Study GuideMicrobiology–The Immune System1.Antigens: The Targets of the Immune SystemOur immune system is designed to recognize and respond toforeign substancesthat enter thebody. These substances are calledantigens, also known asimmunogens.1.1What Are Antigens?Antigens are substances that can trigger an immune response.Most antigens arelarge molecules, such as proteins or complex carbohydrates. However,smallmoleculescan also act as antigens if they first attach to proteins in the body.•These small molecules are calledhaptens.•By themselves, haptens cannot cause an immune response.•When a hapten binds to a body protein, the combined structure (called aconjugate) cantrigger the immune system.So, size matters—but attachment matters even more.1.2How Antigens Start an Immune ResponseFor most immune responses to begin, antigens must betaken up and processed by macrophages(a type of immune cell found in tissues).Here’s what happens:1.A foreign substance enters the body.2.A macrophage engulfs and processes it.3.This processing helps activate other immune cells.However,not every foreign substance causes an immune response. Some substances may bebroken down or destroyed before macrophages can process them, so no immune reaction occurs.Preview Mode
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