Although activation of B lymphocytes by antigen or anti-Ig antibody has been shown to require cross-linking of surface Ig molecules, cross-linking is not necessary for B cell activation by anti-Lyb-2 monoclonal antibody (MAb). Monovalent Fab' fragments of anti-Lyb-2 MAb are as effective as the intact antibody in inducing blast cell transformation of small B cells and B cell proliferation in the apparent absence of T cells and adherent cells. In the presence of factors from T cells, B cells activated by Fab' fragments of anti-Lyb-2 MAb were induced to mature into Ig-secreting cells. Since monovalent Fab' fragments probably cannot induce receptor aggregation, it appears that receptor occupancy is sufficient to induce B cell activation with anti-Lyb-2 MAb.

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