Christiаnа Cаrter had nо оbviоus problems until she was 18 months old, when she stopped gaining weight, her appetite became poor, and she had recurrent episodes of diarrhea. At 24 months, Christiana developed a cough with pulmonary infiltrates unresponsive to treatment with the antibiotics clarithromycin and trimethoprim/sulfamethoxazole. Within 3 months, she developed lymphadenopathy, hepatosplenomegaly, and fevers. A computed tomography scan revealed enlarged mesenteric and para-aortic lymph nodes. A biopsy of an enlarged axillary lymph node revealed acid-fast bacilli, and cultures from the lymph node and blood grew Mycobacterium fortuitum. HIV was ruled out after negative tests by ELISA and PCR. Serologic testing for tetanus antitoxoid antibody showed normal post-vaccination antibody levels. When Christiana’s peripheral blood mononuclear cells (PBMCs) were cultured with interferon-γ plus lipopolysaccharide, the PBMCs showed no significant increase in TNF production. A variety of broad-spectrum and anti-mycobacterial antibiotics were administered, lowering the fever, and over the course of the next 2 months, Christiana began to gain weight but continued to show signs of persistent infection. Which of the following is the most likely explanation for these clinical findings?