Final answer:
The molecule not used by cytotoxic T cells to kill infected cells is lectins. Cytotoxic T cells use perforin, granzymes, and CD95L (Fas ligand) to induce apoptosis of infected cells, preventing the spread of infection.
Step-by-step explanation:
The molecule lectins is NOT used by cytotoxic T cells to kill cells infected with pathogenic microbes. Lectins are a type of protein that binds to certain carbohydrates and plays various roles in the body, but they are not involved in the direct killing of infected cells by cytotoxic T cells.
Cytotoxic T cells, often shortened to CTLs, are key players in the adaptive immune response, specifically targeting infected cells. When these T cells encounter infected cells, they bind through their T-cell receptors (TCRs) to antigen-MHC I complexes on the surface of the infected cells. Once bound, CTLs become activated and release perforin and granzymes, among other substances. Perforin acts by forming pores in the target cell's membrane, and granzymes are proteases that enter through these pores to induce apoptosis—a controlled cell death that effectively eliminates the infected cell without releasing the infective pathogens within it. CD95L, also known as Fas ligand, is another molecule utilized by CTLs; it can engage the Fas receptor of infected cells to trigger apoptosis. The action of these molecules ensures that the spread of the infectious agent is contained.