What triggers an immune response in the body?

An antigen is a substance that the immune system recognizes as foreign or non-self, which triggers an immune response. Antigens are typically proteins or polysaccharides found on the surface of pathogens like bacteria and viruses, as well as on pollen, transplanted tissues, and even some cancer cells. When antigens enter the body, they are detected by immune cells, which then react by producing antibodies, activating T-cells, and initiating a complex cascade of immune reactions aimed at eliminating the threat.

In the context of this question, hormones, antibodies, and neurotransmitters do not independently trigger immune responses. Hormones are signaling molecules that regulate physiological processes but do not directly provoke an immune reaction. Antibodies are produced by the immune system in response to antigens and help neutralize or eliminate them, but they do not trigger the immune response themselves. Neurotransmitters are involved in nerve signaling and are not related to the immune system's activation. Thus, the key role in initiating an immune response is played by the presence of an antigen.