Spontaneous antibody production by different subsets of B cells. CD3⁻CD19⁺CD20⁺CD27⁺CD43⁺CD69⁻ B cells as well as memory B cells (CD3⁻CD19⁺CD20⁺CD27⁺CD43⁻) and plasmablasts (CD3⁻CD19⁺CD20⁻CD27⁺CD43⁺) were analyzed for their capacity to spontaneously secrete IgA, IgM, and IgG antibodies by ELISPOT. Cells were plated at 2 × 10⁴ cells per well and incubated for 12 hours at 37°C. Spot numbers are shown in Table 1.