Problem 12 - Entrance Test

This question focuses on the cellular arm of adaptive immunity against intracellular pathogens, specifically viruses. * Intracellular Pathogens: Viruses replicate inside host cells. Antibodies are effective against free viruses but cannot directly attack infected cells. Cell-mediated immunity is essential. * MHC-I: Major Histocompatibility Complex class I molecules are present on all nucleated cells. Their role is to present endogenous antigens (peptides derived from proteins synthesized within the cell, including viral proteins in an infected cell) to cytotoxic T lymphocytes (CTLs). * Cytotoxic T Lymphocytes (CTLs or CD8+ T cells): These are specialized T cells that recognize viral antigen fragments presented on MHC-I. Upon recognition and activation (often with help from Helper T cells), CTLs become cytotoxic and directly kill infected cells. Let's evaluate the options: A. This describes the humoral immune response, primarily effective against extracellular pathogens or free viruses. It does not directly eliminate infected cells. B. This describes the pathway for activating B cells, which leads to antibody production. Macrophages present on MHC-II, activating Helper T cells (CD4+), which then help B cells. This is part of humoral immunity. Infected cells present on MHC-I. C. This correctly describes the primary mechanism for eliminating virally infected cells by cell-mediated immunity: * Virally infected cell presents viral antigens on MHC-I: The infected cell processes viral proteins and displays fragments on its MHC-I molecules on the cell surface. * Cytotoxic T cells recognize the complex: Specific CTLs (with CD8 coreceptors) recognize the viral antigen-MHC-I complex. * CTLs induce apoptosis in the infected cell: Upon successful recognition and activation, the CTLs release perforin and granzymes, which induce programmed cell death (apoptosis) in the infected cell, preventing further viral replication. D. Natural Killer (NK) cells are part of the innate immune system. They can kill virally infected cells, but they do so by recognizing cells that lack MHC-I or have stress proteins, not necessarily directly recognizing specific viral antigens in the same way T cells do. They do release perforin and granzymes, but phagocytosis by neutrophils is not the direct outcome of NK cell killing. The question asks for a specific immune response, which points to adaptive immunity (T cells, B cells).