When a blood vessel is cut, platelets collect and stick to the vessel at the site of the wound. The platelets then release chemicals that produce a protein called fibrin. Fibrin weaves a network of fibers across the cut that traps blood platelets and red blood cells, as shown in Figure 10. As more and more platelets and blood cells are trapped, a blood clot forms. 

White blood cells
The body's disease fighters are the white blood cells. Like red blood cells, white blood cells are produced in bone marrow. Some white blood cells recognize disease-causing organisms, such as bacteria, and alert the body that it has been invaded. Other white blood cells produce chemicals to fight the invaders. Still, other white blood cells surround and kill the invaders
White blood cells are different from red blood cells in important ways. Many white blood cells move from the marrow to other locations in the body to mature. Unlike red blood cells, there are fewer white blood cells—only about one white blood cell for every 500 to 1,000 red blood cells. Also, white blood cells have nuclei. Finally, most white blood cells live for months or years.
Blood Groups
How do you know what type of blood you have? There are marker molecules attached to red blood cells. These markers are called blood groups, which determine blood type.