T Cell and B Cell Biology

A 35-year-old man from Mumbai with a 10-year history of recurrent bacterial infections (Streptococcus pneumoniae, Haemophilus influenzae) presents with generalized lymphadenopathy and hepatosplenomegaly. Serum protein electrophoresis shows a monoclonal spike in the gamma region. Flow cytometry reveals CD5+ B cells (CD19+/CD5+) comprising 60% of lymphocytes (normal <5%), with markedly reduced CD4+ T cells (15%, normal 40–60%). Serum IgM is elevated at 850 mg/dL (normal 40–230). Bone marrow biopsy shows infiltration by small lymphocytes. What is the most likely diagnosis?

A 28-year-old woman from Delhi presents with recurrent sinopulmonary infections since childhood, chronic diarrhea, and failure to thrive. Physical examination reveals absent lymph nodes and no palpable spleen. Chest X-ray shows a small mediastinal shadow. Laboratory investigations reveal normal hemoglobin and platelet count, but serum immunoglobulin levels are: IgG 150 mg/dL (normal 700–1600), IgA 20 mg/dL (normal 70–400), IgM 30 mg/dL (normal 40–230). Flow cytometry shows CD3+ T cells 5% (normal 60–80%), CD19+ B cells 2% (normal 10–20%), and CD16+/CD56+ NK cells 1% (normal 5–15%). Which of the following best explains the immunological defect in this patient?

Which surface marker is expressed on mature B cells but absent on T cells and is used clinically to identify B cell lineage in flow cytometry?

Which cytokine is primarily secreted by activated T helper cells and is essential for B cell proliferation, differentiation into plasma cells, and antibody class switching?