Background Cis-gender females mount stronger humoral immune responses than cis-gender males in response to infection/vaccination, but are also more likely to develop B cell-driven autoimmune disorders. Murine work suggests a complex interplay between sex chromosomes and hormones leading to this sex-bias. Oestrogen has been shown to enhance class-switch recombination (CSR)- the process by which B cells ‘switch’ to IgG/A/E immunoglobulin isotypes- while testosterone exerts a suppressive effect. This study utilises a unique in vivo human model, with samples from both cis-gender and trans-gender age-matched young healthy volunteers, to investigate the impact of sex-chromosomal complement and hormonal milieu on B cell CSR. Methods Peripheral blood samples were collected from cis-male (n=43) and -female (n=62) volunteers (14-31yrs), and trans-male (n=25) and trans-female (n=23) volunteers (15-19yrs) on GnRH-analogue (“puberty blockers”), +/- testosterone or oestrogen treatment, respectively. PBMC/serum phenotyping was performed using flow cytometry and LEGENDplex™ immunoassay. GraphPad Prism was used for statistical analysis. Results Healthy post-pubertal cis-males had significantly lower levels of class-switched (IgD- CD27+) B cells than cis-females (p=0.002), pertaining specifically to a decrease in IgG+ B cells (p=0.009). Whilst IgG subclasses 1-3 are implicated in autoimmunity and infection control, IgG4 is purported to have immunoregulatory effects. Cis-males had a higher IgG4:IgG1 serum antibody ratio than cis-females (p=0.003). Blocking oestrogen on an XX background in trans-males was sufficient to reduce the proportion of switched B cells compared to cis-females (p=0.005), in-line with the cis-male profile. IgG1:IgG4 ratio however, was unaffected by oestrogen blockade. Testosterone treatment had no further effect on either result. Surprisingly, the addition of oestrogen on an XY background in trans-females showed no overall effect on class-switching compared to cis-males (p=0.250), however, IgG4:IgG1 ratios were decreased significantly (p=0.015). Conclusions Oestrogen differentially affected B cell CSR on XX and XY chromosomal backgrounds. Further work is implicated to establish the mechanisms by which this may occur.