Most commonly due to defective CD40L on Th cells → class switching defect

- X-linked recessive

- Severe pyogenic infections early in life- Opportunistic infection with Pneumocystis, Cryptosporidium, CMV hepatitis- Cryptosporidium enteritis → biliary disease → cholangiocarcinoma

Findings:- normal or ↑ IgM- ↓↓ IgG, IgA, IgE- Failure to make germinal centers

Treatment:- IV immunoglobulin therapy- Prophylactic antibiotics- Recombinant human granulocyte-colony stimulating factor administration- Stem cell transplantation may be curative.