Step 1: Understanding the Concept
An antibody recognises an antigen through variable regions at the tips of its arms.
Step 2: Paratope and epitope
The site on the antibody that fits the antigen is the paratope. The matching site on the antigen is the epitope, also called the antigenic determinant.
Step 3: Conclusion
The paratope is therefore the antigen binding site of the immunoglobulin.
Final Answer:
The paratope is the antigen binding site.
\[ \boxed{\text{(A) }\text{antigen binding site}} \]