Although we have found that B cells also proliferate upon PBMC stimulation with MABSC, that plasmablasts are produced, as indicated by the presence of CD27+ cells, and that specific IgG is detectable in plasma of CF patients with and without MABSC history, the significantly lower rate of CD19+ compared with CD3+ cells among the total formed lymphoblasts suggests a less pronounced role of the B cell response. The gene discussed is CD27; the disease is cystic fibrosis.