CD32

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

CD32, also known as Fc gamma receptor II (FcγRII), is a surface protein found on many immune cells. It binds the Fc portion of IgG antibodies. Its job is to help regulate how immune cells respond to antibodies and to help clear immune complexes—groups of antibodies attached to targets such as bacteria.

There are three main subtypes: CD32A, CD32B, and CD32C. CD32A and CD32C are activating receptors, meaning they turn on immune responses when they bind immune complexes. CD32B is inhibitory and helps dial down antibody production and other responses. All three sit on the cell surface with external parts that grab IgG, and internal parts that send signals into the cell.

Binding specifics: All subtypes can bind IgG1 and IgG3 immune complexes. CD32A binds IgG2. CD32B and CD32C can bind IgG4 but not IgG2. Monoclonal antibodies can distinguish CD32A from CD32B, but CD32A and CD32C are very similar.

Where CD32 is found and what it does:

- CD32A: on platelets, neutrophils, macrophages, dendritic cells, and some activated T cells. It helps platelets take up bacteria, supports platelet activation after vessel injury, and promotes phagocytosis and cytokine release in other cells.
- CD32B: on B cells, dendritic cells, basophils, neutrophils, monocytes, macrophages, and some tissues. It can cross-link with B cell receptors to raise the activation threshold and reduce antibody production, and it helps hold onto immune complexes for later presentation to B cells.
- CD32C: on some B and natural killer cells (about 20% of people). It contributes to antibody-dependent cellular killing, though its exact role is less well understood.

Why CD32 matters: The balance between activating (A/C) and inhibitory (B) signals is crucial for normal immune function. Too little CD32B can lead to excessive antibody responses and autoimmunity, while proper CD32B helps protect against overreaction. Changes in CD32 activity are linked to autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. Some CD32B-related differences can influence malaria risk in certain people.

Therapeutically, targeting CD32B with antibodies can help trigger the destruction of B cell lymphoma cells. Overall, CD32 is a key regulator of antibody responses and how the immune system handles antibody-coated targets.