31.A 14-month-old boy presents with a fever of 102°F. The child has a
longstanding history of recurrent lower respiratory tract infections includ-
ing bronchitis and pneumonia. Chronic diarrhea is a longstanding prob-
lem. His mother reports that she had numerous upper respiratory
infections and chronic diarrhea as a young child. A complete blood count,
lung function tests, and urinalysis values are all within normal range.
Serum immunoglobulin levels are normal for IgG and IgM, but IgA was
25 mg/dL (normal = 40–60 mg/dL). There are numerous neutrophils and
other white cells in the stool sample and the stool is cultured for specific
bacteria. IgA coats pathogens facilitating repulsion of the negative charge
on the cell membrane. That negative charge on the cell membrane is pri-
marily caused by which of the following?
a. Free saccharide groups
b. Glycoprotein
c. Cholesterol
d. Peripheral membrane protein
e. Integrins
98 Anatomy, Histology, and Cell Biology