24 
Transactions Texas Academy of Science. 
two distinct types: (The following classification is borrowed from 
Muir and Ritchie.) 36 
A. Active Immunity — i. e., produced in an animal by an injec- 
tion, or by a series of injections, of non-lethal doses of an 
organism or its toxines. 
1. By injection of the living organisms. 
(i a ) Attenuated in various ways. Examples: — 
(1) By growing in the presence of oxygen, 
or in a current of air. 
(2) By passing through the tissues of one 
species of animal (becomes attenuated 
for another species). 
(3) By growing at abnormal temperatures, 
etc. 
(4) By growing in the presence of weak 
antiseptics, or by injecting the latter 
along with the organism, etc. 
(b) In a virulent condition, in non-lethal doses. 
2. By injection of the dead organisms. 
3. By injection of filtered bacterial cultures , i. e., toxines ; 
or of chemical substances derived from these. 
These methods may also be combined in various ways. 
B. Passive Immunity, i. e., produced in one animal by injection 
of the serum of another animal highly immunized by the 
methods of A. 
1. By antitoxic serum , i. e., the serum of an animal highly 
immunized against a particular toxine. 
2. By antimicrobic serum, i. e., the serum of an animal 
highly immunized against a particular organism in 
the living and virulent condition. 36 
The protective value of active immunity extends through a con- 
siderable period of time, while that of passive immunity is evanes- 
cent. 
An adequate explanation of this vast array of facts is yet before 
us. The explanation in detail cannot be given tonight; that must 
await another time ; but some generalizations must be made. 
1. Pasteur’s Theory of Exhaustion of the pabulum is disproved 
by the fact of passive immunity. 
2. The Theory of Retention will have to be greatly modified 
before it can explain many facts with which it is now in opposi- 
tion. 
