Abstract
Biological sex influences virtually every aspect of immune function, from infection susceptibility and vaccine responses to autoimmunity and cancer immunotherapy. Yet, despite decades of observation that males and females differ in disease outcomes, our mechanistic understanding of these differences remains incomplete. Emerging research reveals that sex differences in immunity arise from layered interactions between sex chromosome dosage, hormone signalling across the lifespan, epigenetic regulation, and tissue-specific immune cell states. At the same time, social and environmental factors intersect with biology, complicating interpretation and translation to treatment.