Platelets are small anucleate cells circulating in the blood. It has been recognized for more than 100 years that platelet adhesion and aggregation at the site of
vascular injury are critical events in hemostasis and
thrombosis; however, recent studies demonstrated that, in addition to these classic roles, platelets also have important functions in
inflammation and the immune response. Platelets contain many proinflammatory molecules and
cytokines (e.g.,
P-selectin,
CD40L, IL-1β, etc.), which support leukocyte trafficking, modulate
immunoglobulin class switch, and germinal center formation. Platelets express several functional
Toll-like receptors (TLRs), such as TLR-2, TLR-4, and TLR-9, which may potentially link innate immunity with
thrombosis. Interestingly, platelets also contain multiple anti-inflammatory molecules and
cytokines (e.g.,
transforming growth factor-β and
thrombospondin-1). Emerging evidence also suggests that platelets are involved in lymphatic vessel development by directly interacting with lymphatic endothelial cells through
C-type lectin-like receptor 2. Besides the active contributions of platelets to the immune system, platelets are passively targeted in several immune-mediated diseases, such as
autoimmune thrombocytopenia,
infection-associated
thrombocytopenia, and fetal and
neonatal alloimmune thrombocytopenia. These data suggest that platelets are important immune cells and may contribute to innate and adaptive immunity under both physiological and pathological conditions.