IMMUNOLOGY 831 



specific factors which are incompatible with or unfitted to the Hfe of the 

 parasite in the unimmunized host. The specificity of parasites lor various 

 hosts (Chapter XVII) is largely an expression of natural immunity. 

 Acquired immunity, on the other hand, denotes the various conditions 

 arising in a host as a result of infection or other immunizing procedure 

 and is generally thought of as resulting in large measure from the pro- 

 duction of antibodies in the host. 



Immunity is the reciprocal of virulence, which in this sense is an ex- 

 pression of the ability of the parasite to invade and parasitize the host. 

 Both immunity and virulence are relative and represent the resultant of 

 the invasive activities of the parasite and the defense activities of the 

 host; they may, therefore, vary in degree from zero to 100 percent. 



THE CELLS INVOLVED IN IMMUNITY 



The defense of the vertebrate body against invading parasites, or 

 even against inanimate foreign material introduced parenterally, appears 

 to be taken care of predominantly by some of the cells of the connective 

 tissue and is a specialized or accentuated aspect of their normal functions. 

 The connective tissue has manifold normal functions, such as respiration, 

 intermediate metabolism, storage, and mechanical support and in its 

 widespread distribution throughout the body consists of the blood and 

 lymph, cartilage, bone, the reticular (blood-forming) tissue of the 

 myeloid and lymphatic organs, and loose and dense connective (includ- 

 ing adipose) tissues associated with the skin, omentum, liver, lung, and 

 so forth. The cells of this tissue arise embryonically from the mesen- 

 chyme and may be either fixed or free. Those of the blood and lymph 

 and of the reticular and loose connective tissues are chiefly concerned in 

 defense. 



The terminology of the connective tissue cells is complicated by 

 the frequent use of several names for the same cell. This condition has 

 arisen ( 1 ) because connective tissue is so widespread and involves so 

 many organs that it has been studied by histologists, hematologists, 

 pathologists, and so forth, some of whom have not correlated the 

 knowledge in fields other than their own; and (2) because investigators 

 have disagreed as to the nature and developmental potencies of various 

 cells. In the following brief review we have defined only those cells of 

 the connective tissue which are known to be involved in defense against 



