What part of the immunoglobulin molecule is responsible for complement activation?

The Fc region of the immunoglobulin molecule is responsible for complement activation. This region is the tail part of the antibody that interacts with cell surface receptors and other molecules in the immune system, including components of the complement system. When an antibody binds to an antigen using its Fab region, the Fc region is exposed and can interact with complement proteins. This interaction initiates the complement cascade, leading to various immune responses, such as opsonization, cell lysis, and inflammation.

The Fab region, on the other hand, is primarily involved in antigen binding and is responsible for the specificity of the antibody, but it does not play a role in activating the complement system. The variable region is part of the Fab region and is unique to each antibody, allowing for the recognition of specific antigens. The constant region refers to parts of the antibody molecule that do not change and are responsible for determining the class of the antibody (e.g., IgG, IgA) and for interactions with other immune components, but it is the Fc part specifically that mediates complement activation.