Endogenous steroid hormones exert robust effects on inflammatory and immune processes, but the immunological activities of steroidogenesis precursors remain largely unknown. Here, we used a systemic approach to examine the relationship between steroid profiles and immune traits in healthy volunteers. Serum concentrations of steroids and their precursors (cortisol, progesterone, testosterone, androstenedione, 11-deoxycortisol and 17-OH-progesterone) were determined by liquid chromatography-tandem mass spectrometry. Immune traits were evaluated by quantifying cellular composition of the circulating immune system and ex vivo cytokine responses elicited by major human pathogens and microbial ligands. The correlation matrix was derived and validated in two large independent cohorts, followed by in vitro validation experiments. We reported that 11-deoxycortisol positively associated with lymphoid cellular subsets numbers and functions after correcting for age, sex and oral contraceptive use in the discovery cohort, which was mainly reflected in the naive and memory cell populations, along with the IL-17 response. The association with lymphoid cellularity was confirmed in the validation cohort. Experiments showed that 11-deoxycortisol promoted T cell proliferation and Candida-induced Th17 polarization in vitro. These results suggest that 11-deoxycortisol could be a positive immunomodulator in physiological conditions.