Questions: Antibody Structure and Biological Functions

Question 1

A monoclonal antibody neutralizes a virus by physically blocking the receptor-binding domain on the viral surface protein. Which structural region of the antibody is responsible for this neutralization?

• A. The Fc region, which activates complement
• B. The hinge region, which provides flexibility
• C. The Fab region (variable domains / CDRs), which directly binds antigen
• D. The CH2 domain, which engages Fc receptors on phagocytes

Answer: (C)

Antigen binding — and therefore direct neutralization — is mediated by the variable domains in the Fab arms of the antibody, specifically the complementarity-determining regions (CDRs). The Fc region does not bind antigen; it mediates downstream effector functions by engaging Fc receptors on immune cells or activating complement.

Question 2

The Fc region of an antibody is responsible for recognizing and binding to specific antigens.

• T. True
• F. False

Answer: False

Antigen binding is the job of the variable (Fv) domains in the Fab arms. The Fc (Fragment crystallizable) region is the constant stem of the Y-shaped antibody and mediates effector functions: it binds Fc receptors on phagocytes and NK cells, activates complement, and determines tissue distribution and serum half-life. Confusing Fab and Fc is one of the most common errors in immunology.

Question 3

Why is it functionally advantageous that antigen recognition and effector function are physically separated into distinct domains of the antibody?

Model answer: Separating recognition (Fab/variable domains) from effector recruitment (Fc/constant domains) allows the immune system to use the same set of downstream killing mechanisms against any antigen, simply by changing the variable domain sequence. It also allows different antibody classes (isotypes) to engage different effector systems without altering antigen specificity.

This modular design is a core feature of adaptive immunity. B cells can undergo class switching — replacing the constant (Fc) region while keeping the same variable region — to redirect the same antigen-specific antibody toward different effector pathways (e.g., switching from IgM to IgG for better complement activation, or to IgA for mucosal secretion). Therapeutic antibody engineering exploits this modularity to tune effector function independently of target specificity.