A 7-Month-Old Boy, the Only Child of Second-Degree Cousins, Saw

Multiple Choice

A 7-month-old boy, the only child of second-degree cousins, saw a pediatrician for immunologic evaluation after developing Pneumocystis carinii pneumonia. Serum IgG, IgM, and IgA levels were normal. Blood cell count showed 10,600 leukocytes/mm³ and 80% lymphocytes; 90% of the lymphocytes were TCR αβ⁺ CD4⁺. In vitro lymphocyte-proliferative responses to PHA and anti-CD3 were absent, and the pattern of tyrosine-phosphorylated cytoplasmic proteins after anti-CD3 treatment of the T cells was distinctly abnormal. This boy most likely carries homozygous mutations in the gene encoding which one of the following proteins?

A) Zap-70
B) RAG-1
C) CD3
D) Pre-Tα
E) TCRα