|Multiple sclerosis is a complex, heterogeneous disorder of the CNS.An unresolved issue is whether similar or different mechanisms contribute to the different MS subtypes. Most think that peripheral immune mechanisms contribute to attacks in RRMS but not in PPMS imbalance between regulatory and effector arms of the immune system may initiate attacks in RRMS. PPMS, favoured hypotheses include a neurodegenerative process or a neuroinflammatory process confined within the blood-brain barrier . outlined below, cumulative data from various investigators suggest an alternate, unifying concept, that premature thymic involution leads to a shift in peripheral T-cell homeostasis, which contributes to the pathogenesis of both RRMS and PPMS . Homeostastic mechanisms maintain body systems in equilibrium in the face of change . T-cell homeostasis balances thymic output of newly generated naive T-cells, delivery of death and survival signals, differentiation of naive T-cells into memory T-cells, and homeostatic proliferation. Homeostatic proliferation compensates for the progressive thymic involution and decreasing thymic output that occurs with age.