What is categorized as an antigen in the immune response?

An antigen is defined as a substance that triggers an immune response, making it capable of eliciting the production of antibodies or activating specific immune cells. This definition is central to understanding how the immune system identifies and responds to foreign substances, such as pathogens like bacteria, viruses, and other harmful entities. The recognition of antigens by immune cells is crucial for mounting an effective immune response.

In this context, any molecule that can bind specifically to a receptor on T cells or B cells and elicit a response qualifies as an antigen. This includes proteins, polysaccharides, and even some lipids, depending on their structure and the context of the immune response. Their role is vital in distinguishing between self and non-self, facilitating the body's ability to react appropriately to infections or other challenges.

While the other options relate to aspects of the immune system, they do not accurately capture the full definition of an antigen. For example, a substance that solely stimulates a T cell response pertains to a specific aspect of immune activation, but it lacks the broader function of triggering a comprehensive immune response. An antibody neutralizing pathogens is an outcome of the immune response but is not itself an antigen. Lastly, a molecule that inhibits immune cell activation is contrary to the function of