98 THE POPULAR SCIENCE MONTHLY 



the body-cells begin making antitoxin the}'' make a great deal more 

 than is needed to neutralize all the toxin which the invaders have 

 manufactured. 



Hence it is that a person who has successfully come through some 

 infectious disease, smallpox or scarlatina, for instance, can not, for some 

 time thereafter, be reinfected with the poison of that disease ; his blood 

 contains an excess of the antitoxin of that disease so that any toxin of 

 that kind happening to be produced within him is immediately neutral- 

 ized. He is immune from or refractory to this infection for a certain 

 time, it may be years. He has fought a good fight microchemically, 

 and his tissues now rest from their labors. 



Man has taken advantage of this natural chemical immunity to 

 confer an artificial immunity on himself. When a person gets over an 

 attack of diphtheria, it is because his body-cells, stimulated by the 

 poison of diphtheria (diphtheritin), produced sufficient anti-diph.tb.eri- 

 tin to neutralize the poison; but it is clear that if he can get anti- 

 diphtheritin ready made, the diphtheritin in his body will be neutral- 

 ized all the quicker. He makes use of the horse. A horse, which has 

 recovered from an attack of diphtheria and thus has in his blood plenty 

 anti-diphtheritin (specific antitoxin) has some of his blood drawn off. 

 If a little of this blood, specially treated, be injected into the person 

 suffering from diphtheria, the person will recover, or if it be injected 

 into a person about to go into the infection of the disease, that person 

 will not take the disease. This is conferred immunity ; it has been con- 

 ferred on man by the horse's blood-serum. 



Thus we have three kinds of immunity from infection : 



I. An original, congenital refractoriness towards the disease 

 which may be called natural immunity; 

 II. Actively acquired immunity, the ordinary condition of having 

 come successfully through an infectious illness. 



III. Artificially or passively acquired immunity, or conferred 

 immunity, one of the latest triumphs of biological science. 



All these varieties are chemical means of defence. 



Coming under the head of chemical means of defence, we have the 

 existence of an acid in the gastric juice. It is well known that when 

 the acid (hydrochloric) is present in the stomach in the proper quan- 

 tity, it is uncommon to be infected by microorganisms through the 

 alimentary canal. The author knew of an officer who had come through 

 a severe epidemic of cholera in the West Indies, and who, on being asked 

 if he had been afraid, said : " I had no fear as long as I knew that my 

 digestion was not out of order." We and the other mammals are not 

 the only animals whose alimentary canals are guarded by a free acid; 

 there has been discovered in the Mediterranean a mollusc (Dolium 



