MEDizzy - A 4-year-old boy has a history of recurrent sinopulmonary

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Immune System Diseases

A 4-year-old boy has a history of recurrent sinopulmonary infections with Staphylococcus aureus and Streptococcus pneumoniae since age 17 months. He also developed an arthritis that diminished with intravenous immunoglobulin therapy. On physical examination, he is at the 30th percentile for height and weight. His temperature is 37.9° C. There is no lymphadenopathy, and lymph nodes are difficult to palpate. There is no hepatosplenomegaly. Laboratory studies show total serum protein of 5.1 g/dL and albumin of 4.6 g/dL. A lymph node biopsy specimen shows rudimentary germinal centers. Over the next 10 years, the child develops arthralgias and erythematous skin rashes and has a positive ANA test result. Which of the following types of cells has failed to develop in this patient?

ExplanationX-linked agammaglobulinemia of Bruton is a condition in which B cell maturation stops after the rearrangement of heavy-chain genes, and light chains are not produced. Complete immunoglobulin molecules with heavy and light chains are not assembled and transported to the cell membrane. The lack of immunoglobulins predisposes the child to recurrent bacterial infections after maternally derived antibodies diminish following infancy. Because T cell function remains intact, viral, fungal, and protozoal infections are uncommon. CD4-positive and CD8-positive lymphocytes differentiate from precursors in the thymus, which is not affected by the BTK gene mutation that gives rise to Bruton agammaglobulinemia. Follicular dendritic cells are a form of antigen-presenting cell that is not affected by B cell and T cell disorders. Monocytes may leave the circulation to become tissue macrophages, a process not dependent on B cell maturation