Page 624 - Veterinary Immunology, 10th Edition

VetBooks.ir Inhibitory Receptors

A key feature of the adaptive immune system is that, while poised
to launch a potent array of destructive mechanisms against

invaders, the body maintains control of the process. It is critically
important to limit and eventually terminate a response by
inactivating or eliminating pathways that are no longer required.
This regulation involves the extensive use of inhibitory receptors.
These suppress the activity of lymphocytes once they have

completed their task and so provide a crucial safeguard against
inappropriate immune responses. Thus activation and inhibition
must be paired to initiate and terminate immune responses. In

some cases, activating and inhibitory receptors recognize similar
ligands, so the net outcome is a product of the relative strength of
these signals. Loss of inhibitory signals is often associated with
autoimmunity or hypersensitivity.
An excellent example of an inhibitory receptor is CD32b

(FcγRIIb) expressed on B cells. Any antibodies present will occupy
these receptors. If these receptor-bound antibodies are cross-linked
to a BCR through an antigen, the BCR and CD32 come together

(Fig. 20.9). As a result, their signal transduction pathways interact,
and BCR signaling is blocked. This prevents B cell activation and
triggers its apoptosis. The CD32 pathway is a feedback mechanism
whereby B-cell activation is suppressed by antibody and
uncontrolled B-cell responses are prevented. Since another receptor,

FcγRIII, stimulates B cells, B cell responses can be regulated by
altering the ratio of FcγRIIb to FcγRIII. Macrophage activation is
regulated in a similar manner, and activated macrophages have a

high FcγRIII-to-II ratio.

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