168 ESSENTIALS OF PHYSIOLOGY. 



derived, it bias been used in medico-legal cases to ascertain whether 

 blood, for example on clothes or weapons, is of human origin or not. 



Again, the poisons (toxins) formed by bacteria give rise, when 

 introduced into the body, to antibodies which neutralise the toxin. 

 If, for example, a minute amount of diphtheria toxin is injected at 

 intervals into an animal, the latter forms antitoxin in considerable 

 amount. This antitoxin is able to neutralise diphtheria toxin, and such 

 an animal will now survive the injection of a huge dose of toxin, many 

 times larger than that which would previously have killed it ; and it 

 is said to be immune to that toxin. In this example the toxin and 

 antitoxin combine directly with one another. In other cases, however, 

 the antibody which is formed does not itself destroy the antigen, but 

 forms a link between the antigen and a substance present in normal 

 serum and known as complement ; the complement, thus linked on to 

 the antigen, is able to destroy it. Antigens of this kind include 

 bacteria, red blood corpuscles and tissue cells, and the antibodies are 

 called lysins. The formation of hcemolysin (p. 157) is an example of 

 this process. 



The capacity to form antibodies, which can destroy or neutralise 

 bacteria and their toxins, is one of the fundamental means by which 

 human beings are enabled to resist or to recover from diseases of 

 bacterial origin. 



Anaphylaxis. If a small dose of an antigen, such as egg-white, is 

 injected into an animal, and after an interval of sixteen days or more, 

 a second, even smaller dose is given, the animal becomes extremely ill 

 and may die in a few minutes. In guinea-pigs, which are particularly 

 sensitive, there is a marked fall of blood pressure, extreme constriction 

 of the bronchioles, and convulsions. This hypersensitiveness of an 

 animal to a second dose of an antigen is known as anaphylaxis. 



The Reaction of the Blood. The acid characters of a substance 

 such as hydrochloric acid in solution are due to the presence in it of 

 free hydrogen ions (H ions) ; similarly the alkaline characters of an 

 alkali such as caustic potash depend upon the presence of free hydroxyl 

 ions (OH ions). In a perfectly neutral solution, the two kinds of ions 

 are present in equal amount. In any aqueous solution, whatever its 

 reaction, the product of the proportion of H and OH ions present is a 

 constant figure, but in an acid solution the H ions will be in excess, 

 whereas in an alkaline solution they will be relatively few as compared 

 with the OH ions. It is customary to express the reaction of a fluid 

 in terms of its concentration in H ions, this being indicated by the 

 formula C H . In an acid solution the relative concentration of H ions 

 is large, whereas in an alkaline solution it is small. 



