Cytotoxic T Cells and Macrophages: Direct and Bystander Damage | AI Learning Guide

The comparison on this page turns on one question: does the damaged cell have to display the antigen? For the cytotoxic T cell, yes. It sees a self-peptide on MHC class I, forms a synapse, and kills that specific cell with perforin and granzymes. For the macrophage, no. Once activated by IFN-gamma or innate signals, it releases TNF, interleukins, reactive oxygen and nitrogen species, and matrix metalloproteinases onto whatever is nearby. That is why macrophage damage is broader, and why it can continue even after the original antigen is cleared.

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Cytotoxic T cell versus macrophage effector damage

Cytotoxic T cell

Requires antigen displayed on MHC class I of the target cell
Kills through perforin pores and granzyme-induced apoptosis
Damage is focused on the antigen-bearing cell

Activated macrophage

Activated by IFN-gamma and innate signals, not by target antigen
Releases TNF, IL-1, IL-6, reactive oxygen and nitrogen species, and matrix metalloproteinases
Damage spreads to nearby tissue regardless of antigen

The distinction is not academic. If the injury is cytotoxic T cell killing, removing or hiding the antigen-bearing cells should stop it. If the injury is macrophage bystander damage, the tissue can keep being destroyed even after the initiating antigen is gone, because the macrophage has already been activated and does not need to re-recognize anything.