Which region of an immunoglobulin has an identical amino acid sequence across different types?

The constant region of an immunoglobulin is characterized by having an identical amino acid sequence across different types of antibodies, such as IgG, IgA, IgM, IgD, and IgE. This region is responsible for the effector functions of antibodies, including their interaction with various receptors on immune cells and the complement system.

Unlike the variable region, which differs between antibodies to allow for the specific binding to a diverse array of antigens, the constant region remains relatively uniform among different classes of immunoglobulins. This homogeneity is crucial for maintaining consistent functionality across these antibody types, even as the variable regions change to adapt to different antigenic challenges.

The Fab region, which includes both variable and constant domains, varies in the variable part, while the Fc region does have differences among antibody classes, particularly in their effector roles and structure. However, the constant region itself is the one that exhibits the same sequence across the antibody classes, making it the correct choice in this context.