What is the primary characteristic of an antigen?

The primary characteristic of an antigen is that it is a substance that can provoke an immune response in the body. Antigens are typically proteins or polysaccharides found on the surface of pathogens such as bacteria, viruses, and other foreign substances. When detected by the immune system, antigens stimulate the production of antibodies, which are proteins designed to identify and neutralize foreign invaders.

While the other options present different biological concepts, they do not define the nature of antigens. For instance, a biological catalyst refers to substances like enzymes that speed up chemical reactions without being consumed, genetic material pertains to DNA or RNA that carries the information necessary for inherited traits, and cellular energy usually relates to molecules like ATP that provide energy for cellular processes. None of these options provide the direct connection to immune response that is critical to the definition of an antigen.