Here's why:

* Antigens are substances that the body's immune system recognizes as foreign. This recognition triggers an immune response, leading to the production of antibodies to neutralize the antigen.

* Viruses, bacteria, and toxins are all examples of antigens.

* Viruses: These are infectious agents that can only replicate within the cells of a living host.

* Bacteria: These are single-celled organisms that can cause infections.

* Toxins: These are poisonous substances produced by living organisms, such as bacteria.

Let me know if you have any other questions!