Which cell type is primarily responsible for directly killing virus-infected host cells?
ACD4⁺ helper T cells
BB cells
CCD8⁺ cytotoxic T cells
DPlasma cells
CD8⁺ cytotoxic T cells (CTLs) directly kill any host cell displaying a foreign peptide on MHC class I — including cells infected by viruses. CD4⁺ helper T cells orchestrate the response but do not directly kill infected cells. B cells differentiate into plasma cells that secrete antibodies, which act on extracellular pathogens rather than infected cells. Plasma cells are antibody factories, not killers.
Question 2 True / False
Antibodies primarily defend against pathogens by directly lysing (rupturing) them.
TTrue
FFalse
Answer: False
Antibodies work by three main mechanisms: neutralization (blocking pathogen entry into cells), opsonization (coating pathogens to promote phagocytosis by macrophages and neutrophils), and complement activation (triggering a protein cascade that can damage pathogen membranes). Direct lysis by antibodies alone is minimal — most pathogen killing is achieved by downstream effectors (phagocytes, complement, NK cells) that antibodies recruit or enhance.
Question 3 Short Answer
HIV specifically infects CD4⁺ helper T cells. Why does this single cellular target make HIV so devastatingly immunosuppressive?
Think about your answer, then reveal below.
Model answer: CD4⁺ helper T cells coordinate both arms of adaptive immunity: they provide signals that activate CD8⁺ cytotoxic T cells (cellular arm) and that stimulate B cells to produce antibodies (humoral arm). Destroying this central coordinator collapses both arms simultaneously, leaving the immune system unable to mount effective responses against virtually any pathogen.
This question targets the misconception that losing one cell type would only impair one function. Because CD4⁺ T helpers sit at the hub of adaptive immune coordination, their loss creates a cascading failure. AIDS patients succumb to opportunistic infections — normally harmless organisms — because without CD4⁺ help, neither cellular nor humoral immunity can be fully activated against new antigens.