Frequencies of preexisting CD4 T cells specific to KLH and Epo in the blood of normal donors. The estimate of protein-specific T-cell precursors was done on the basis of the percentage of negative wells and the number of distributed CD4 T cells according to the following formula of the Poisson law: frequency = −Ln(number of negative wells/total number of wells tested)/(number of CD4⁺ T cells/well). (A) Number of KLH- and human Epo-specific CD4 T cells was estimated from 9 HLA-typed normal donors (top panel). (B) Estimates were performed from 14 donors for ovalbumin (OVA), 3 for glutathione S-transferase (GST), 11 for insulin (INS), 4 for α-antitrypsin (TRY), and 3 for antithrombin III (THR).