Analysis of 8,822 older adults revealed substantial sex differences in 39 of 48 immune cell biomarkers, with variations ranging from 2-38% between men and women. Nine specific immune markers—particularly monocyte, B cell, and NK cell populations—showed significant associations with mortality risk. However, these immune differences contributed minimally to explaining why men have consistently higher mortality rates than women. The immune cell variations had less than 5.4% impact on mortality hazard ratios when sex was factored into models. This challenges assumptions that sex-based immune system differences drive the well-documented male mortality disadvantage. The finding suggests other biological, behavioral, or environmental factors likely play larger roles in sex-based mortality gaps. From a longevity perspective, this indicates that targeting immune system optimization may benefit both sexes similarly, rather than requiring sex-specific approaches. The research also highlights how biological differences don't always translate into meaningful health outcome disparities. As a preprint awaiting peer review, these conclusions require validation, but they offer important insights into the complex relationship between immune function, sex, and aging outcomes in older populations.