In contrast, both patients displayed a defective IgM responsiveness to T-independent bacterial polysaccharide antigens such as 23-valent pneumococcal capsular polysaccharides, either after vaccination with Pneumo 23 “Merieux” (patient A) or following natural exposure/infection (patient B) (Figure 4A, left panel), or tetravalent unconjugated meningococcal vaccine (Mencevax, patient A, data not shown). The gene discussed is CD40LG; the disease is infection.