Page 380 - Veterinary Immunology, 10th Edition

cells and stimulate dendritic cell maturation and antigen
VetBooks.ir presentation.

CD16 (FcγRIII) binds IgG with low affinity and will therefore
only bind immune complexes. It is found on granulocytes, NK cells,

and macrophages, but not on B cells. Signaling through CD16 can
trigger NK cell activation.
Mice have an additional receptor for IgG called FcγRIV, and
related proteins are found in humans, chimpanzees, rats, dogs, cats,

pigs, and cattle. This receptor binds IgG2 antibodies with moderate
affinity, but it does not bind IgG1 or IgG3. It is expressed
exclusively on neutrophils, macrophages, and dendritic cells.
Cattle and sheep also have a unique FcR called Fcγ2R. It is not

related to other mammalian FcγRs but belongs to a novel gene
family that includes FcαRI (CD89) and the KIRs (Chapter 19). It is
expressed on myeloid cells and binds only IgG2. It is important in
promoting phagocytosis of antibody-opsonized bacteria in these

species.
FcαRI (CD89) is expressed on neutrophils, eosinophils,
monocyte-macrophages, and dendritic cells. It binds IgA and
mediates its endocytosis and recycling. FcεRI is a high-affinity IgE

receptor found on mast cells and discussed in Chapter 29. It plays
an important role in allergies. CD23, or FcεRII, in contrast, is a low-
affinity IgE receptor expressed on activated B cells, platelets,
eosinophils, macrophages, NK cells, dendritic cells, and possibly

even T cells. Activated B cells can secrete soluble CD23, which then
regulates allergic responses.
PIgR and FcRn are Fc receptors involved in immunoglobulin
transport across epithelial surfaces. They are described in Chapters

22 and 23.

Complement Receptors

There are four major complement receptors on lymphocytes (CR1 to
CR4). B cells and activated T cells express CR1 (CD35), which binds
C3b and C4b, and CR2 (CD21), which binds C3d and C3bi. CR2 is

closely associated with the BCR and regulates B cell responses to
antigen. NK cells express CR3 and CR4.

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