Question

Antigen binding site in an antibody is found between
A. Two light chains
B. Two heavy chains
C. Variable regions of heavy and constant regions of the light chains
D. Variable regions of light and heavy chains

Answer 1

Hint: Antibodies are immunoglobulin proteins. B-lymphocytes secretes the antibodies after the attack of the pathogens or antigens in the body. B-lymphocytes get differentiated into plasma cells. Plasma cells secrete a large number of antibodies in response to antigen into our blood.

Complete answer: Antibodies are responsible for antibody-mediated immunity or humoral immunity in our body. As the B-lymphocytes get activated and differentiated to form plasma cells, the undifferentiated B-lymphocyte remains as memory cells. An antibody molecule consists of four peptides, among them two are long chains called heavy chains or H-chains, and the remaining two are short chains called light chains or L chains. The molecular weight of the heavy chain is 50,000 Da and the molecular weight of the light chain is 25,000 Da. The regions which are responsible for the antigen-binding are extremely variable while other regions are relatively constant. Thus every heavy and light chain consists of the variable and constant region. The physicochemical properties or isotype of immunoglobulin is determined by the constant region. The terminal end of antibodies that binds to the antigen is called a Fab portion. Fab is a fragment of the molecule to which antigen binds, it is also known as a paratope. The other end is called Fc, as it can be crystallizable and responsible for effector function.
The antigen-binding site consists of the variable region of the light chain and the variable region of the heavy chain.

Hence, the correct answer to the given question is option D, i.e., Variable regions of light and heavy chains.

Note: Constant regions of the heavy chain are connected by a disulphide linkage. The heavy and light chains are also connected by a disulphide linkage. The representation of antibodies is represented by H2L2. Different types of antibodies are present in our body such as IgG, IgA, IgM, IgE, IgD.