What are antigens?

• A. Cells that destroy pathogens
• B. Proteins that can stimulate an immunocyte reaction
• C. Substances that provide energy
• D. All of the above

Answer: (B)

Antigens are specifically defined as any substance that can provoke an immune response, particularly by stimulating the activity of immunocytes such as B cells and T cells. These substances are typically proteins (though they can also be polysaccharides or other macromolecules) that are recognized by the immune system as foreign, triggering the production of antibodies and the activation of various immune cells.

The choice highlighting proteins that can stimulate an immunocyte reaction accurately captures the essence of what antigens are and underscores their role in initiating the adaptive immune response. They are not cells that destroy pathogens; rather, they are recognized by immune cells, which then mount a response. Additionally, antigens do not provide energy, which is a function associated with nutrients rather than immune activity. As such, the option stating that antigens are proteins stimulating an immunocyte reaction is the most precise definition.