304 Microbes and You 



INTERNAL RESISTANCE FACTORS 

 The Blood 



In addition to the usual cellular elements and fluids found in 

 the blood, there are protein elements called antibodies which have 

 been manufactured by selected tissues of the body and have spilled 

 over into the blood and other body fluids. Since these antibodies 

 are specific and work against the bacteria, or antigens, which 

 stimulated their production, individuals who have had microbial 

 diseases, or who have been injected with vaccines containing weak- 

 ened or dead organisms, will possess protective antibodies to com- 

 bat the specific bacteria. This is a form of immunity, and this 

 topic will be discussed later in the chapter. 



The Lymph 



This fluid is similar to blood in its composition but it lacks the 

 cellular elements found in blood. Lymph seeps out of the finer 

 blood vessels (capillaries) and bathes the surrounding tissue 

 spaces, eventually gathering at large drainage points from where 

 it is returned to the blood vessels. 



The Tissues 



The liver and the spleen serve as filtering c;evices for the re- 

 moval of circulating microorganisms. These organs are included 

 in the sites generally accepted for antibody production, and hence 

 they may be better able to cope with the microbes which find 

 their way into these organs. 



FACTORS AFFECTING RESISTANCE TO DISEASE 



HEREDITY AND CONGENITAL FACTORS 



When a geneticist speaks of heredity he usually means that the 



individual acquired certain characteristics directly through the 



genes. Microbial diseases, therefore, are not inherited. After 



fertilization of an egg takes place, the embryo or fetus may be 



