A 5-month-old boy presents with fever and severe cough. A silver-stained specimen shows Pneumocystis jirovecii. What cell type is most likely decreased?

The presence of Pneumocystis jirovecii, particularly in a young infant, suggests an immunocompromised state, most often associated with a deficiency in CD4+ T lymphocytes. This organism is a common opportunistic pathogen that typically does not cause disease in individuals with a robust immune system. However, in cases where there is a significant reduction in CD4+ T cells, such as in HIV infection or primary immunodeficiency conditions, Pneumocystis pneumonia can develop.

CD4+ T lymphocytes play a crucial role in orchestrating the immune response against various pathogens, including fungi like Pneumocystis jirovecii. These cells help activate B cells to produce antibodies and stimulate macrophages and other immune cells to enhance their responses to infections. A decrease in CD4+ T lymphocyte count impairs the body's ability to effectively combat opportunistic infections, leading to the presentation of respiratory symptoms and the finding of Pneumocystis on silver-stained specimens.

Dendritic cells, monocytes, and natural killer cells are also important components of the immune system, but they do not specifically correlate with the susceptibility to Pneumocystis jirovecii infections in the same manner as CD4+ T lymph