What is a large protein molecule produced in response to an antigen and binds specifically to it?

• A. Nucleotide
• B. Antibody
• C. Enzyme
• D. RNA

Answer: (B)

Antibodies are the large protein molecules produced by B cells in response to an antigen, and they bind specifically to that antigen. Each antibody has unique binding sites formed by the variable regions of its heavy and light chains, creating a precise fit to an epitope on the antigen. This specific binding allows the immune system to neutralize the pathogen directly or tag it for attack by other components like phagocytes or the complement system. Nucleotides and RNA are nucleic acids, not proteins, and while enzymes are proteins, they don’t inherently specialize to bind a particular antigen in the same targeted way antibodies do. So the description matches antibodies.