Primary Immunodeficiencies

A 3-year-old boy from rural Maharashtra presents with recurrent pyogenic infections (otitis media, pneumonia, skin abscesses) since age 6 months. Physical examination reveals absence of palpable lymph nodes and no splenomegaly. Serum immunoglobulin levels: IgG 150 mg/dL (normal 600–1600), IgA 20 mg/dL (normal 70–400), IgM 40 mg/dL (normal 40–230). Flow cytometry shows CD19+ B cells 2% (normal 10–20%), CD3+ T cells 65% (normal 60–80%), CD4+ cells 40% (normal 35–45%), CD8+ cells 20% (normal 20–30%). Chest X-ray shows absent thymic shadow. Which is the most likely diagnosis?

A 2-year-old girl from Delhi presents with a 4-month history of chronic diarrhea, failure to thrive, and recurrent oral candidiasis. She had a severe varicella infection at age 18 months requiring hospitalization. Physical examination shows hepatosplenomegaly, generalized lymphadenopathy, and oral thrush. Chest X-ray reveals an absent thymic shadow. Laboratory findings: CD3+ T cells 800/µL (normal 1500–3000), CD4+ cells 200/µL (normal 800–1500), CD8+ cells 150/µL (normal 400–1000), CD19+ B cells 600/µL (normal 1000–2000). Serum IgG 300 mg/dL (low-normal), IgA <10 mg/dL, IgM 25 mg/dL. Which is the most likely diagnosis?

Which enzyme is deficient in adenosine deaminase (ADA) deficiency, and what is the primary mechanism of lymphocyte death?

Which of the following is the most common form of primary immunodeficiency and is characterized by defective B-cell maturation due to mutations in the BTK gene?