There are, on the other hand, a few species of bacteria which may be able to retain their lodgment in the body m spite of this attempt of the individual to get rid of them.  These, of course, constitute the pathogenic species, or so called “disease germs”.  Only such species as can overcome this first resistance can be disease germs, for they alone can retain their foothold in the body.

But how do these species overcome the poisons, which kill the other harmless bacteria?  They, as well as the harmless forms, find these alexines injurious to their growth, but in some way they are able to counteract the poisons.  In this general discussion of poisons we are dealing with a subject which is somewhat obscure, but apparently the pathogenic bacteria are able to overcome the alexines of the body by producing in their turn certain other products which neutralize the alexines, thus annulling their action.  These pathogenic bacteria, when they get into 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 bacterial growth The invaders can now multiply rapidly enough to get a lasting foothold in the body and then soon produce the abnormal symptoms 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 resistance to their parasitic life.

Even if the bacteria do thus overcome the alexines the battle is not yet over, for the individual 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-corpuscles (Fig. 33 a, b).  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 engulf 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 attacks of pathogenic bacteria Very commonly they collect in great numbers in the region of the body where invading bacteria are found.  Such invading bacteria exist upon them a strong attraction, and the corpuscles leave the blood-vessels and sometimes form a solid phalanx completely surrounding the invading germs.  Their collection at these points may make itself seen externally by the phenomenon we call inflammation.