Active immunity is due to

Options

A. Memory cells
B. Killer T-cells
C. Helper T-cells
D. Suppresosr T-cells

Answer

### Step-by-Step Solution:

1. **Understanding Active Immunity**:
- Active immunity is defined as the immunity that develops after exposure to an antigen. This exposure can occur through infection or vaccination.

2. **Role of Immune Cells**:
- When the body is exposed to an antigen for the first time, various immune cells are activated. These include killer T-cells, helper T-cells, and suppressor T-cells. These cells work together to fight off the infection.

3. **First Exposure and Immune Response**:
- During the first exposure to the antigen, the immune system generates a response, which includes the production of antibodies. This process involves the activation of the immune cells that help to combat the infection.

4. **Formation of Memory Cells**:
- After the initial infection, the immune system creates memory cells. These cells "remember" the specific antigen and remain in the body for a long time.

5. **Subsequent Exposure**:
- If the body encounters the same antigen again, these memory cells quickly recognize it and mount a faster and more effective immune response, producing antibodies to neutralize the pathogen.

6. **Conclusion**:
- Therefore, active immunity is primarily due to the presence of memory cells that are formed after the first exposure to an antigen. These cells enable the body to respond more efficiently to future infections by the same pathogen.

### Final Answer:
Active immunity is due to memory cells.