PfelttCIPLES OP Bacteriology 
eo 
These varieties of immunity are graphically repre¬ 
sented in the following diagram. 
Immunity- 
Natural (C o n- 
g-enital or 
inherited). 
Species — (fowl 
to anthrax). 
Racial — (White 
men more re¬ 
sistant to tu- 
b e r c u 1 osis 
than Indians 
or negroes). 
Individual. 
Acquired 
/ 
Naturally Ac¬ 
quired (b y 
having had 
one attack 
of the in¬ 
fectious dis¬ 
ease). 
Artificially Acquired 
(through protective substances) 
/ \ 
Active 
(when the pa¬ 
tient produces 
his own pro¬ 
tective sub¬ 
stances, as, e. 
g., following 
typhoid vacci¬ 
nation). 
Passive 
(when the pa¬ 
tient receives 
protective sub¬ 
stances pro¬ 
duced in some 
animals; as, e. 
g., when he re¬ 
ceives diph¬ 
theria anti¬ 
toxin). 
To make this still clearer let us take as an example 
the typhoid fever and various types of immunity which 
may exist against it: if a person is born immune to it, 
this will be natural immunity; if he develops immun¬ 
ity after birth, this is acquired immunity; if immunity 
was the result of this person’s having had an attack of 
typhoid fever, we will call this naturally acquired im¬ 
munity ; if the person developed it after he had received 
an injection of dead typhoid bacilli, he has the active 
artificial immunity; but if he had been injected with 
the blood serum of an animal which had received dead 
typhoid bacilli, then his would be the passive artificial 
immunity. 
IV. Protective Substance or Immune Bodies 
Now that we have considered the various types of 
immunity we have to study just what the protective 
substances, which constitute the immunity, are: 
