Foreign substances that elicit an immune response are termed:

Step 2: Review the definitions of the options provided: - Cytokines: These are signaling proteins released by cells, especially immune cells, to regulate immunity, inflammation, and hematopoiesis. They do not directly refer to foreign substances. - Antigens: These are molecules or substances (often proteins or polysaccharides) on the surface of pathogens or foreign particles that are recognized by the immune system and elicit an immune response. - Phagocytes: These are immune cells (e.g., macrophages) that engulf and digest foreign particles, pathogens, or debris. They are not foreign substances themselves. - Antibodies: These are proteins produced by B cells in response to antigens. They bind specifically to antigens to neutralize or mark them for destruction.