Which of the following immune cells/molecules are most effective at destroying intracellular pathogens?
a. T helper cells
b. B cells
c. Antibodies
d. Complement
e. T cytolytic cells
The correct answer and explanation is :
Correct Answer: e. T cytolytic cells
Explanation:
Intracellular pathogens—such as viruses, some bacteria (e.g., Mycobacterium tuberculosis), and certain protozoa—reside and replicate within host cells, making them largely inaccessible to components of the immune system that target extracellular threats. Among the immune cells and molecules listed, T cytolytic cells (also called cytotoxic T lymphocytes or CD8+ T cells) are the most effective at detecting and eliminating infected cells that harbor these intracellular pathogens.
T cytolytic cells recognize infected cells by detecting peptide fragments of the pathogen that are presented on MHC class I molecules on the surface of all nucleated cells. Once recognition occurs, these T cells are activated and induce apoptosis (programmed cell death) in the infected cells. They release perforin, which forms pores in the target cell membrane, and granzymes, which enter the cell and trigger the apoptotic cascade, effectively destroying the pathogen’s intracellular niche.
Here’s why the other options are less effective against intracellular pathogens:
- a. T helper cells (CD4+ T cells): These cells are crucial for orchestrating immune responses and activating other cells (like B cells and macrophages), but they do not directly kill infected cells.
- b. B cells: These produce antibodies, which mainly target extracellular pathogens. They are not directly involved in killing infected host cells.
- c. Antibodies: Although antibodies can neutralize viruses before they enter cells or tag infected cells for destruction, they cannot access pathogens inside cells directly.
- d. Complement system: This is part of the innate immune system and works mainly in the extracellular environment, lysing pathogens or tagging them for phagocytosis, but it is ineffective against intracellular organisms.
Thus, T cytolytic cells (CD8+ T cells) are the primary effectors against intracellular infections, making option (e) the correct choice.