An antigen is

A. a molecule that reacts specifically with an antibody or immune lymphocyte.

B. the variable region of an antibody

C. the constant region of an antibody

D. a cytokine.

E. None of these choices are correct.

Answer: A. a molecule that reacts specifically with an antibody or immune lymphocyte.

Explanation:

An antigen is a foreign body that enters inside the other host body it initiates an immune response. As a result of action of immune response the antibodies are produced. The antibodies are the proteins which fights against the antigen. The antibodies are produced by the white blood cells or lymphocytes. The antigen can be in the form of chemical agents, pollen, pathogens such as bacteria and viruses.