A 63-year-old man comes to the office due to 3 months of increasing cough with occasional hemoptysis, night sweats, and unintentional weight loss. He recently emigrated from South Africa, where he worked for many years in the gold mines. He has a history of silicosis diagnosed 10 years ago. Vital signs show a low-grade fever. Physical examination reveals diffuse, fine crackles with right upper lobe predominance. Chest x-ray shows diffuse, small nodules; hilar adenopathy with prominent calcifications; and a cavitary lung lesion in the right upper lobe. A sputum sample is sent for appropriate staining and culture. This patient's increased susceptibility to the current infection is best explained by which of the following factors?
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