PARASITIC BACTERIA. 151 



the body, give rise at once to a group of bodies 

 which have been named lysines. These lysines 

 are as mysterious to us as the alexines, but they 

 neutralize the effect of the alexines and thus 

 overcome the resistance the body offers to bac- 

 terial growth. The invaders can now multiply 

 rapidly enough to get a lasting foothold in the 

 body and then soon produce the abnormal symp- 

 toms which we call disease. Pathogenic bacteria 

 thus differ from the non-pathogenic bacteria 

 primarily in this power of secreting products 

 which can neutralize the ordinary effects of the 

 alexines, and so overcome the body's normal re- 

 sistance to their parasitic life. 



Even if the bacteria do thus overcome the 

 alexines the battle is not yet over, for the indi- 

 vidual has another method of defence which is 

 now brought into activity to check the growth of 

 the invading organisms. This second method of 

 resistance is by means of a series of active cells 

 found in the blood, known as white blood-cor- 

 puscles (Fig. 33 a, fr). They are minute bits of 

 protoplasm present in the blood and lymph in 

 large quantities. They are active cells, capable 

 of locomotion and able to crawl out of the blood- 

 vessels. Not infrequently they are found to take 

 into their bodies small objects with which they 

 come in contact. One of their duties is thus to en- 

 gulf minute irritating bodies which may be in the 

 tissues, and to carry them away for excretion. 

 They thus act as scavengers. These corpuscles 

 certainly have some agency in warding off the at- 

 tacks of pathogenic bacteria. Very commonly 

 they collect in great numbers in the region of 

 the body where invading bacteria are found. Such 

 invading bacteria exert upon them a strong attrac- 



