Teclistamab treatment reduces peripheral blood B cells and decreases serum IgG levels. (A-C) Longitudinal data representation of (A) absolute B cell counts, (B) CD4⁺ T cell counts, and (C) CD8⁺ T cell counts over time in the peripheral blood. Peripheral blood was obtained from 135 unique patients during treatment with teclistamab. There is a reduction of sample numbers over time because of study discontinuation. Most patients discontinued study treatment because of progressive disease (n = 58; 43.0%), and only 5 patients (3.7%) discontinued because of adverse events, including 2 (1.5%) due to infections. (D) Serum levels of uninvolved, polyclonal IgG were analyzed using an immunoturbidimetric assay, at baseline (priming dose 1), before the first full dose, and directly before initiation of each subsequent treatment cycle in 44 patients with non-IgG RRMM who received teclistamab monotherapy (RP2D). Serum samples were obtained directly before the teclistamab administration. Time points after patients received IVIG supplementation were removed. The black dotted line represents the lower limit of normal (7 g/L); the gray dotted line represents the LLOD (0.7 g/L). Data are depicted as box plots, indicating the distribution, including median and interquartile range. Wilcoxon rank sum test was used to compare each mean with priming dose 1. ns, not significant. ∗P < .05; ∗∗P < .01; and ∗∗∗P < .001.