A 32-year-old man from Delhi presents with a 3-month history of progressive weight loss (8 kg), chronic diarrhea, and oral candidiasis. He reports unprotected sexual contact 18 months ago. On examination, he has generalized lymphadenopathy and hepatosplenomegaly. CD4+ count is 85 cells/µL. A rapid HIV test is positive, confirmed by Western blot. Which of the following best explains the severe immunodeficiency seen in this patient?