This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'Components of innate immunity that are active against viral cells includes: \nSelect and explain. \nA. NK cells, B. Cytotoxic T cells, C. B cells, D. Memory B cells', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Properties of natural killer cells\xa0\n\nLarge granular lymphocytes.\nLack T-cell receptor, CD3 proteins, and surface IgM\xa0and IgD.\nPrior exposure does not increase the activity.\nThymus is not required for development.\nNumber remains normal in severe combined immunodeficiency\xa0disease.\xa0\n\nFunctions of natural killer cells\xa0\n\nKill virus-infected cells and tumor cells.\nNonspecific killing of virus-infected cells and tumor cells.\nKilling is independent of antigen presentation by MHC proteins.\nMechanism of killing is by perforins and granzymes, which cause apoptosis of target cell.\nKilling is activated by failure of a cell to present antigen with class I MHC or by reduction of class I MHC proteins on the cell surface.\n\nParija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:131', 'role': 'assistant'}]