Blood cells and platelets. Colored scanning electron micrograph (SEM) of human blood showing red and white cells and platelets. Red blood cells (erythrocytes) have a characteristic biconcave-disc shape and are numerous. These large cells contain hemoglobin, a red pigment by which oxygen is transported around the body. They are more numerous than white blood cells one of which is visible in this sample. White blood cells (leukocytes) are rounded cells with microvilli projections from the cell surface. Leucocytes play an important role in the immune response of the body. Platelets are smaller cells that play a major role in blood clotting.