**Abstract**

Multiple sclerosis is a relapsing and eventually progressive disorder of the central nervous system that continues to challenge researchers who try to understand the pathogenesis of the disease and prevent its progression. Interferon-beta is the most widely prescribed treatment for MS. Peripheral blood seems to mirror the immunological disturbances that underlie MS, which could represent the migration patterns between periphery and other tissues according to the clinical phase of the disease. Based on this assumption, several studies point to significant alterations in peripheral blood homeostasis of different subpopulations of T cells, like γδ T cells or Th1, Th2 and Th17 functional subsets; of B cells subpopulations; and of innate cells like monocytes and dendritic cells. The main goal of this chapter is to make an in-depth review of the major findings described in the literature that correlate specific alterations on different leukocytes subpopulations with disease status, and which therefore have the potential to constitute a peripheral biomarker of disease progression.

**Keywords:** biomarkers, T cells, B cells, dendritic cells, monocytes
