Platelets

Platelets contribute greatly to immune and/or inflammatory responses, in addition to having prominent roles in hemostasis and thrombosis. Platelets have numerous immune-associated molecules in their granules, rapidly express receptors such as P-selectin, CD40 ligand, and integrins on their surface upon activation, and secrete soluble mediators such as chemokines, growth factors, and lipid mediators. In this way, platelets affect the function of endothelial cells and leukocytes such as neutrophils, T cells, natural killer cells, B cells, monocytes, and dendritic cells via direct cell-cell contact and/or soluble mediators. In cutaneous inflammatory disorders including atopic dermatitis, psoriasis, and infectious diseases, platelets circulate in an activated state and are involved in the pathogenesis of these diseases. On activation, platelets bind leukocytes via P-selectin on the surface of platelets in circulating blood. The platelet-leukocyte complexes then roll along the endothelium and transmigrate into subendothelial tissue. After reaching inflamed skin tissue, platelets release soluble mediators such as chemokines at the sites of inflammation, leading to aggravation of inflammatory responses. Platelets can also recognize bacteria pathogens through interactions via Toll-like receptors, with subsequent elimination of the bacteria directly by release of microbicidal proteins or by aggregation of platelets around the bacteria. Thus, platelets play important roles in immunity and inflammation in skin through interactions with cells such as leukocytes, other platelets, and the endothelium.