What triggers an immune response in the body?

An immune response in the body is specifically triggered by antigens. Antigens are foreign substances or molecules that the immune system recognizes as non-self, which can include proteins on the surface of pathogens such as bacteria or viruses, as well as other foreign materials. When the immune system encounters an antigen, it initiates a complex cascade of events designed to neutralize or eliminate the threat.

Understanding this, while bacteria, pathogens, and viruses can all provoke an immune response because they often carry antigens, they are not the direct triggers of the immune response. Instead, they are sources of antigens, and it's the antigens' presence that specifically activates the immune system. This distinction is key in immunology, as it highlights the role of antigen recognition in initiating the body's defense mechanisms.