FCER2 and rheumatoid arthritis: The results showed that the percentages of activated B cells (CD19+CD23+, CD19+CD21+), mature B cells (CD19+IgD+), and plasma cells (CD19-CD138+; P < 0.05) were increased and the percentage of immature B cells (CD19+IgM+IgD-) was slightly decreased (though not significantly) when co-cultured with IgD (10 μg/ml) or PHA (4 μg/ml) in RA patients.