Ig production was measured either after incubation of PBMCs (5 × 10⁵ cells/500 μL), isolated CD19⁺B cells alone (0.5 × 10⁵ cells/200 μL), or in coculture with 2 × 10⁵ CD4⁺ T cells or 2 × 10⁵ CD19-depleted PBMCs (CD19⁻PBMCs) for 8 days. The results are presented for patient no. 1 (CVID group B according to Bryant et al9) and patient no. 2 and patient no. 3 (both CVID group A). See Table 2 for comparison with healthy donors and “Patients, materials, and methods” for further details. The numbers of B cells incubated per test are recorded in the second column. In patient no. 2, the low peripheral B-cell number is likely to be the main reason for failure of a detectable IgM production. It normalized after increasing the number of incubated B cells. PBMCs of patient no. 3, however, contained a sufficient number of B cells, but IgM production was only detectable after cocultivation of purified CD4⁺ T cells and CD19⁺ B cells. The missing IgM production after adding CD19-depleted PBMCs to isolated B cells suggests a suppressive effect of the remaining populations.