A 1 yr old boy presents with history of recurrent pneumonia. He also has eczema and a history of prolonged bleeding after circumcision. Peripheral smear shows thrombocytopenia with small platelets. Which of the following is most likely to be deficient?

The history here is highly suggestive of Wiskott-Aldreich syndrome. IgM levels are decreased in this condition. Wiskott-Aldrich syndrome It is an X-linked disease characterised by a traid of clinical manifestations: R - Recurrent infections E - Eczema T - Thrombocytopenia with small platelets. Serum IgM levels are low but IgE and IgA levels are normal or elevated. IgG is usually normal Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition