Compromised antiviral immunity in an MM patient with hypogammaglobulinemia. (A) Total IgG and IgM levels of patient ID359 on the day of COVID-19 diagnosis, as determined by immunoelectrophoresis. M indicates the level of total monoclonal protein in the γ region, and dotted lines indicate the reference range. (B) Reciprocal IgG titers against tetanus toxoid (TT; Millipore-Sigma, cat. no. 582231) and influenza H1N1 nucleoprotein (FLU; Sino Biological, cat. no. 11675-V08B) in patient ID359 plasma at COVID-19 diagnosis, as well as in 4 healthy donors (HD), as determined by ELISA. (C) IgG responses, expressed as optical density (OD) readings, of 6 COVID-19 patients (PAT) and 5 healthy donors (HD) against SARS-CoV-2 proteins S1 (ACROBiosystems; cat. no. S1N-C52H3), S2 (expressed in Expi293 cells), and N (BioVision; cat. no. P1523-50), the S1 RBD expressed in mammalian cells (mRBD; RayBiotech; cat. no. 230-30162) or produced synthetically (sRBD; LifeTein), positive-control proteins TT and FLU, and negative-control glutathione-S-transferase (GST; expressed in Expi293 cells), as determined by ELISA. Data represent the mean, and circles indicate technical replicates.