Page 172 - The Cell in 40 Topics

here is a system in the body that protects it at all times.
Complement proteins, one component of that system, are pro-
grammed to attack virtually every cell in the body.
This is really astonishing. Although they exist to protect the body,
they regard all the cells that comprise that body as hostile. Complement
proteins are manufactured in the liver, from where they enter the blood-
stream and under normal conditions, travel through the bloodstream at
random, producing no effects. When stimulated, however, they suddenly
decide to eradicate all the cells they encounter.
This stimulus they receive is disseminated throughout the entire body
by way of a single complement protein. They make no distinction between
friend and foe.
However, harmless cells belonging to the body have been created in
such a way as to defend themselves against complement proteins. As soon
as complement proteins make contact with cells belonging to the body, the
cells neutralize those proteins. On the other hand, any foreign organisms
that have entered into the body, will be subjected to an unexpected assault
from these security guards (Figure 137).

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