Platelets, not white blood cells are the first responders in an immune attack

Platelets, not white blood cells are the first responders in an immune attack

Platelets play a much more important role in our immune system than previously thought, according to a study published by researchers in the journal Proceedings of the National Academy of Sciences (PNAS). In addition to their role in coagulation and healing, platelets also act as the immune system's first responders when a virus, bacterium, or allergen enters the bloodstream.

When a foreign body enters the blood for the first time, it causes the formation of antibodies, explained lead author. The next time these antibodies encounter the same foreign body, they quickly attach to its surface to form antigen-antibody complexes that trigger an inflammatory response.

Platelets have receptors that recognize these complexes, and this is what led researchers to suspect that platelets were involved in the inflammatory process. To test their hypothesis, they created antigen-antibody complexes in the blood of mice using a virus, a bacterial toxin, and an allergenic protein.

The results were similar in all three cases. The mice showed the classic symptoms of septic or anaphylactic shock, namely, a drop in body temperature, tremors, impaired cardiac function, vasodilation, and loss of consciousness. "We repeated the tests on mice with almost all platelets removed and on mice with no antigen-antibody complex receptors on their platelets. These mice had no physiological response, which clearly demonstrates the key role of platelets in the process. Platelets, and not white blood cells, are first on the scene during an immune response," said the lead author.

The researchers established that the mice went into shock because the platelets had released serotonin. "It's the same molecule as the neurotransmitter in the brain, but the molecule in the platelets is produced by cells in the intestine. Platelets store serotonin--they contain 90% of the body's entire serotonin supply--and release it in certain situations," author explained. During shock, platelets were sequestered in the lungs and brain and returned to the blood circulation after their degranulation.

One of the study's clinical implications is that platelet transfusion for patients in septic or anaphylactic shock could aggravate their condition by increasing the amount of serotonin in the blood.

"Transfusion remains important, especially since those patients often show low platelet levels, but in order to prevent the problem the antigen-antibody complex receptors on the platelets should be blocked before transfusion," lead author said.