THE TRANSPORTATION SYSTEM 



515 



parasites are protein in nature and therefore 

 induce antibodies when they get into the 

 blood stream and other tissues. The para- 

 site is toxic to the host, so antitoxins are 

 produced that will either destroy the para- 

 site or render it helpless so that the leuco- 

 cytes may engulf and destroy it more easily. 

 Often the parasite gives off a protein prod- 

 uct which is also toxic to the host and again 

 an antitoxin is built up against this product, 

 so that it is neutralized and can no longer 

 harm the body. The end products of this 

 reaction are removed from the body through 

 various channels. 



The production of antitoxins is spoken of 

 as immunity, a very familiar term. Once the 

 antibodies have been produced, they are 

 active for some time against a second inva- 

 sion of the parasite. How long they are ac- 

 tive seems to be specific for the parasite. 

 For example, immunity against typhoid 

 fever may last a year or two, whereas one 

 may never expect to have a second case of 

 whooping cough, the antibodies for which 

 last throughout the lifetime of the individ- 

 ual. Immunity built up by the actual par- 

 ticipation of the parasite in question is 

 called active immunity. A similar immunity 

 can be produced artificially by introducing 

 a weakened or even a dead strain of the 

 parasite into the body so that it will bring 

 about antibody formation but will not cause 

 the disease or, if it does, only in a very mild 

 form. This is the method employed in im- 

 munizing against smallpox, for example. 

 Another method is to employ an altered 

 toxin, such as that used in building up ac- 

 tive immunity against diphtheria. The toxin 

 taken from the diphtherial organism is 

 treated so that it has lost none of its anti- 

 genic properties but is no longer toxic to 

 tissues in the body. This substance is 

 called toxoid. Upon receiving a small quan- 

 tity of toxoid, antibodies (antitoxins) are 

 produced, so that if at any subsequent time 

 the diphtherial organism enters the body 

 its effect will be neutralized at once and it 

 will be unable to obtain a foothold. This 



type of treatment has made cases of diph- 

 theria very rare and they could be non- 

 existent if everyone were thus protected. 



Sometimes, as in the case of advanced 

 tetanus, it is necessary to build up the sup- 

 ply of antibodies immediately. There is not 

 sufficient time to allow the body to produce 

 them in the usual slow manner. It is pos- 

 sible, then, to add them directly by injec- 

 tions of antitoxin that has been previously 

 produced in a horse. This type is called 

 passive immunity, because the person him- 

 self contributes nothing toward the produc- 

 tion of the antibody. Passive immunization 

 is short-lived, which is its chief disadvan- 

 tage, but in certain diseases it can save a 

 life. It is wiser to prevent the appearance 

 of the disease by active immunization rather 

 than attempt to cure the disease once it has 

 struck — hence the popularity of immuniza- 

 tion programs in our schools and the abso- 

 lute enforcement of such programs in the 

 armed forces. 



Acid-base balance. All of the cells of the 

 body are very sensitive to the amount of 

 acid or base that is present in their environ- 

 ment, and can withstand only very slight 

 changes in the concentration of hydrogen 

 ions. Since the blood controls the internal 

 environment, it follows that it, too, must be 

 very constant. Such is indeed the case. 

 In whatever part of the body the hydrogen 

 ion concentration or pH of the blood is 

 measured, it will be found to be remarkably 

 constant, being slightly alkaline (pH 7.45). 

 This may seem difficult to understand in 

 view of the many compounds being 

 "dumped" into and withdrawn from the 

 blood continuously. It is made possible by 

 substances in the blood, appropriately called 

 buffers, which maintain a constant pH. 

 They combine with both acids and bases so 

 as to prevent any important change in the 

 relative number of hydrogen and hydroxyl 

 ions. The most important buffers in the 

 plasma are the proteins, phosphates, and 

 carbonates. Acids that form in the cells as 

 a result of metabolic activity are passed 



