blood cell

blood cell One of several different types of cells that make up the blood. The red blood cells (erythrocytes) contain hemoglobin, which carries oxygen in the blood. The white blood cells (leukocytes) are a blood-borne part of the immune system. The platelets help blood to clot. Together, these three types of cells make up about half of the volume of blood. The remainder is made up of plasma. See also erythrocyte; leukocyte; plasma; platelet.