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Immune responses against bovine serum albumin by chickens were dependent upon thymusderived cells. Thirty-five of seventy chickens that had been neonatally thymectomized and subsequently immunized with bovine serum albumin produced lgM antibodies, but not IgG antibodies, against the antigen. T cells (IgM-T cells) of such chickens were able to help B cells to produce IgM antibody responses but were not able to help them to switch IgM- to IgG-antibody responses. Helper activity of the IgM-T cells was much less susceptible to the cytotoxic effect of anti-thymus cell serum and complement than was that of normal T cells. The introduction of the IgM-T cells into normal chickens at the same time as the initiation of immunization of the chickens did not affect immune responses by them at all, indicating the absence of suppressor T cells in the IgM-T cell preparations. Injection of chicken thymus factor into immunodeficient chickens transplanted with normal B cells and IgM-T cells developed the capability to help B cells to switch IgM- to IgG-antibody responses. On the basis of these findings the authors propose the existence of helper T cells which are characterized by peripheralization in early periods of ontogeny, the possession of helper activity for only IgM-antibody responses, the lack of helper activity for the switch from IgM- to IgG-antibody responses and relative insusceptibility to the cytotoxic effect of anti-thymus cell serum and complement. [ABSTRACT FROM AUTHOR]