Passive immunization with anti-HOD mAb IgG2a or IgG2c enhances alloantibody production upon HOD RBC transfusion. Recipient mice received a passive immunization of individual anti-HOD mAb subtypes followed by an RBC transfusion. RBCs and sera were collected at multiple time points over 21 days. (A) The survival of allogeneic HOD RBCs, as a function of control B6 RBCs, was calculated. (B) The percentage of circulating HOD RBCs was determined. The percentage of HOD RBCs in PBS-treated animals were normalized to 100% across all time points. Sera were analyzed by flow crossmatch against HOD and control B6 RBCs for IgM (C), Igs (D), IgG1 (E), IgG2a (F), IgG2b (G), IgG2c (H), and IgG3 (I) anti-HOD alloantibodies. (J) Representative flow plots of flow crossmatch data are provided. Each experiment was performed 3 times with 3 to 5 mice per group. Representative data are shown. Data were analyzed with a repeated measures 2-way ANOVA with Dunnett multiple comparisons to PBS-treated animals. *P ≤ .05, **P ≤ .01, ***P ≤ .001, ****P ≤ .0001.