IMMUNOLOGY. IMMUNITY AND IMMUNIZING AGENTS 247 



as cut, abrasion or other marked injury. In a more or less severe abrasion 

 of the skin there is a tearing of the tissues as epidermis, derma and the 

 lesser arterioles and capillaries, with more or less extravasation of red 

 blood corpuscles and serum. Under ordinary conditions such a wound 

 is always infected, primarily and principally by the staphylococcus group 

 which are present everywhere, less commonly also by the streptococcus 

 group, and occasionally by the tetanus bacillus. The latter being 

 anaerobic is more apt to develop in closed wounds or deep wounds. It 

 is furthermore a spore bearer. These organisms find the serum of the 

 blood and the tissue cell juices a very suitable food supply and an active 

 invasion is thus set up. However, the body defenders, the leucocytes, are 

 at once despatched to the scene of action to repel the bacterial invasion. 

 After the hemorrhage which was the direct result of the mechanical injury, 

 has ceased, there still continues an exudation of blood serum carrying 

 with it numerous leucocytes, and these leucocytes immediately begin the 

 work of seizing upon and devouring the invading bacteria. Vast numbers 

 of the leucocytes are killed and become mixed with the serum exudate, 

 with broken down tissue cells and with bacteria, constituting the pus. 

 The dying, bacteria gorged leucocytes, gradually lose the power of 

 ameboid movement and finally assume a fixed spherical form and consti- 

 tute the characteristic pus cells. 



Ordinarily or normally the leucocytes gradually gain the upper hand, 

 pus becomes more and more scant, finally ceasing to form altogether. 

 Rejuvenescence of tissue cells begins and finally the damage is entirely 

 repaired and the wound is said to have healed. The source and origin of 

 the cells concerned in regenerative activities is as yet not fully deter- 

 mined. It is known that in the case of the infected skin injuries, numerous 

 leucocytes and lymphocytes migrate to the injured area and are largely 

 concerned in phagocytic activities, but these body cells are also concerned 

 in the healing processes, assisted by the endothelial cells. These several 

 cellular elements constitute the so-called inflammatory lymph which enters 

 into the formation of cicatricial or scar tissue. 



The older writers on surgery and pathology (fifty to sixty years ago) 

 distinguished between "laudable pus" and "sanious pus." It was be- 

 lieved that pus formation was unavoidable and when the pus was of a 

 whitish color and creamy in consistency it was a favorable sign as indicat- 

 ing a "normal" healing process and such pus was said to be "laudable." 

 On the other hand, if the pus gradually became watery, blood tinged and 

 foul smelling, it was designated "sanious" and the wound condition 

 was considered unfavorable. We now know that the change hi pus 

 formation designated by "sanious" is the result of the gain of the in- 

 vaders, the pus organisms, more especially the streptococcus group. 



