widely studied in reproductive endocrinology, immune regulation, and pituitary disease, contributing to understanding of hormone signaling pathways and their roles in human health and disease.