An **endosome** is a **membrane-bound compartment** inside eukaryotic cells that plays a key role in **sorting and transporting internalized material**. It's part of the **endocytic pathway**, which is how cells take in molecules from their surroundings.

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### 🧬 What does an endosome do?

Endosomes are responsible for:

- **Sorting internalized molecules** (like nutrients, receptors, and pathogens)
- **Transporting cargo** to different destinations:
  - Back to the **plasma membrane** (recycling)
  - To the **Golgi apparatus**
  - To **lysosomes** for degradation

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### 📦 Types of Endosomes

1. **Early endosomes**
   - First station after internalization
   - Mildly acidic
   - Decide whether cargo gets recycled or degraded

2. **Late endosomes**
   - More acidic
   - Fuse with lysosomes for degradation

3. **Recycling endosomes**
   - Return cargo (like receptors) back to the plasma membrane

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### 🦠 Role in Immunity

Endosomes are crucial in **nucleic acid-mediated signaling**, especially:

- **TLR3, TLR7, TLR8, and TLR9** are located in **endosomal membranes**
- They detect **viral RNA or DNA** inside endosomes, triggering **immune responses**

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### 🧪 Clinical relevance

- Pathogens (like viruses) may **exploit endosomes** to enter cells
- Dysregulated endosomal trafficking is linked to **neurodegenerative diseases** and **autoimmunity**
- Therapeutic delivery systems (e.g., **mRNA vaccines**) often target **endosomal uptake**