The oral sphingosine 1-phosphate (S1P) receptor (S1PR) modulator fingolimod has been shown to be effective in the treatment of patients with relapsing multiple sclerosis (MS). The drug binds with high affinity to 4 of the 5 G-protein-coupled S1P receptors (S1P(1-5)). After binding, the receptors are internalized, degraded, and thus functionally antagonized by fingolimod. Under physiologic conditions, S1P(1) mediates the egress of lymphocytes from secondary lymphoid organs to the peripheral circulation. Functional antagonism of S1P(1) by fingolimod results in a reduction in peripheral lymphocyte counts by inhibiting egress of lymphocytes, including potentially encephalitogenic T cells and their naïve progenitors that would otherwise be present within the circulation. Despite the fingolimod-mediated reduction of lymphocyte counts, fingolimod-treated patients with MS have been shown to have few infections and related complications and were able to mount antigen-specific immune responses in vaccination studies.