Final answer:
Perforin is the antimicrobial peptide secreted by cytotoxic T lymphocytes (CTLs) that form pores in the membranes of infected cells, leading to cell death through the induction of apoptosis.
Step-by-step explanation:
The anti-microbial peptide secreted by CTLs (Cytotoxic T Lymphocytes) when they encounter an MHCI-foreign antigen complex is called perforin. CTLs detect antigens presented by MHCI on the surface of infected cells through their TCRs (T-Cell Receptors). If these CTLs recognize foreign antigens, they become activated to release perforin and granzymes. Perforin forms a pore in the target cell's membrane, allowing granzymes to enter and induce apoptosis (controlled cell death). This mechanism helps in the destruction of infected cells, similar to the function of NK (Natural Killer) cells. Moreover, CTLs release cytokines, such as interferons, which enhance the immune response and inhibit the spread of the infection.