Multiple sclerosis (MS) is an autoimmune disease targeting the central nervous system (CNS) and the most common neurological cause of disability in young adults. In most cases, the disease course is characterised by the cycling of relapses and remissions, so called relapsingremitting MS (RR-MS). Although extensively studied, the underlying mechanisms are not fully elucidated, yet CD4+ T cells have been shown to be of importance in disease pathology. A range of treatments are available; the most effective to date being natalizumab, a monoclonal antibody directed against the adhesion molecule VLA-4 on the lymphocyte surface, thereby preventing entry into the CNS. The aim of this thesis was to assess the nature of lymphocyte populations in MS. This was achieved by studying CD4+ T helper cells (TH) and regulatory T cells (TREG) in peripheral blood. In addition, the influence of natalizumab was also investigated, both regarding the effect of the drug on the composition of the peripheral lymphocyte compartment as well as its effects on CD4+ T cells in vitro. We showed an imbalance in the mRNA expression of CD4+ T helper cell lineage specific transcription factors in peripheral blood. While TH1 and TH17 associated TBX21 and RORC expression was comparable in MS and healthy individuals, the TH2 and TREG associated GATA3 and FOXP3 expression was decreased in RR-MS. Given the reciprocally inhibitory nature of TH subsets, this might imply not only diminished function of TH2 and TREG cells but also a permissive state of harmful TH1 and TH17 cells. The size of the peripheral TREG population was unaltered in RR-MS. When analysed in detail, activated and resting TREG were distinguished, showing clear differences in FOXP3 and CD39 expression. Furthermore, when investigating these subpopulations functionally, the ability of activated TREG to suppress proliferation of responder T cells was found to be decreased in RR-MS patients compared to controls. To further investigate this defect, the global gene expression of TREG was compared between patients and controls. Gene set enrichment analysis revealed an enrichment (overexpression) of chemokine receptor signalling genes in RR-MS TREG, possibly suggesting a role for chemokines in TREG function. A sizable effect of natalizumab treatment was seen in the composition of peripheral lymphocyte populations after one year of treatment. While the number of lymphocytes increased over all, the largest increase was seen in the NK and B cell compartments.