Mini MCAT passage: Fighting infection with adaptive immunity

The immune system consists of innate and adaptive immunity. Adaptive immunity consists of B and T cells. When activated, B cells produce antibodies, otherwise known as immunoglobulin (Ig).

A doctor is seeing a patient who seems to have a weakened immune system. The patient has had repeated infections of the middle ear (otitis media) and lungs (pneumonia). When he comes in to see the doctor, he has not been sick for some time because he has taken care to isolate himself from pathogens. The patient is twenty-five years old, and has two brothers who are around the same age. Neither brother recalls having infections of this sort in the past couple of years.

In order to test immune function, the doctor first compares the immune function of the patient with the immune function of the other two brothers. In order to do so, he administers a vaccine for pneumococcus to all three of the brothers. None of them has received the vaccine before. He then measures the levels of immunoglobulin in the blood at three intervals after administration of the vaccine: ($1$‍) on the first day, one hour after vaccine administration, ($2$‍) on the fifth day, and ($3$‍) on the fifteenth day. The results are shown below: