What triggers an immune response in the body?

An immune response is primarily triggered by the presence of an antigen. Antigens are foreign substances, typically found on the surface of pathogens (such as bacteria, viruses, and fungi) or toxins, that provoke an immune reaction. When the immune system detects an antigen, it recognizes it as a potential threat and activates various immune cells, including lymphocytes, to eliminate it from the body. This process involves the production of antibodies specifically designed to bind to the antigen, marking it for destruction.

While pathogens and toxins can indeed prompt an immune response, they are not the direct triggers; rather, it is the antigens associated with them that actually initiate the immune response. Vaccines work by introducing a harmless form of an antigen into the body, stimulating the immune system to produce a response without causing disease, hence their effectiveness in providing immunity. Therefore, antigens play a crucial role in activating the immune system and are the correct answer to the question.