The antibody idiotype is dictated by which part of the immunoglobulin?

The correct answer is indeed the variable regions of heavy and light chains. The idiotype of an antibody refers to the unique antigenic specificity and is primarily determined by the structure of the variable regions of both the heavy and light chains. These regions comprise a unique combination of amino acid sequences that create a specific binding site for a particular antigen.

Antibodies consist of two heavy chains and two light chains, and within each chain, there are variable and constant regions. The variable regions at the ends of the heavy and light chains are highly diverse and responsible for the specificity of the antibody. This diversity arises from a complex process of somatic recombination and hypermutation that occurs during B cell development, allowing the immune system to recognize a wide array of antigens.

In contrast, the constant regions of the heavy and light chains are responsible for the antibody's effector functions, such as recruitment of immune cells and complement activation, but they do not dictate the specific binding characteristics to antigens. As for the hydrophobic regions, while they play roles in the stability and structure of the antibody, they do not contribute to its antigen specificity.

Understanding this relationship is crucial for grasping how antibodies function in the immune response and their role in recognizing and neutralizing pathogens.