MS is an immunologically mediated disease, as determined by observation of the response to immunotherapy and the existence of an animal model, experimental autoimmune encephalitis. Interferon (IFN) beta-1b, IFN beta-1a, and glatiramer acetate, the therapies used for relapsing or remitting MS, have mechanisms of action that address the immunologic pathophysiology of MS. The IFNs bind to cell surface-specific receptors, initiating a cascade of signaling pathways that end with the secretion of antiviral, antiproliferative, and immunomodulatory gene products. Glatiramer acetate, a synthetic molecule, inhibits the activation of myelin basic protein-reactive T cells and induces a T-cell repertoire characterized by anti-inflammatory effects. Although the two classes of drugs have some overlapping mechanisms of action, the IFNs rapidly block blood-brain barrier leakage and gadolinium (Gd) enhancement within 2 weeks, whereas glatiramer acetate produces less rapid resolution of Gd-enhanced MRI activity. IFN beta has no direct effects in the CNS, but glatiramer acetate-specific T cells are believed to have access to the CNS, where they can exert anti-inflammatory and possibly neuroprotective effects.