Complement receptor

Receptors that bind complement proteins

Complement Receptors

Complement receptors are integral membrane proteins that interact with components of the complement system, a crucial part of the immune system. These receptors play a significant role in the immune response by recognizing and binding to complement proteins, thereby facilitating various immune functions such as phagocytosis, inflammation, and clearance of immune complexes.

Types of Complement Receptors

Complement receptors are classified based on the specific complement components they bind to. The major types include:

CR1 (Complement Receptor 1)

CR1, also known as CD35, is a receptor for the complement components C3b and C4b. It is expressed on the surface of erythrocytes, leukocytes, and kidney cells. CR1 plays a role in the clearance of immune complexes and the regulation of complement activation.

CR2 (Complement Receptor 2)

CR2, or CD21, binds to the complement fragment C3d. It is primarily found on B cells and follicular dendritic cells. CR2 is involved in the enhancement of the humoral immune response by lowering the threshold for B cell activation.

CR3 (Complement Receptor 3)

CR3, also known as CD11b/CD18 or Mac-1, binds to iC3b, a degradation product of C3b. It is expressed on macrophages, neutrophils, and natural killer cells. CR3 facilitates phagocytosis and cell adhesion.

CR4 (Complement Receptor 4)

CR4, or CD11c/CD18, is similar to CR3 and also binds to iC3b. It is found on dendritic cells, monocytes, and granulocytes. CR4 is involved in phagocytosis and immune cell signaling.

Function of Complement Receptors

Complement receptors mediate various immune functions, including:

• Phagocytosis: By binding to opsonized pathogens, complement receptors enhance the ability of phagocytes to engulf and destroy microbes.
• Immune Complex Clearance: Complement receptors on erythrocytes help transport immune complexes to the liver and spleen for removal.
• Cell Activation: Binding of complement receptors can activate immune cells, leading to the production of cytokines and other inflammatory mediators.
• Regulation of Complement Activation: Some complement receptors, like CR1, help regulate the complement cascade to prevent excessive inflammation and tissue damage.

Clinical Significance

Dysfunction or deficiency of complement receptors can lead to immune system disorders. For example, defects in CR3 and CR4 are associated with leukocyte adhesion deficiency, a condition characterized by recurrent infections due to impaired phagocyte function.

Related Pages

• Complement system
• Immune system
• Phagocytosis
• Leukocyte adhesion deficiency