What cells are included in human blood and how do they differ?

Human blood consists of blood plasma and three types of cells: erythrocytes, platelets and leukocytes. Erythrocytes carry out the function of carrying oxygen to the cells of the body. They do not have a nucleus and their concentration is very high. It is they who give the blood a red color. Platelets are needed for blood clotting when injured. Without them, the blood would simply flow out of the wound and would not stop. Leukocytes perform the function of the body’s defenders and attack viruses, bacteria and foreign particles that enter the human body.