CASE HISTORY
Nathan was a ten-month-old male who had suffered much in his short life.Since birth he had experienced recurrent serious extracellular bacterial infections,all of which were resolved by antibiotic treatment.On Christmas Day,Nathan's mother discovered he had another high fever and a nonproductive cough (no mucus).He was also having trouble breathing.Fearing one more serious infection,Nathan's mother rushed him to the hospital.An X-ray showed the lung infiltrate with the appearance of ground glass.A tube was placed through Nathan's nose and into his lung,and a solution was used to wash out deep-lung contents for microscopic examination (bronchoalveolar lavage).The fluid yielded microscopic organisms.Nathan was hospitalized with pneumonia caused by Pneumocystis jirovecii,a yeastlike fungus.Although most of us have been infected with this organism,our immune systems almost always prevent disease.Finding the disease in Nathan suggested a serious immune dysfunction.An immunological workup revealed mild neutropenia (lower than normal numbers of neutrophils)but normal numbers of T and B cells,normal levels of complement,and normal complement activity.However,Nathan's blood exhibited exceptionally high levels of IgM but very low IgG and IgA levels.The physician suspected a specific defect in Nathan's T cells.
Based on his findings regarding antibody production,which defect do you think Nathan's doctor suspects is causing Nathan's immunodeficiency?

Question

Quizplus · Accepted Answer

Nathan's condition, characterized by high levels of IgM with very low levels of IgG and IgA, suggests a class-switch recombination defect. CD40 is crucial for this process, as it is involved in the interaction between T cells and B cells that is necessary for isotype switching from IgM to other antibody classes like IgG and IgA. A mutation affecting CD40 would impair this interaction, leading to the observed immunoglobulin profile and immunodeficiency.

CASE HISTORY Nathan Was a Ten-Month-Old Male Who Had Suffered Much in Much