The exact part of the antigen that reacts with the immune system is called as:
High-Yield Explanation
Ans. is 'b' i.e., Epitope(Ref: Ananthanarayan, 9th/e, p. 88 and 7th/e, p. 81; Harrison, 18th/e, p. 2673 and 17th/e, p. 2036)* The part of the antibody or T cell receptor that interact with the epitope.