Germinal center (GC) reaction and somatic mutation in the spleen in response to protein antigens is absolutely dependent on help from antigen specific CD4+ T lymphocytes and stimulation of B cells via the CD19 co-receptor. However, the absence of CD4 T cell help apparently does not abrogate the GC formation in the Peyers patch. The present study makes use of normal, CD4-null and CD19-null mice to test the hypothesis that activation of somatic mutation in P. patch is independent of conventional co-stimulation. It was determined that the GC formation in the P. patches was dependent on help from...