542 VERTEBRATE LIFE AND ORGANIZATION 



be triggered by the breakdown ol platelets on exposure to rough and 

 injured tissue. A clot within a vessel is known as a thrombus, and it 

 can be very serious ii it plugs a vessel that supplies a vital area. In the 

 hereditary disease hemophilia the platelets do not readily break down, 

 clots do not form and die slightest scratch may lead to fatal bleeding. 

 This disease attracted special attention because it appeared in several 

 different European royal families and was apparently inherited from 

 Queen Victoria of England. 



231. White Blood Cells 



Five types of white blood cells, or leukocytes, can be recognized- 

 lymphocytes, monocytes, neutrophils, eosinophils and basophils (Fig. 

 3.14). They differ in the size and shape of the nucleus, and in the 

 amount and granulation of the cytoplasm. Collectively they are not as 

 numerous as erythrocytes, for there are only about 7000 per cubic milli- 

 meter in human blood, and their life span is much shorter. They are 

 produced in the lymph nodes, the spleen and red bone marrow, and 

 live from one to four days. Although they are passively carried by the 

 blood, most leukocytes can also creep about by sending out cytoplasmic 

 processes in ameboid fashion. This enables them to squeeze between 

 the cells of the capillary walls, and many are lost from the body by 

 escaping through the capillaries in the lungs, digestive tract and kidneys. 



Their primary function is that of protecting the body against dis- 

 ease organisms. They are apparently attracted by chemicals released by 

 invading bacteria, move to the site of the injury, and phagocytize the 

 foreign microorganisms. Frequently, the leukocytes are themselves de- 

 stroyed, and the products of their breakdown contribute to the forma- 

 tion of pus in an infected wound. 



232. Immunity 



Leukocytes also protect the body by producing substances known as 

 antibodies, which can neutralize or destroy foreign proteins (antigens) 

 that may enter the body. Many of the plasma globulins are antibodies 

 synthesized by leukocytes, plasma cells or the liver. Although any foreign 

 protein may act as an antigen, the antigens with which we are perhaps 

 most familiar are the microorganisms that cause infectious diseases. 

 Viruses, bacteria and the toxins that they produce are all antigenic. The 

 body responds to their presence by forming antibodies which combat 

 the antigens in one of several ways. The antibodies may combine with 

 the antigens and neutralize them; they may cause the invading micro- 

 organisms to clump, or agglutinate, thereby effectively preventing a 

 further penetration of the body; they may attack the invading micro- 

 organisms and cause them to break up and dissolve (a phenomenon 

 known as lysis); or they may make the invaders more susceptible to 

 phagocytosis. 



The antigen-antibody reaction is generally very specific. Antibodies 



