What all can act as an antigen?

An antigen can be any substance that elicits an immune response, including proteins, polysaccharides, lipids, and nucleic acids. Common examples include pathogens like bacteria and viruses, as well as non-infectious substances such as pollen, certain foods, and transplanted tissues. Even small molecules, when attached to larger carrier proteins, can function as haptens and act as antigens. Overall, antigens are diverse and can originate from both external and internal sources.