118 
E. P. NILES. 
THE THEORIES OF IMMUNITY. 
By E. P. Niles, D.V.M., Blacksburg, Va. 
Read before the Virginia State Veterinary Medical Association at Richmond, Va., 
January 2, 1896. 
No branch of the science of biology is receiving at the pres¬ 
ent time more deserved attention than immunity. It may also 
be said that less is known about this part of the science than of 
any other. 
It is not my intention to offer anything new on the subject. 
If I even succeed in placing before you in an intelligent manner 
the views of our eminent bacteriologists I shall feel that my fee¬ 
ble effort has not been in vain. In the beginning of this paper 
I shall not attempt to offer a definition of the term immunity, 
but shall confine my remarks to a discussion of the theories of 
immunity, and a few of the experiments brought to bear in 
support of the same. 
Immunity may be discussed under two heads, viz., natural 
and acquired. It is a well-known fact that certain bacterial 
diseases affect a certain class of animals, while others are ex¬ 
empt from the action of these germs. Glanders is essentially a 
of the equine species, while the bovine species show per¬ 
fect immunity to the disease. The dog and rat are decidedly 
immune to anthrax, while it is readily communicated to the 
equine, bovine and human races by inoculation. Even age in 
animals of the same species seems to influence the degree of im¬ 
munity. In the human family scarlet fever and whooping- 
cough are considered children’s diseases, since they are much 
more prevalent in them than adults. Diphtheria is also more 
frequently met with in children. At the same time, however, 
I have heard physicians assert that a young baby will not con¬ 
tract the disease. If this be true, it can only be explained by 
the supposed germicidal action of the thymus gland, which 
rapidly disappears after birth, and possibly some influence of the 
mother’s milk. In the lower animals anthrax is known to be 
