A 2-year-old boy has had almost continuous infections since he was 6 months old. These infections have included otitis media, pneumonia, and impetigo. Organisms cultured include Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus aureus. He also has had diarrhea, with Giardia lamblia cysts identified in stool specimens. The family history indicates that an older brother with a similar condition died because of overwhelming infections. The boy's two sisters and both parents are not affected. Which of the following laboratory findings would most likely be seen in this boy?