Human red blood cells, two T lymphocytes and platelets, composite coloured scanning electron micrograph (SEM). Red blood cells (RBCs), or erythrocytes, are the most common type of blood cell in vertebrates. They are involved in delivering oxygen to the body tissue. Lymphocytes (leukocytes, white blood cells) are involved in the specific immune response. Pre-T cells circulate in the blood before migrating to the thymus where they develop into specialized cells (helper T cells and killer T cells) that are able to identify antigens and infected tissue cells. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair. Magnification: x1, 245 when shortest axis printed at 25 millimetres.