When the immune system of host are exposed to antigen and produces antibodies it is called as:-

Answer

**Step-by-Step Solution:**

1. **Understanding the Immune Response**: The immune system is the body's defense mechanism against pathogens such as bacteria and viruses. When an antigen (a substance that the immune system recognizes as foreign) enters the body, it triggers an immune response.

2. **Role of Antigens**: Antigens can be components of pathogens, such as proteins on the surface of bacteria or viruses. When these antigens are detected by the immune system, they stimulate a response.

3. **Activation of Lymphocytes**: The immune system has specialized cells called lymphocytes, which are a type of white blood cell. There are two main types of lymphocytes: B cells and T cells. In this context, we focus on B cells.

4. **Production of Antibodies**: Upon exposure to an antigen, B cells are activated and begin to produce antibodies. Antibodies are proteins that specifically recognize and bind to antigens, marking them for destruction or neutralization.

5. **Humoral Immunity**: The process described above is known as humoral immunity. It is called "humoral" because it involves substances (antibodies) that are found in the body fluids (humors), such as blood and lymph.

6. **Conclusion**: Therefore, when the immune system of a host is exposed to an antigen and produces antibodies, it is referred to as humoral immunity.

**Final Answer**: Humoral immunity

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