Susan and Joe were worried. Their little boy, Daniel, had beenhaving an awful lot of bacterial infections and he was barely ayear old. It seemed that the antibiotics cleared up one bacterialrespiratory infection only to have another follow shortly. Thescary thing was that Daniel had just fought off a case of pneumoniacaused by Pneumocystis carnii, a fungal infection that was usuallyfound in people with HIV. Waiting for the test results of an HIVtest for their little boy was one of the worst waits ever. Thankgoodness it came back negative.

However, it seemed that their troubles were just beginning.After this last lung infection, the fungal one, and a negative HIVtest, their doctor had ordered a number of other blood tests,including a genetic test that Susan didn’t fully understand.Apparently the doctor was worried about Daniel’s immune systemfunctions. Susan had also met with a genetic counselor whocollected a family history of any immune disorders. The detailswere vague, but Susan’s mother, Helen, knew that one of her threebrothers had died young from an unexplained lung infection.Unfortunately, Grandma Ruth had passed away a few years ago,leaving them with numerous unanswered questions.

Susan and Joe had an appointment with their doctor thatafternoon to go over the results. When they arrived Dr. Dresdnerled them into an office where Ms. Henchey, the genetic counselor,waited. This can’t be good, thought Susan. The doctor began byexplaining that they had analyzed Daniel’s blood and found thatwhile he had normal levels of B cells and T cells, his antibodylevels were anything but normal. The levels of IgG, IgA, and IgEwere very low, almost undetectable, and Daniel had abnormally highlevels of IgM and IgD. He went on to explain the nature of thesedifferent categories of immunoglobulins.

Ms. Henchey, the genetic counselor, explained thatDaniel had a genetic mutation in the gene for the CD40ligand.

How does a deficiency in CD40 ligand explain Daniel’simmunological deficiency?