198 ELEMENTARY BIOLOGY 



227. Opsonins. Although the white corpuscles (or rather 

 certain kinds of them, for there are really several kinds of 

 colorless corpuscles in our blood) will " eat " foreign particles, 

 including many kinds of bacteria, they will do this only under 

 certain chemical conditions. A substance which thus makes the 

 bacteria "attractive" to the corpuscles is called an opsonin, 

 which comes from a Greek word meaning " to prepare a meal." 

 The activity of the cell-eating corpuscles depends upon the 

 presence of a suitable quantity of the right kind of opsonin. 

 The production of opsonins may be stimulated by the introduc- 

 tion of small quantities of dead or living bacteria of various 

 kinds. 



228. Vaccines and vaccination. We have seen that the 

 blood meets the invasion of foreign bodies or foreign substances 

 in several different ways, all of them depending upon the vital 

 properties of the cells of the body, and especially of the white 

 corpuscles. In recent years various methods for increasing the 

 resistance of the body to specific diseases have been developed 

 by using in some cases one of the blood's reactions, in other 

 cases another. All of these are roughly classed as vaccination. 



In some kinds of vaccination cytolysins are produced ; in 

 other kinds, opsonins or antitoxins. 



229. Immunity and susceptibility. Individuals differ so 

 greatly that some are much more sensitive or much less sensitive 

 to certain substances than others. Now we find that sometimes, 

 or in some people, the blood is extremely sensitive to foreign 

 substances of a particular kind. It is a matter of common observa- 

 tion that some people catch colds more easily than others ; some 

 more frequently have boils or pimples ; some are more suscep- 

 tible to typhoid or to diphtheria. 



Not only do individuals differ one from another ; there are 

 also racial differences. Thus, the dark-skinned races are less 

 susceptible to malaria and to hookworm than the white races ; 

 on the other hand, the white races are less susceptible to 

 tuberculosis and measles than the dark races. 



