Platelets: are special cells in the blood that help form clots when repair is necessary. Normal platelet levels help your blood clot properly and keep you protected from excessive bleeding. Low platelets (thrombocytopenia) can increase your risk of easy bruising, prolonged bleeding, or nosebleeds. High platelets (thrombocytosis) can increase the risk of blood clots, which may affect your heart, brain, or other organs.