These are small granular a nucleated cells, derived from mega karyocytes. Usually, they are about 3pm in diameter and about 150,000 to 350,000 platelets in a micro litre of blood. Besides their role in blood clotting they are involved in hyper sensitivity and inflammatory reactions.
In addition to receptors for factor VIII they express MHC class molecules, they express IgG receptors and low affinity receptors for Ig E on their surface. Following injury platelets aggregate and adhere to damaged vascular epithelial cells and release their secretions. Histamine and serotonin of the released substances cause inflammatory response.
In people with a low platelet count, bleeding is more likely to occur, even after a slight injury. When the platelet count is below 20,000 (very low), massive bleeding may occur even when there is no injury. Aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), and antihistamines may interfere with how platelets function, although the platelet count remains normal.