CD4 and myeloid sarcoma: Under physiological conditions, lymphocyte traffic across the BBB is low, but during inflammatory diseases of the CNS—such as multiple sclerosis (MS) or its animal model, experimental autoimmune encephalomyelitis (EAE)—circulating CD4+ and CD8+ lymphocytes gain access to the CNS leading to inflammation, demyelination, and neurodegeneration [1–3].