Naturally active acquired immunity is given by:

Options

A. Vaccination
B. Antiserum
C. NK cells
D. Infections

Answer

### Step-by-Step Solution:

1. **Understanding Naturally Active Acquired Immunity**:
- This type of immunity is developed when a person is exposed to a live pathogen. The immune system responds to this exposure by generating an immune response.

2. **Immune Response Activation**:
- When a pathogen enters the body, it triggers the immune system. This involves the activation of T-cells and B-cells, which are crucial components of the immune response.

3. **Role of T-cells and B-cells**:
- T-cells help in directly attacking infected cells, while B-cells produce antibodies that neutralize the pathogens. This process is known as the primary immune response.

4. **Outcome of the Immune Response**:
- After the immune response, the body develops a memory of the pathogen. This means that if the same pathogen tries to infect the body again, the immune system can respond more quickly and effectively.

5. **Long-lasting Immunity**:
- The immunity gained from this process is long-lasting, often for a lifetime. This is because the immune system retains a memory of the pathogen, allowing for a faster response upon re-exposure.

6. **Examples of Naturally Active Acquired Immunity**:
- A common example is the immunity developed after an infection with the hepatitis A virus. Once a person recovers from the infection, they typically have lifelong immunity against future infections by the same virus.

7. **Identifying the Correct Option**:
- Given the options:
1. Vaccination
2. Anti-serum
3. NK cells
4. Infections
- The correct answer is option 4: infections, as naturally active acquired immunity is specifically acquired through exposure to pathogens.