How does an NK cell recognize and kill a virally-infected cell or a tumor cell?

Eosinophils, natural killer lymphocytes (or NK cells), and neutrophils can accomplish extracellular killing. Eosinophils primarily attack parasitic helminths by attaching to their surface and secreting toxins. Eosinophilia, an abnormally high number of eosinophils in the blood, is often indicative of helminth infestation. Eosinophils are also able to kill bacteria by expelling mitochondrial DNA and coating the bacteria with it; how this process kills bacteria is being investigated. NK cells secrete toxins onto the surfaces of virally infected cells and cancerous tumors. Neutrophils can destroy nearby microbial cells without phagocytosis by creating chemicals that kill nearby invaders or by producing extracellular fibers that bind to and kill bacteria.

<a rel="nofollow" href="https://http://www.microbelibrary.org/images/kaiser/apoptosis_NK.swf" target="_blank">https://http://www.microbelibrary.org/images/kaiser/apoptosis_NK.swf</a>

Slides 1 and 2 show protease enzymes called caspases destroying the protein structural cytoskeleton of the cell, degrading the cell's nucleoprotein, and activating enzymes that degrade the cell's DNA. As a result, the infected cell breaks into membrane-bound apoptotic fragments.

In slides 3 and 4, a phagocyte is shown engulfing and destroying the apoptotic fragments. Killing the infected cell by apoptosis reduces inflammation and also precludes the release of viruses that have assembled within the infected cell preventing their spread to other cells.