Humans, like many other species, show striking sex differences in immune function. While the proximate determinants of sex differences across species can include both chromosomal and hormonal differences, the ultimate evolutionary determinants will be rooted in differences in investments in competing and caring between the sexes. Ultimate explanations have classically been framed in terms of quantitative sex differences in immune function (with males typically the ‘weaker’ sex), but have failed to account for qualitative sex differences in immune function. We model immune trade-offs that might underpin these differences. Once the dangerous side of immunity is accounted for, two distinct immunological profiles emerge as peaks on a fitness landscape, one aligned with investment in pathogen detection and the other with investment in pathogen killing. Intriguingly, alignment of these fitness peaks with observed sex differences in birds and mammals – where females typically favor detection – cannot be explained by selection pressures emerging from models including immunity trade-offs alone. Age-specific patterns of infection, and/or maternal transfer of immunity (e.g., maternal antibodies) are also required. The characterization of immune function underlying these predictions is necessarily a simplified caricature of the true complexity. We develop projections of the scope of expectations of this simplified caricature in terms of the development of immunity across the life course in the two sexes.