Q.25 Which one of the following features is NOT appropriate for Fab fragment of IgG ?

(A) Contains antigen binding site

(B) Contains an intact L chain

(C) Two fragments are formed from one IgG molecule

(D) Mediates complement fixation in the intact IgG molecule

The correct answer is (D) Mediates complement fixation in the intact IgG molecule. Fab fragments of IgG lack the Fc region required for complement fixation, making this feature inappropriate for them.​

Option Analysis

** (A) Contains antigen binding site **
Fab fragments retain the variable regions (VH and VL) from one heavy and one light chain, forming the complete antigen-binding site. This feature is appropriate.​

** (B) Contains an intact L chain **
Each Fab includes the full light (L) chain paired with the VH and CH1 domains of the heavy chain, linked by a disulfide bond. This feature is appropriate.​

** (C) Two fragments are formed from one IgG molecule **
Papain digestion of one IgG yields two identical Fab fragments and one Fc fragment by cleaving above the hinge region. This feature is appropriate.​

** (D) Mediates complement fixation in the intact IgG molecule **
Complement fixation requires the Fc region of intact IgG to bind C1q; Fab fragments exclude Fc and cannot mediate this. This feature is NOT appropriate for Fab.​

The Fab fragment of IgG plays a key role in antigen binding but lacks effector functions like complement fixation. Understanding its structure helps in exams like CSIR NET Life Sciences.​

IgG Structure Basics

IgG consists of two heavy and two light chains forming a Y-shape, with Fab arms for antigen binding and Fc stem for immune effector functions. Papain cleaves above the hinge, producing two Fab (50 kDa each) and one Fc.​

Key Fab Features

• Retains one antigen-binding site per fragment.​

• Includes intact light chain and partial heavy chain (VH + CH1).​

• Monovalent, ideal for blocking without crosslinking.​

Why No Complement Fixation?

Fab excludes CH2-CH3 (Fc), which binds C1q for complement activation in intact IgG. This makes option (D) incorrect for Fab.​