Which is incorrect about the FC region of an immunoglobulin?
It forms an antigen binding site
It is called the crystallizable fragment
It determines the class to which the immunoglobulin belongs
It contains an infector molecule that combined cells such as macrophages and mast cells
It contains an effector molecule that can fix complement
The correct answer and explanation is:
Correct Answer: It forms an antigen-binding site
Explanation:
The Fc region of an immunoglobulin (antibody) does not form an antigen-binding site. Instead, the antigen-binding sites are located in the Fab (Fragment antigen-binding) regions of the antibody. These Fab regions consist of the variable domains of the heavy and light chains, which are specifically structured to recognize and bind to a unique antigen.
The Fc region, on the other hand, is the constant portion of the antibody molecule. Its primary roles are not related to antigen recognition but to mediating immune system interactions and determining the antibody’s effector functions. The term “Fc” stands for “Fragment crystallizable,” reflecting its ability to form crystals during certain experimental conditions. This region determines the class (or isotype) of the immunoglobulin, such as IgG, IgA, IgE, IgM, or IgD, and influences its interaction with other components of the immune system.
Functions of the Fc region include:
- Effector Function: The Fc region can bind to Fc receptors on immune cells such as macrophages, mast cells, and natural killer (NK) cells, triggering various immune responses such as phagocytosis or degranulation.
- Complement Activation: In certain classes of immunoglobulins (e.g., IgG and IgM), the Fc region can interact with complement proteins to initiate the complement cascade, a critical mechanism for pathogen destruction.
The incorrect statement—”It forms an antigen-binding site”—confuses the function of the Fc region with that of the Fab region. Understanding the distinct roles of the Fab and Fc regions is crucial for comprehending antibody structure and function in the immune response. This knowledge is especially important in immunology and the design of therapeutic antibodies.