**Question:** Variable portion of antibody molecule is -

A. Fc fragment
B. Fab fragment
C. Variable region
D. Constant region

**Core Concept:**
Antibodies are glycoproteins produced by B cells in the immune system. They play a crucial role in combating pathogens by binding to specific antigens on their surface. Antibodies contain two identical heavy chains and two identical light chains, which are connected by disulfide bonds. The variable region is responsible for antigen binding, while the constant region is involved in effector functions.

**Why the Correct Answer is Right:**
The correct answer, **C. Variable region**, refers to the part of the antibody molecule responsible for binding to antigens. This region is unique to each individual antibody and allows it to recognize and neutralize specific pathogens. The variable region is composed of variable heavy chain (VH) and variable light chain (VL) domains. These domains are responsible for the unique antigen-binding properties of the antibody and are the basis for the concept of antigen specificity.

**Why Each Wrong Option is Incorrect:**

A. **Fc fragment (Fc)** is the fixed region of the antibody molecule, which is common to all antibodies. It is involved in effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), complement activation, and opsonization. The Fc fragment is derived from the joining (J) chain, constant heavy chain (CH), and constant light chain (CL) regions.

B. **Fab fragment (Fab)** is another fixed region of the antibody molecule, derived from the heavy chain variable region (VH) and light chain variable region (VL). It is involved in antigen recognition, unlike the Fc fragment which is involved in effector functions.

D. **Constant region (C)** refers to the common regions of the antibody molecule, including the joining (J), heavy chain constant (CH), and light chain constant (CL) regions. These regions play a role in effector functions but not in antigen recognition.

**Clinical Pearls:**
1. The variable region is essential for the unique antigen-binding properties of antibodies, which is a key concept in understanding antibody-mediated immunity and specific immune responses.
2. The distinction between Fc fragment, Fab fragment, and constant region is crucial for understanding the functional differences between these regions and their roles in the immune system.