Upon antigen challenge, T-helper cells differentiate into two functionally distinct subsets, Th1 and Th2. Th1 cells produce IL-2, IFN-gamma and lymphotoxin-b that augment the cell-mediated immune response, while Th2 cells secrete IL-4, IL-5, and IL-10 that enhance humoral immunity. The function of T-helper cells is regulated by cytokines. A novel cytokine receptor was recently identified and cloned. It is a new member in the type I cytokine receptor family and designated TCCR for T-cell cytokine receptor and WSX-1. TCCR deficient mice had impaired Th1 responses to protein antigen challenge, including decreased levels of IFN-gamma and Th1-dependent antibody IgG2a. TCCR is predominantly expressed in thymus, spleen, lymph nodes and peripheral blood leukocytes.