Q.Explain the mechanism of cell-mediated immunity.

Cell-mediated immunity is the arm of the specific immune response carried out directly by cells rather than by circulating antibodies, and it is the main defence against pathogens hiding inside host cells (such as virus-infected cells) and against abnormal cells such as tumour cells.

An antigen-presenting cell processes and displays fragments of an intracellular antigen on its surface, and this activates matching cytotoxic T-lymphocytes (CTLs, or killer T-cells). An activated CTL binds directly to the infected or abnormal target cell and releases two key proteins into the narrow space between them: perforin, which inserts into the target cell's membrane and punches pores through it, and granzymes, protease enzymes that pass through these pores into the target cell and trigger it to undergo apoptosis (programmed cell death) — destroying the target cell, and the pathogen inside it, in a controlled way that limits damage t …