fibrinogen into fibrin. Fibrin forms a network of shrinking fibers through which serum (plasma without fibrinogen) is forced. The serum, then, forms a hard clot on the fibrin fiber network.
Leukocytes are found in both blood and lymph. There are five different types of leukocytes produced either in both red bone marrow or lymphoid tissues. The different types of leukocytes all help the body fight disease and infection. Phagocytic leukocytes, capable of amoeboid movement, engulf and destroy damaged tissue cells and invading cells such as bacteria. Some leukocytes release enzymes that destroy invading cell's surface membranes, thus, destroying the invaders. Lymphocytic leukocytes destroy foreign substances, called antigens, by making molecules, called antibodies, capable of inactivating the invading antigens.
Erythrocytes, small, biconcave, disc-shaped cells, are produced from rapidly dividing connective tissue cells in red bone marrow. Late in their genesis, erythrocytes lose their nuclei and gain the oxygen carrying pigment hemoglobin. Erythrocytes are so numerous that there are approximately 5 million per cubic millimeter of blood. Each erythrocyte normally survives 120 days before being destroyed by phagocytic cells in the liver and spleen which together destroy over 2 million red blood cells every minute.
