IMMUNITY 151 



efforts to combat disease, is the degree of immunity which the 

 host shows to the parasite. The word ' immunity ' is used in a 

 wide sense to cover a number of different modes of reaction. At 

 the one extreme, host restriction and other difficulties in infection 

 may be determined simply by the fact that the ordinary conditions 

 of the environment are not suitable for the development of the 

 parasite. Very often the egg-shells or cyst-cases of intestinal 

 parasites must be digested so that the embryo is liberated ; 

 we have seen that this is so in tapeworms, and it is true also of 

 many Protozoa, such as the sporozoan genus Eimeria, various 

 species of which are common parasites of domestic animals, 

 with no very great degree of host restriction. The cysts of Eimeria 

 from fish, however, when ingested by man, are unaffected by his 

 digestive enzymes, and pass right through and remain alive in the 

 faeces. Live proglottides of tapeworms cannot infect man, because 

 the egg-shells need the successive action of acid and bicarbonate, 

 and unless the proglottis is first killed, the eggs are not liberated, 

 and so exposed to the digestive juices, until the intestine is 

 reached and the acid of the stomach has been left behind. 



At the other extreme, there may be an active warfare of the 

 host against the invading parasite, and it is this that we most 

 commonly think of as immunity. It takes two chief forms. In 

 the first, which may be called the cellular mechanism, amoeboid 

 cells of the host, called phagocytes, actively ingest and digest the 

 parasites. Phagocytes are found chiefly in the blood (the poly- 

 morphonuclear leucocytes ; see pp. 524-25) and in connective 

 tissue (macrophages; see p. 515). In the second, the chemical 

 or humoral mechanism, the presence of the parasite causes the 

 production of substances which have a specific action on the things 

 which bring them into being. This is a particular example of the 

 general effect of the introduction of a foreign protein or carbo- 

 hydrate of large molecule into the tissues. Such a substance, called 

 an antigen, induces the formation of a specific antibody. This may 

 act in any of three main ways ; it may merely neutrahse the objec- 

 tionable effect of the antigen (the antitoxic effect), it may render 

 the parasite more vulnerable to the attack of phagocytes 

 (opsonification), or it may itself bring about the death and 

 destruction of the invader (lysis). Antibodies acting in these ways 

 are called antitoxins, opsonins, and lysins respectively. 



There are also substances which act in the same way called 

 natural antibodies, because they are naturally present, and they 



