Immunology of the male reproductive system - from testicular immune privilege to seminal plasma-induced reproductive tolerance

Male reproduction poses a distinctive immunological problem. Meiosis and post-meiotic differentiation generate germ-cell antigens after central self-tolerance has been established, yet these antigens must be protected without rendering the reproductive tract defenseless against infection. The solution is not simple sequestration. It is a spatially organized and dynamically regulated immune state created by the blood-testis barrier, Sertoli and Leydig cells, peritubular myoid cells, tissue-resident macrophages, dendritic cells, lymphocytes, soluble mediators, endocrine signals and local metabolic programs. This state permits robust spermatogenesis while retaining context-dependent innate and adaptive responses. Beyond the testis, the epididymis and accessory glands add further layers of antimicrobial defense and tolerance. Seminal plasma carries cytokines, prostaglandins, extracellular vesicles and paternal antigens into the female reproductive tract, where it initiates a controlled inflammatory response followed by antigen-specific immune adaptation. Transforming growth factor beta (TGF-β) is prominent in this signaling network and, together with prostaglandin E (PGE) and other seminal mediators, contributes to dendritic-cell conditioning, leukocyte recruitment and expansion of regulatory T cells (Tregs). This review integrates the cellular and molecular basis of immune privilege in the male reproductive tract with the immunomodulatory function of semen at conception. It also considers how infection, sterile inflammation, autoimmunity, metabolic disease and disruption of barrier integrity impair spermatogenesis and sperm quality. The male reproductive system is best understood not as an immunologically isolated organ but as one component of a bidirectional immunological interface that links paternal physiology to maternal tolerance and reproductive outcome.