Active immunity is due to

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Active Immunity**: Active immunity refers to the immunity that develops as a result of the body’s own immune response to an antigen. This can occur through natural infection or vaccination.

2. **Role of Antigens**: When an antigen (a substance that the immune system recognizes as foreign) enters the body, it triggers the immune response. This is the first step in the process of developing active immunity.

3. **Immune System Activation**: The immune system responds to the antigen by producing specific immune cells, including T helper cells and B cells. These cells are crucial for recognizing and responding to the antigen.

4. **Formation of Memory Cells**: In response to the initial exposure to the antigen, the immune system creates memory cells, specifically T memory cells. These cells "remember" the specific antigen and remain in the body for a long time.

5. **Secondary Response**: Upon re-exposure to the same antigen, these memory cells quickly recognize it and mount a faster and more effective immune response. This is known as the secondary response, which involves the rapid production of antibodies against the antigen.

6. **Conclusion**: Therefore, active immunity is primarily due to the formation of memory cells, which allow for a quicker and more efficient response upon subsequent exposures to the same antigen.

### Final Answer:
Active immunity is due to the formation of memory cells in response to antigens.

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