Multiple sclerosis (MS) is an immune-mediated demyelinating disease of the central nervous system (CNS) which has a complex aetiology that involves the interplay of both genetic and environmental factors. Although more than 200 susceptibility loci have been identified through genome-wide association studies, our understanding of the molecular mechanisms underlying these associations remains limited. My predecessor in the laboratory established that the MS associated variants rs9282641 and rs4810485 respectively influence the expression of CD86 and CD40 in freshly collected (ex-vivo) B cells. In my research, I sought to extend this work by investigating the impact of these variants in the context of activation in an effort to refine understanding of the molecular mechanisms underlying the effects of these genotypes on the risk of developing MS. In the first phase of my work I processed venous blood samples from 108 healthy volunteers, who were recruited from the Cambridge BioResource (CBR) to enable a balanced representation of each possible genotype. I isolated peripheral blood mononuclear cells (PBMCs) from these samples and then activated these with CD40L-transfected L cells. In line with the ex vivo results, I found that the expression of CD86 and CD40 remained associated with the genotypes of the respective MS associated variants. I then went on to investigate the effects of these stimulated cells on co-cultured naïve T cells by studying a second cohort of 28 volunteers recruited in pairs from the CBR; the individuals in each pair carrying either the AA or the GG genotype at rs9282641. In these experiments I co-cultured naïve T cells with purified B cells that had been activated with CD40L-transfected L cells. In these 12-day co-cultures, the T cells from subjects homozygous for the risk allele (GG) at rs9282641 demonstrated significantly enhanced proliferation and displayed significantly higher IFN-γ and IL-17 production, and lower IL-10 production. In my PhD I have therefore shown that the altered expression of co-stimulatory molecules on B cells induced by MS risk variants results in enhanced proliferation of pro-inflammatory T cells.