The IgA effector sites such as the salivary glands and the intestinal tract contain several distinct T cell subsets which possess unique biologic characteristics. Freshly isolated CD3+ T cells from the salivary glands, the LP region of the small intestine and IELs all harbor T cells which spontaneously produce Th1 (IFN-gamma)- and Th2 (IL-5 and IL-6)-type cytokines. Interestingly, a high frequency of IL-5-producing Th2-type cells is always associated with the occurrence of increased numbers of IgA plasma cells (e.g., the salivary glands and the LP region of the small intestine). Further, the salivary gland CD3+ T cells can be divided into three distinct subsets including those of CD4+, CD8- (12-23%), CD4-, CD8+ (18-25%) and DN (6-16%) T cells. In terms of TCR expression, CD4+, CD8- and DN T cells exclusively expressed alpha/beta TCR and gamma/delta TCR, respectively. One of the unique features of the salivary gland T cells is that like IELs, relatively high numbers of gamma/delta TCR-bearing cells are seen in the CD4-, CD8+ T cell fraction. Since our study has provided important new evidence that these gamma/delta TCR-bearing T cells from IELs of mice orally immunized with TD antigen possess the capability of abrogating oral tolerance to antigen-specific immune responses including the IgA isotype, one can visualize that gamma/delta TCR+ T cells can be essential regulatory T cells which protect (or enhance) alpha/beta TCR+, CD4+ Th cells for maximum IgA responses at IgA effector tissues including the salivary glands, and the gastrointestinal tract in the presence of an active state of systemic unresponsiveness or oral tolerance.