Nonsplenectomized ALPS patients display a low serum IgM antibody response after immunization with a nonconjugated pneumococcal vaccine. (A) Serum anti-pneumococcal IgM (“IgM antipneumo”) levels measured 3 to 4 weeks after immunization with a nonconjugated vaccine were measured in 16 healthy adult controls, 3 asymptomatic MPRs, and 9 nonsplenectomized ALPS patients. *P < .05; ***P < .001. Horizontal bars represent mean values. (B) Levels of anti-phosphatidylcholine IgM antibodies were measured in healthy controls and ALPS patients. Horizontal bars represent mean values. OD, optic density; P.C., phosphatidylcholine. (C) Isohemaglutinin IgM antiA and antiB measured in 20 patients (total of 37 values) showing the percentage of measures with a titer of ≤1:8, 1:16, or ≥1:32. Normal values in healthy controls are ≥1/32.