RA induces Ig secretion. CD27⁺ B cells (0.5 × 10⁶/mL) were cultured in the presence or absence of CpG (1 μg/mL) and RA (100 nM). At consecutive days, cell supernatants from 100 000 cells were harvested and subjected to ELISA for the determination of IgM (A) and IgG (B). The data represent mean ± SEM of duplicate determinations (*P < .05, paired sample t test; **P ≤ .01, paired sample t test; n = 7).