Human red blood cells, monocytes (white blood cells), activated platelets and fibrin thread, composite coloured scanning electron micrograph (SEM). Red blood cells (RBCs), or erythrocytes, are the most common type of blood cell in vertebrates. 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. Monocytes are a type of white blood cell (leukocyte).They are part of the innate immune system of vertebrates. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair, and can also activate certain immune responses. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair. Blood cells collect in the entangled fibrin and platelet mass forming a blood clot. Magnification: x1, 000 when shortest axis printed at 25 millimetres.