- GENERAL BACTERIOLOGY 
61 
After all the conditions necessary for a successful in¬ 
fection have been satisfied (see the section on Infec¬ 
tion) and the patient has become a victim of an in¬ 
fectious disease, his organism makes an attempt to 
overcome it by producing the substances which will 
destroy the infective agent, i. e., the bacteria and their 
poisons. 
These protective substances are called immune bodies 
or antibodies; just where in the body they are pro¬ 
duced (that is, in what particular organ or organs) is 
unknown, but they are always found in the blood 
serum. 
A few words as to the blood: the blood of the living 
person is always liquid and consists of a fluid part called 
plasma; in this plasma are three kinds of blood cells: 
the red cells ( erythrocytes, from Greek erytheos, meaning 
red, and Icy t os, meaning cell), the white cells (leucocytes, 
from Greek leulcos, meaning white), and the blood plate¬ 
lets. 
When blood is shed it ceases, after a few minutes, 
being liquid and becomes solid, or, as we usually say, 
it clots (coagulates) ; after a few hours a straw-colored 
liquid begins to separate from the part which remains 
clotted, this liquid part of the clotted part is called 
the blood serum; so that the liquid part of living (i. 
e., unshed) blood is plasma, while the liquid part of the 
clotted blood is blood serum. 
One of the most interesting things in immunity is 
the fact that if the blood should be shed and not be 
permitted to clot (e. g., by receiving into a solution 
of such chemical substances as potassium oxalate or 
citrate), so that the plasma should be obtained, no 
trace of any immune bodies can be found; yet allow 
this blood to clot and obtain the blood serum, we shall 
find all the immune bodies that the organism contains. 
