The immune response to HTLV infection in humans is char-acteristic and depends on whether the patient develops malignancies or neuropathy. HTLV infection is characterized by the development of circulating antibodies within 4–8 weeks, which remain positive for the rest of the life. Paradoxically, the patients who have got high level of antibodies appear to be at a greater risk for HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). These antibodies do not confer any protective immunity in patients.
Cell-mediated immunity (CMI) fails to eradicate HTLV infection possibly due to ability of the virus to spread without replication, through cell-to-cell contact. The CMI through its cytotoxic T-cell lymphocytes appears to contribute to the intra-medullary degeneration of the central nervous system. On the other hand, cytotoxic T-cell lymphocyte responses in patients with HTLV are depressed, which leads to increase in the number of transformed cells.