CD19 and plasma cell myeloma: Here, a CD19+ B-cell count of median 33.5/μL (r: 1–696/μL) was seen in the overall myeloma patient cohort; in patients with negative SARS-CoV-2 SP-AbT, the median CD19+ B cell numbers were significantly lower compared to patients with positive titers (median CD19+ B cells: 2.0 vs. 52.5/μL, p = 0.005).