# Antibody and Antigen Binding

> An IgG antibody is a Y-shaped protein. The tips of its two arms fit a specific part of an antigen, and its tail signals other immune cells to deal with whatever is bound.

Web page: https://cellnaut.com/topics/antibody-antigen-binding

![Teaching illustration: Antibody and Antigen Binding](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/antibody-antigen-binding-teaching.jpg)

## Labelled parts

1. **Antigen particle**: A foreign particle, such as part of a virus or bacterium, that the immune system can recognise.
2. **Surface antigen**: A molecule on the particle's surface that antibodies can bind.
3. **Antigen epitope**: The small part of an antigen that an antibody actually recognises.
4. **Fab arm**: Each arm of the Y (fragment antigen-binding) ends in a variable tip.
5. **Antigen-binding site**: The tip's shape and charge match one epitope.
6. **Hinge region**: A flexible joint that lets the arms swing to reach two antigens.
7. **Heavy and light chains**: An IgG is built from two heavy and two light chains joined by disulfide bonds.
8. **Fc region**: The tail, which signals immune cells such as macrophages to engulf the bound target.

## Key points

- Binding is specific: the shape and charge of the tip match one epitope, like a key in a lock.
- Each IgG has two identical antigen-binding sites, so it can link two antigens together.
- The Fc tail is recognised by immune cells such as macrophages, which then engulf the target.

## Check yourself

**1. Which part of an antibody recognises the antigen?**

- A. The Fc region
- B. The tips of the Fab arms
- C. The hinge
- D. The disulfide bonds

**Answer:** B. The tips of the Fab arms - The variable tips of the Fab arms form the antigen-binding sites.

**2. How many antigen-binding sites does one IgG antibody have?**

- A. 1
- B. 2
- C. 3
- D. 4

**Answer:** B. 2 - One at the tip of each arm.

## Related lessons

- [Cell Communication and the Neuron](https://cellnaut.com/learn/cell-communication)

## See it in 3D

- [White Blood Cell](https://cellnaut.com/studio/whiteBlood)