Sex has a major role in determining susceptibility and progression of disease, ranging from communicable diseases (such as viral infections) to autoimmune disorders (such as rheumatoid arthritis). Fundamentally, the immune system of males and females are known to differ, but sex differences are generated through the interaction of numerous genes, biological and environmental factors, including age and life events. As such, it remains to be understood which of these genes and factors contribute the most to differences in disease and which could help in diagnosis and therapies. In this work, we aimed to characterize differences in the immune system across the sexes at single-cell resolution to help better tease apart these factors. We have analysed 1,267,758 cells (PBMCs) from 982 individuals to give a clearer picture of a populations immune landscape. We assessed cell-type proportions and sex-and age- biased gene expression. We confirm previous findings of differences in immune cell-types across age and sex (such as NK cells), but at a greater resolution as single-cell transcriptomics afforded multiple levels of cell-type classification. These preliminary findings suggest analysing data with sex, age and cell-type as key biological variables will improve future disease discoveries.