Fcr

Overview

Fc receptors (FcRs) are a class of cell surface receptors that bind to the Fc region of immunoglobulins (antibodies). These receptors play a crucial role in the immune system by mediating the effects of antibodies, including phagocytosis, antibody-dependent cellular cytotoxicity (ADCC), and the release of inflammatory mediators.

Structure

Fc receptors are typically composed of multiple subunits, including a ligand-binding α-chain and one or more signaling subunits. The α-chain is responsible for the specific binding to the Fc region of antibodies, while the signaling subunits, often containing immunoreceptor tyrosine-based activation motifs (ITAMs), are involved in signal transduction.

Types of Fc Receptors

Fc receptors are classified based on the type of immunoglobulin they bind:

Fcγ Receptors

Fcγ receptors bind to the Fc region of IgG antibodies. They are further divided into:

• FcγRI (CD64): High-affinity receptor for IgG.
• FcγRII (CD32): Includes FcγRIIA, FcγRIIB, and FcγRIIC, with varying affinities and functions.
• FcγRIII (CD16): Includes FcγRIIIA and FcγRIIIB, involved in ADCC.

Fcε Receptors

Fcε receptors bind to the Fc region of IgE antibodies. They are primarily involved in allergic responses and include:

• FcεRI: High-affinity receptor for IgE, found on mast cells and basophils.
• FcεRII (CD23): Low-affinity receptor, involved in regulation of IgE levels.

Fcα Receptors

Fcα receptors bind to the Fc region of IgA antibodies. The main receptor is:

• FcαRI (CD89): Found on myeloid cells, involved in phagocytosis and inflammatory responses.

Fcμ and Fcα/μ Receptors

These receptors bind to the Fc region of IgM and IgA/IgM, respectively. They are less well-characterized but are involved in immune regulation.

Function

Fc receptors mediate a variety of immune functions, including:

Phagocytosis

Fc receptors on phagocytes such as macrophages and neutrophils recognize and bind to antibody-coated pathogens, leading to their engulfment and destruction.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

Fc receptors on natural killer cells recognize antibody-coated target cells, leading to the release of cytotoxic granules and the destruction of the target cell.

Release of Inflammatory Mediators

Binding of antibodies to Fc receptors on mast cells and basophils triggers the release of histamine and other inflammatory mediators, contributing to allergic reactions.

Clinical Significance

Fc receptors are involved in various autoimmune diseases, allergic reactions, and immunotherapy strategies. Modulation of Fc receptor activity is a therapeutic target in conditions such as rheumatoid arthritis, systemic lupus erythematosus, and cancer.

Research and Therapeutic Applications

Research into Fc receptors has led to the development of monoclonal antibodies and Fc fusion proteins that exploit Fc receptor pathways to enhance immune responses against tumors and infectious diseases.

See Also

• Antibody
• Immune system
• Phagocytosis
• Allergy