After puberty, sex-related differences emerge in the memory B-cell compartment, with cisgender women (assigned female at birth) showing higher frequencies than age-matched cisgender men. As these cells are essential for long-term humoral immunity and rapid antibody responses upon antigen re-exposure, this difference may have functional relevance. The attenuation of this difference after menopause points to an important hormonal contribution. This interpretation is further supported by observations in transgender individuals: suppression of endogenous oestrogens in transgender men with an XX chromosomal complement was accompanied by a reduction in memory B-cell frequencies, whereas oestrogen administration to transgender women with an XY karyotype did not induce a comparable increase. A similar pattern has been reported in postmenopausal women, in whom hormone replacement therapy (HRT) was associated with higher memory B-cell counts than in untreated counterparts [21]. Reprinted from [22] under a Creative Commons CC-BY license.