Page 116 - Veterinary Immunology, 10th Edition

by blocking C5b678 insertion and C9 polymerization.
VetBooks.ir Complement Receptors (CR)

Cells express five receptors for C3 or its fragments. These are called
CR1 (CD35), CR2 (CD21), CR3 (CD11a/CD18), CR4 (CD11c/CD18),

and CRIg.
CR1 is found on primate red cells, neutrophils, eosinophils,
monocytes, macrophages, B cells, and some T cells. It is a receptor

for C3b and C4b as well as for the C3b breakdown product, iC3b.
Red cell CR1 accounts for 90% of all CR1 in the blood. In primates,
CR1 removes immune complexes (antigen-antibody-complement
complexes) from the circulation. These immune complexes bind to
CR1 on red cells, and the coated red cells are then removed in the

liver and spleen (Chapter 32). Deficiencies of complement
components or their receptors may allow circulating immune
complexes to accumulate in the kidney and cause damage. For

example, some patients with the autoimmune disease systemic
lupus erythematosus have a CR1 deficiency and are thus unable to
remove these immune complexes effectively. C3-deficient dogs
develop immune complex–mediated kidney lesions for the same
reason (Chapter 32).

CR2 (CD21) is found on B cells. It is the receptor for C3d. CR2
associates with another B cell surface protein called CD19. This
CD21/CD19 complex regulates B cell responses (see Fig. 15.11). In

order to respond optimally to foreign antigens, B cells must be
stimulated by C3d acting through CR2. When C3d binds to CR2 on
B cells, it reduces their activation threshold 1000-fold. As always,
mice are not a good guide to the situation in other mammals.
Bovine CR2 consists of four distinct receptors that are generated by

alternative splicing. They include two variants that are homologs of
mouse CR1 and CR2 and each is expressed in a short and long
form.

CR3 (CD11a/CD18) is an integrin that binds iC3b. It is found on
macrophages, neutrophils, and natural killer cells. An inherited
deficiency of CR3 (leukocyte adherence deficiency, LAD) occurs in
humans, cattle, and dogs. Affected individuals die from
overwhelming infections (Chapter 39).

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