Immune function varies widely across humans. Biological sex is a key factor underlying human immune variability, with men presenting with more severe infections and increased cancer rates, while women exhibit higher vaccine responses and prevalence of autoimmunity. Intrinsic biological sex differences arise from varying contributions of chromosomal sex, and sex hormone sensing and downstream signaling to different cell types. This complex regulation presents a unique opportunity for the exploration of human immune sex differences using systems-level methods of investigation. Here we analyze the current literature and the applications of systems immunology in elucidating the immune sex differences in humans. We examine mechanisms of biological sex modulation of human immunity via sex chromosomes, and particularly emphasize the role of sex hormones. We then focus on how systems immunology has been advancing our understanding of how sex impacts the healthy immune system at steady state, ranging from cell composition, transcriptomics, epigenomics, metabolomics, spatial and cell-cell interactions, to plasma proteomics. We also examine systems-level applications to investigating sex differences upon immune perturbations and give an overview of key future directions for the field. Systems immunology provides a powerful framework to decode biological sex-regulated pathways in immunity, paving the way for more precise, sex-informed therapeutic interventions to address sex differences in immune-related conditions.