ILEMOLYSIS AND IMMUNITY 109 



is the toxin, the key the haptqphor, the lock the receptor. If the man happened 

 t o have left his key fitted in a loose lock he would not be able to open his door on 

 reaching home. So the toxins meeting the loose receptors in the blood (antitoxin) 

 become bound to these and cannot attack the cells. The neutralization of the toxin 

 by the antitoxin can be demonstrated in vitro. It is stated to be a chemical process 

 taking place in definite proportions with the liberation of a small amount of heat. 

 Neither toxin nor antitoxin is destroyed in the process; they are simply linked to- 

 gether, and in some cases, at any rate, can be separated again by appropriate means. 

 The reaction is accelerated by warmth, slowed by cold, and occurs more rapidly in 

 strong than in weak solutions. 



The chemical nature of the antitoxin is unknown. It is closely related to serum 

 globulin, being carried down when this is precipitated, but is not necessarily protein, 

 although its general properties are those of a colloid. Unlike an enzyme, it is not 

 carried down by an indifferent precipitat3. A toxin seems to be a simpler body 

 than an antitoxin, for in the case of the haemolysis produced by the Megatherium 

 bacillus, the toxin can be pressed through a porcelain filter impregnated with gelatin, 

 while the .antitoxin cannot pass it. Toxic effects are sometimes produced by the 

 injection of antitoxic sera. These effects must not be ascribed to the antitoxins, but 

 to other bodies contained in the foreign serum which is injected. 



There are many bacteria which do not liberate soluble toxins into the plasma, 

 but have endotoxins which accumulate within them, and only become liberated 

 when the bacteria are disintegrated. The body elaborates no antitoxins for 

 such as these. Under this class come cholera and typhoid bacilli. If an animal 

 be injected with appropriate doses of typhoid bacilli, alive or dead (the bacterial 

 vaccines prepared for man are sterilized by heat), the blood acquires bactericidal 

 properties which are specific for the typhoid bacillus, ft is then found to contain 

 an agglutinin which causes the bacilli to stick together or agglutinate, and, if motile, 

 to become motionless. This is seen to take place when a drop of serum is mixed 

 with a drop of culture fluid containing living typhoid bacilli. Further, it contains 

 an opsonin which renders the bacilli more "tasty," so that they are eaten or 

 destroyed by the white leucocytes, known as phagocytes. In some cases the plasma 

 also acquires anti-enzymes which counteract the action of the enzymes contained 

 in the bacteria. The plasma naturally contains anti-enzymes which neutralize 

 the enzymes in the body, such as thrombin, pepsin, etc. Lastly, by virtue of 

 precipitins it obtains the power of precipitating the bacterial proteins. All these 

 reactions are specific against the bacteria injected, and are quite distinct from each 

 other. 



The mode of action of bactericidal serum is different to that of antitoxic serum. 

 If a bactericidal serum for example, a serum taken from an animal which has re- 

 ceived repeated injections of non-lethal doses of cholera vibrios be heated to 55 C. 

 for fifteen minutes, it is found to have lost its power of destroying these bacteria. 

 Yet if now inactive normal serum be added to this inactivated heated serum, it again 

 becomes bactericidal. By the process of immunization the blood has obtained some 

 new substance not destroyed by heating to 55 C., which is unable by itself to kill 

 bacteria, but is able to do so when associated with another body, which is contained 

 in normal serum and is destroyed by heating to 55 C. Just as is the case with haemo- 

 lytic sera, there are two bodies concerned in this process of bacterial immunity 

 the one developed during immunization and not destroyed by 55 C., the immune 

 body, or ambocsp'.or , the other present in normal serum and susceptible to heat 

 the complement. Any protein which provokes the production of an haamolysin, 

 antitoxin, antivenin, precipitin, immune body, etc., is called an aat'g}. The action 

 of the immune body and complement is explained on the supposition that the immune 

 body is a haptophor which unites- the bacteria to the complement. The complement 

 takes on the role of toxin (toxic to the bacteria). The immune body differs from 

 antitoxin in having two affinities one for the bacteria and one for the complement; 

 for this reason the immune body is termed aTib.coptor. There are some sera which 

 naturally possess amboceptors apart from any process of immunization. The term 

 immune body is reserved for amboceptors produced by immunization. 



The Immune Body. The immune body is apparently formed in all the tissues of 

 the body, particularly the connective tissues. It is not destroyed by temperatures 

 which are fatal to the complement. Thus, twenty hours' heating at 60 C. scarcely 

 injures it, but at 100 C. it is destroyed almost at once. It is resistant to putrefaction, 

 and has been kept for as long as eight years. It seems to be closely associated with, 

 or absorbed to, the serum globulins, and on this account is not dialyzable. 



