THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 99 



to a wound destroys the platelets and thus sets in motion the clotting process. 

 Hemophilia is transmitted through the female (who is not affected) to the male 

 descendants by a mechanism which is discussed in our account of genetics. 



DEFENSE AGAINST DISEASE-PRODUCING ORGANISMS 



Although the circulatory system plays a major role in defense against 

 disease, it is by no means the sole agency. Blood, skin, and the tissues 

 in general all play a part, and we shall need to consider the defensive 

 mechanisms of the body as a coordinated whole. 



The microorganisms capable of producing disease are myriad and ever- 

 present, ready to invade the body at any opportunity. They include 

 bacteria, fungi, protozoa, and the quasi-living viruses. Many of them 

 normally live in the outer environment, and are only casual or accidental 

 invaders. Others can multiply only in the human body or in the bodies of 

 man and other animals, and some of these have insect or other hosts that 

 aid in their transmission. These microorganisms produce their injurious 

 effects in two ways : Some of them, like the diptheria and tetanus organ- 

 isms, liberate poisonous substances called toxins. Others, including many 

 kinds of streptococci and staphylococci, the typhoid bacilli, and the 

 bacillus of tuberculosis, produce their effects by growing in the tissues 

 and thereby interfering with normal cell life. 



The body has multiple defenses against invasion by microorganisms. 

 The first of these is a sort of general chemical defense called immunity. 

 The newborn baby has no immunity save that which it gained passively 

 from the mother's circulatory system. But as soon as it enters the world 

 it comes into contact with a multitude of microorganisms and begins to 

 fight off their attacks. In repelling each invasion the tissues develop 

 substances (or the capacity to produce substances) that are specific 

 antagonists of the invading cells, and these substances become available 

 for fighting later invasions by the same type of organism. 



Many or all of the cells and tissues of the body show these immunity 

 reactions — and not merely to viruses and living microorganisms. The 

 same type of response is elicited by suspensions of dead bacteria, or by 

 any kind of foreign protein such as egg albumin or a protein mixture such 

 as horse serum. Evidently the production of immunity is a chemical 

 phenomenon. Any type of substance capable of calling forth this response 

 is called an antigen, and the substance produced by the cells which reacts 

 with the antigen and renders it harmless is called an antibody. When the 

 antigen is a toxin, the antibody is called an antitoxin. The relation be- 

 tween antigen and antibody is highly specific; a given antibody is fully 

 effective only against the antigen that produced it. An animal that has 

 developed antibodies against particular antigens is said to be immunized 

 against those antigens. 



