Antibody fragments: How many antigen-binding sites does a single Fab fragment contain?

Introduction / Context:
IgG antibodies are Y-shaped with two antigen-binding arms. Enzymatic cleavage yields Fab and Fc fragments, each with distinct properties used in research, therapeutics, and diagnostics.

Given Data / Assumptions:

• Fab is the fragment antigen-binding region derived from one arm of IgG.
• An intact IgG has two Fabs (bivalent overall).

Concept / Approach:
Each Fab comprises one light chain and the corresponding portion of one heavy chain, forming a single paratope that binds one epitope. Therefore, a single Fab is monovalent for antigen.

Step-by-Step Solution:
Start from whole IgG: two identical binding sites. Isolate one arm by papain → one Fab. Recognize each arm binds one epitope. Answer: a Fab has one antigen-binding site.

Verification / Alternative check:
Structural studies (X-ray, cryo-EM) and binding assays demonstrate monovalent Fab interactions, in contrast to bivalent whole IgG or F(ab')2.

Why Other Options Are Wrong:
Two sites describes whole IgG or F(ab')2; antibody-binding site is not meaningful here; zero sites contradicts function.

Common Pitfalls:
Confusing bivalent antibody with monovalent Fab; mixing terms Fab versus Fc.

Final Answer:
One antigen-binding site.