Antigens are surface molecules on pathogens that trigger immune responses

Steps:

• Recall that antigens are foreign molecules recognized by the immune system.
• Identify that pathogens display antigens on their outer surfaces for immune detection.
• Note macrophages process and present pathogen-derived antigens, not their own.
• Eliminate options not matching the primary location of antigens.

Why A is correct:

• By immunological definition, antigens are specific molecules, like proteins or polysaccharides, located on the pathogen's surface to be recognized by antibodies and T-cells.

Why the others are wrong:

• B duplicates A exactly, making it incorrect as a distinct choice.
• C confuses antigen presentation (macrophages display processed pathogen antigens) with the antigen's origin.
• D ignores the established location on pathogens, as antigens are not absent from them.

Final answer: A