The small piece of a pathogen that generates an immune response is called an antigen.

Here's a breakdown:

* Pathogen: A disease-causing organism (like a virus, bacteria, or parasite).

* Antigen: A substance that triggers an immune response, specifically the production of antibodies. Antigens can be proteins, carbohydrates, lipids, or even nucleic acids.

Think of it like this: Your immune system is like a security guard. It needs to know what to look for to protect you from invaders. Antigens are the "wanted posters" of the immune system, giving it the information it needs to identify and attack pathogens.