CD19 and rheumatoid arthritis: The study chose surface markers (CD19, CD27, and CD38) based on previous studies to label B lymphocytes, and divided B cells into native B cells (CD19+CD27−CD38−/+), memory B cells (CD19+CD27+CD38−), and plasmablasts (CD19+CD27++CD38++) to explore the distribution of B cell subsets in RA patients at different disease stages.