DISCOVERY 



67 



Bacteria are harmful to the body because they 

 produce poisonous substances, not of mahce but by 

 chance. The enormous majority of bacteria excrete 

 nothing which is manifestly injurious to the higher 

 animals, and it appears to be accidental that the waste 

 products of a few of them are among the most poisonous 

 substances known. These poisons belong to the class 

 of complex nitrogenous substances found in live things 

 known as proteids, and are similar in character to a 

 number of other poisons produced by higher animals 

 and plants, as, for e.xample, snake venom, spider poison. 

 and the ricin present in castor-oil seeds. Their precise 

 chemical constitution is not yet known, and none of 

 them have been made artificially. The whole class of 

 proteidpoisonshave this peculiarity, that they reproduce 

 the phenomena of immunity or resistance to infectious 

 diseases. Thus an animal given a dose of the poison 

 produced by the diphtheria bacillus, or of cobra venom, 

 or of ricin less than that necessary to kill it, acquires 

 resistance, not to proteid poisons in general, but to the 

 particular poison with which it has been treated. 

 Starting with small doses and graduall}- increasing them 

 week by week, it is possible in this way to prepare an 

 animal so that it will tolerate several thousand times 

 the quantity of poison which would kill it in its natural 

 untreated state. Nothing of the kind happens with 

 simpler poisons of known chemical composition, whether 

 organic such as morphia or hyoscine, or inorganic such as 

 arsenic. It is true that experience enables us to smoke 

 without being sick, that assiduous practice enables 

 people to consume surprisingly large quantities of 

 morphia with relative impunity, that girls in the Tyrol 

 are said to eat arsenic like salt to improve their com- 

 plexions. But closer examination shows that these 

 habituations are of quite a different order from the im- 

 munities which are produced to proteid poisons. The 

 largest dose of morphia which de Quincey could take 

 is by no means necessarily a fatal dose for a normal 

 person without previous experience of the drug, and 

 careful experiments have failed to train animals to 

 withstand more than about one and a half times the 

 fatal dose of alkaloids, such as nicotine and morphia : 

 there is no real reason to suppose that the girls in 

 Styria (Austria) can take more arsenic without ill-effects 

 than girls from Stafford. That proteid poisons had 

 these special and peculiar relations to the animal body 

 was one of the great biological discoveries. Who made 

 it cannot be precisely affirmed, for discovery is a process 

 and not an event ; but Sir Thomas Frazer, of Edinburgh, 

 had a great deal to do with its precise formulation. 



Further inquiries have, indeed, shown that the 

 reactions aroused by these proteid poisons depend on 

 their being proteids rather than poisons. Analogous 

 phenomena occur when the white of a hen's egg is intro- 

 duced into a rabbit or an extract of cabbage leaves into 



a guinea-pig. These substances arc not poisonous in any 

 ordinary sense of the word, but they produce the same 

 type of reaction as do the proteid poisons, as can be 

 demonstrated in a variety of ways appropriate to each 

 particular case. As far as we know, any proteid will 

 initiate the response if it is " foreign," and if it reaches 

 the tissues of the animal in an unaltered or " native " 

 state. And each kind of animal treats as ' ' foreign ' ' 

 the proteids of any other species, and by digestion so 

 decomposes them that they cease to be " native " and 

 lose their foreign characteristics. Ox blood, for example, 

 is foreign if it is injected under the skin of man, but not 

 if it is injected into a cow. If man eats it in a sausage 

 his digestive juices break it up, so that, when the remains 

 are absorbed into the blood and reach the tissues, they 

 are no longer recognised as bovine in origin, and are not 

 treated as foreign. The parasites which cause infectious 

 diseases are always of different species from the hosts 

 whose bodies they invade ; hence, they are always 

 liable to set up this peculiar reaction to foreign proteids, 

 among which, in this particular connection, the poisons 

 are the most conspicuous. 



What, then, does the body do to defend itself against 

 these poisons produced by bacteria and other para- 

 sites ? Its activities run along two main lines. In the 

 first place it deals or tries to deal directly with the 

 poison by producing substances which combine with 

 the poison and render it inert and harmless, just as 

 alkali will neutralise an acid and remove its caustic 

 qualities. These substances are known as " anti- 

 toxins," because they neutralise the poisons or 

 " toxins " ; in more general terms they are "antibodies," 

 i.e. proteid substances produced by the tissues in 

 response to foreign proteids or " antigens." Anti- 

 toxins are found in the blood of animals which have 

 acquired resistance to a proteid poison. Thus, if a 

 horse is treated with gradually increasing doses of the 

 poison produced by the diphtheria bacillus till it will 

 tolerate much more than the quantity which would 

 originally have killed it — or, to use a more technical 

 phrase, it has been immunised to diphtheria toxin — 

 it will be found that its blood will neutralise diphtheria 

 toxin in a test-tube outside the body, so that, when the 

 mixture is injected into a guinea-pig, it does it no harm. 

 The same blood will neutralise the same toxin when it 

 is being produced inside an infected animal by the 

 diphtheria bacillus, and is used with considerable suc- 

 cess in the treatment of diphtheria in man. Similarly 

 the blood of a horse (or any other mammal) immunised 

 to cobra venom will neutralise cobra venom, and so 

 forth. But the parallel with ordinary chemical " neu- 

 tralisation " holds no longer. For, while any alkali 

 will neutralise any acid, an antitoxin will neutralise 

 only the toxin which has stimulated its production, just 

 as measles protects only against measles. 



