Formaldehyde reacts with protein side chains by combining with which group?

• A. Amino group
• B. Carboxyl group
• C. Sulfhydryl group
• D. Hydroxyl group

Answer: (A)

Formaldehyde fixation mainly cross-links proteins by reacting with amino groups. The primary amine groups, especially the ε-amino group of lysine residues and N-termini, react with formaldehyde to form a hydroxymethyl adduct, which can then couple with another amino group to create a methylene (cross-)link between proteins. This cross-linking stabilizes and preserves tissue structure during processing. While carboxyl, sulfhydryl, and hydroxyl groups can react with formaldehyde to some extent, they are not the primary sites responsible for the characteristic cross-linking seen in standard fixation, so the amino group is the best answer.