Biological sex differences refer to differences between males and females caused by the sex chromosome complement (that is, XY or XX), reproductive tissues (that is, the presence of testes or ovaries), and concentrations of sex steroids (that is, testosterone or oestrogens and progesterone). Although these sex differences are binary for most human individuals and mice, transgender individuals receiving hormone therapy, individuals with genetic syndromes (for example, Klinefelter and Turner syndromes) and people with disorders of sexual development reflect the diversity in sex-based biology. The broad distribution of sex steroid hormone receptors across diverse cell types and the differential expression of X-linked and autosomal genes means that sex is a biological variable that can affect the function of all physiological systems, including the immune system. Sex differences in immune cell function and immune responses to foreign and self antigens affect the development and outcome of diverse diseases and immune responses.

生物性别差异是指由性染色体补体（即XY或XX）、生殖组织（即睾丸或卵巢的存在）和性类固醇浓度（即睾酮或睾酮）引起的男性和女性之间的差异。雌激素和黄体酮）。尽管这些性别差异对于大多数人类个体和小鼠来说是二元的，但接受激素治疗的跨性别个体、患有遗传综合征（例如克兰费尔特综合征和特纳综合征）的个体以及患有性发育障碍的个体反映了基于性别的生物学的多样性。性类固醇激素受体在不同细胞类型中的广泛分布以及 X 连锁基因和常染色体基因的差异表达意味着性别是一个生物变量，可以影响包括免疫系统在内的所有生理系统的功能。免疫细胞功能以及对外来和自身抗原的免疫反应的性别差异影响多种疾病和免疫反应的发展和结果。