Page 42 - Veterinary Immunology, 10th Edition

The rejection of a first graft is slow and relatively mild. The rejection
VetBooks.ir This accelerated rejection is due to the presence of memory cells.
of a second graft is rapid and accompanied by severe inflammation.

Notice the similarity of this response to that in Fig. 1.7.

However, graft rejection differs from antibody-mediated
immunity in that it cannot be transferred from a sensitized to a
normal animal by serum. The ability to mount a secondary reaction

to a graft can only be transferred between animals by living T cells.
These T cells are found in the spleen, lymph nodes, or blood, and
they are responsible for organ graft rejection. It is a good example

of a cell-mediated immune response.

Mechanisms of Adaptive Immunity

In some ways the adaptive immune system may be compared to
systems in a totalitarian state in which foreigners are expelled,
citizens who behave themselves are tolerated, but those who

“deviate” are eliminated. While this analogy must not be carried
too far, clearly such regimes possess a number of characteristic
features. These include border defenses and a police force that
keeps the population under surveillance and promptly eliminates

dissidents. In the case of the adaptive immune system, the
antibody-mediated responses would be responsible for keeping the
foreigners out, whereas the cell-mediated responses would be
responsible for stopping internal dissent. Organizations of this type

also tend to develop a pass system, so that invading foreigners or
dissidents not possessing certain identifying features are rapidly
detected and dealt with.
Similarly, when foreign antigens enter the body, they first must

be trapped and processed so that they can be recognized as being
foreign. If so recognized, then this information must be conveyed
either to the antibody-forming B cells or to the T cells of the cell-
mediated immune system. These cells must then respond by

producing specific antibodies and/or cytotoxic T cells that can
eliminate the antigen. The adaptive immune system must also
generate long-lived B or T memory cells that can remember this
event so that the next time an animal is exposed to the same

antigen, these cells will respond faster and with greater efficiency.
In our totalitarian state analogy, the police force would be trained

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