A lymphocyte population that expresses surface markers found on T cells and natural killer (NK) cells secretes large amounts of interleukin-4 (IL-4) immediately after T cell receptor ligation. These NK-like T cells are thus thought to be important for the initiation of type 2 T helper cell (T(H)2) responses. CD1-deficient mice were found to lack this lymphocyte subset, but they could nevertheless mount a protypical T(H)2 response; after immunization with antibody to immunoglobulin D (IgD), CD1-deficient mice produced IgE. Thus, although dependent on CD1 for their development, IL-4-secreting NK-like T cells are not required for T(H)2 responses.