What defines an antigen?

An antigen is defined as a substance that is recognized by the immune system as foreign or non-self. This recognition prompts an immune response, as the body identifies the antigen as a potential threat, such as pathogens like bacteria, viruses, or other foreign substances. Antigens are typically proteins or polysaccharides found on the surfaces of these foreign materials.

Option B is correct because it accurately reflects the fundamental role of antigens in stimulating the immune response. When the immune system identifies an antigen, it activates various cells and processes to neutralize or eliminate the threat, highlighting the crucial function of antigens in immunology.

The other options pertain to immune system components but do not accurately describe an antigen. Healthy tissue within the body does not qualify as an antigen, as it is recognized as self. Proteins produced by immune cells, such as antibodies, work against antigens but do not define what an antigen is. Finally, natural body defenses against diseases encompass a broader scope of immune responses and mechanisms, rather than focusing on the specific definition of an antigen itself.