A 48-year-old man with AIDS is admitted to the hospital with a fever of 38.7degC (103degF). The patient has a 2-week history of persistent cough and diarrhea. Laboratory studies show that the CD4+ cell count is less than 500/mL. A sputum culture reveals acid-fast organisms, which are fuher identified as Mycobacterium avium-intracellulare. This patient's pneumonia is characterized by extensive pulmonary infiltrates of which of the following cell types?