How do immune cells talk to each other, and when does it go too far?

Cytokines are small proteins your cells use to send messages to each other. One immune cell releases a cytokine, it drifts to a neighboring cell, locks onto a matching receptor, and changes what that cell does next: multiply, mature, attack, or calm down. Interferons, interleukins and tumour necrosis factors are all cytokines; so are the molecules that cause a fever during an infection.

Interferons were discovered in 1957, when Alick Isaacs and Jean Lindenmann noticed that cells already exposed to one flu virus somehow blocked a second one. They traced the effect to a protein the cells released, and named it interferon, for its ability to interfere with a virus copying itself. Beyond blocking viruses directly, interferons push nearby cells to display more of their internal contents on their surface, making hidden infections easier for patrolling immune cells to find.

Cytokines can also misfire. Normally one activated immune cell releases cytokines that recruit a few more, which calm down once the threat is cleared. But if too many immune cells release cytokines at once, each new batch recruiting still more cells, the loop can spiral into a cytokine storm: fever, crashing blood pressure, organ strain. It can follow a severe infection, and it is also a known side effect of CAR-T cell cancer therapy, where modified T cells are deliberately unleashed against a tumor, though severe cases are rare.