What type of substance can serve as an antigen?

The correct response identifies that any substance capable of triggering an immune response can serve as an antigen. Antigens are typically complex molecules recognized by the immune system, which include a wide range of substances such as proteins, polysaccharides, nucleic acids, and even some lipids. The key characteristic of an antigen is its ability to elicit an immune response; this can occur whether the antigen is a natural component like a pathogen or something synthetic, like a vaccine.

This broad definition encompasses various natural and artificial substances. For example, viruses, bacteria, and cancer cells can all act as antigens. In addition, some small molecules can act as haptens, which may require binding to larger proteins to be recognized as antigens by the immune system.

In contrast, the other options are limited in scope. One option restricts antigens to only proteins, overlooking other types of molecules that can elicit an immune response. Another option limits the definition to just large molecules, excluding smaller antigens and haptens. Lastly, stating that only synthetic compounds can serve as antigens completely disregards the multitude of natural substances that can function in this role. Thus, the broad definition captures the diverse nature of antigens in the immune response effectively.