68 



DISCOVERY 



In the second place, the body attacks the parasite 

 which is producing the poison in a variety of ways 

 which mav be conveniently grouped under two main 

 headings. Firstly, the tissues are stimulated by the 

 proteids of the bodies of the parasites to generate 

 various substances which have influences on the 

 parasites. These antibodies are found in the blood like 

 antito.xins. Some of them dissolve the bodies of the 

 parasites, some so alter the surface of the bacteria that 

 they run together into masses, as would inorganic 

 particles of comparable size, some form obvious preci- 

 pitates with the bacterial proteids ; one way and 

 another the antibodies are unfavourable to the para- 

 sites. Secondly, the tissues are stimulated to destroy 

 the parasites by eating and digesting them as an amoeba 

 eats a diatom. The cells in which this capacity for 

 what is technically called "phagocytosis" is most 

 highly developed are partly fixed cells in and close to 

 the blood and lymph channels, and partly the wandering 

 cells, or leucocytes, which move freely in the blood 

 stream, and actively crawl about in the tissues of their 

 own proper movement. The realisation of the im- 

 portant part which this process plays in the defence of 

 the body was the great contribution of Metchnikoff ^ to 

 biology ; with the fundamentals of our knowledge of 

 the unorganised antibodies we associate the name of 

 Ehrlich.2 The two points of view, for long treated as 

 divergent, found a harmonious meeting-place in the 

 discovery that among the antibodies are substances 

 which combine with bacteria, and so render them more 

 susceptible to phagocytosis. 



It is important to realise that these phenomena 

 also are not peculiar to harmful parasites : the reactions 

 in disease are simply a special case of general reactions 

 to cells from other species of animals and plants. If a 

 guinea-pig is inoculated with typhoid bacilli, which are 

 harmful to it, it will develop the various sorts of anti- 

 bodies which have been indicated, and its wandering 

 cells will eat the bacteria ; it will do the same if it 

 is dosed with some harmless bacillus, or with the 

 innocuous particles of brown pigment which colour the 

 hair and skin, or with the red blood corpuscles of some 

 other species of animal. These are all foreign bodies. 

 But, if it is given its own blood corpuscles or those of 

 another guinea-pig, it treats them as harmonious to its 



1 Elie Metchnikoff (1845-1916), Russian zoologist to whom, 

 in 1883, first occurred the idea that the eating of foreign microbes 

 bv the cells of the body might be a defence against disease (a 

 process for which Claus, of Vienna, invented the word phago- 

 cytosis). In 1888 he joined Pasteur in Paris and worked there 

 till his death. See the splendid Life by his wife (Constable & 

 Co., 1921). 



2 Paul Ehrlich (1S54-1915). German pathologist who, with 

 the Belgian Bordct, is chiefly responsible for the fundamental 

 experiments on which is based our knowledge of the formation 

 of antibodies and their mode of action. 



economy and takes none of these steps to attack them. 

 What happens to a parasite, then, happens simply 

 because the parasite is not the same kind of live thing 

 as the host. And if Linnaeus' had known of these 

 modern elaborations of his passion for putting the 

 world into a tidy and orderly arrangement, he might 

 well have knelt down in a laboratory, instead of on 

 Berkhamsted Common, and thanked God for making 

 the world so beautiful. 



It seems, therefore, that these reactions are a general 

 proposition as regards the relation of the lowest forms of 

 animal and vegetable life to mammals and birds, and 

 of the different kinds of mammals and birds to one 

 another. For the great bulk of our knowledge has been 

 derived from experimental studies in these fields. It is 

 reasonable to suspect that the proposition might well 

 extend so as to include the relations of all forms of 

 live things to one another — that, for example, if a 

 mammal stimulated by a bacillus produces an anti- 

 body, so the bacillus stimulated by the antibody (which 

 is to it a foreign proteid) wdll produce an anti-antibody. 

 It is, fortunately, unnecessary to think about the ulti- 

 mate complications of an action and reaction of this sort, 

 because there are no reasons to think that the first step 

 actually occurs. Bacteria, it is true, take steps to with- 

 stand antibodies and phagocytosis, but they proceed 

 by an entirely different method. Like other small 

 organisms, bacteria occur in enormous numbers and 

 multiply very rapidly. If \fou are counting bacterial 

 populations, 1,000 millions is a convenient miit with 

 which to work, and any ordinary laboratory culture 

 may contain a good many of these units in a few cubic 

 centimetres ■ under favourable conditions a single 

 pair of rats will increase to some 200 in a twelvemonth, 

 while a single bacillus may become about 8 million 

 in twelve hours. The importance of these features of 

 bacterial populations is this. The larger the absolute 

 numbers of organisms, the more likely it is that a given 

 population will contain individuals in which fluctuating 

 characters are most liighly developed. Just as a giant 

 or a saint is more likely to be found in a town of a 

 million inhabitants than in a village with a hundred, 

 in Manchester rather than Toller Porcorum, so indi- 

 vidual bacteria with a giant capacity to resist phago- 

 cytosis and antibodies are commoner than individual 

 men with an exceptional capacity to resist bacteria, 

 simply because the bacteria are infinitely more numerous 

 than the men. The body of a person with typhoid 

 fever contains many more typhoid baciUi than there 

 are people in the British Isles. Using the advantages 

 which their niunbers give them, the bacteria do nothing 



3 Carl von Linne (1707-1778), Swedish naturalist who reduced 

 the animal and vegetable kingdoms to order, systematically 

 described them, and devised the methods of classification and 

 nomenclature now in use. 



