Blood cells, computer artwork. Red blood cells (red), or erythrocytes, are biconcave disc-shaped cells that are responsible for supplying tissues with oxygen. They are about 7 micrometres in diameter. They are the most abundant type of cell in the blood. White blood cells (grey and multicoloured), come in several varieties and sizes according to their function. Shown are small lymphocytes, which play an important role in the immune system. They have knobbly protrusions or microvilli on their surfaces.