Anatomy Practice Exam 2025 - Free Anatomy Practice Questions and Study Guide

An antigen is defined as a foreign substance that induces an immune response in the body. This typically includes molecules such as proteins, polysaccharides, or other substances found on the surface of pathogens like bacteria, viruses, or allergens. When these foreign substances enter the body, they are recognized by the immune system, which may trigger the production of antibodies to neutralize or eliminate the threat.

The immune response can vary in its intensity and duration depending on the type and quantity of the antigen, as well as the individual's immune system status. Antigens are crucial for the body's ability to recognize and defend against infections and diseases, playing a central role in the adaptive immune response.

Other options describe different biological elements. For example, blood cells are important for various functions within the immune system, but they are not defined as antigens. The lymphatic system components play a role in immune function but do not directly correspond to what an antigen is. Hormones that regulate glucose, such as insulin, pertain to metabolic processes rather than immune responses. Thus, the definition of an antigen as a foreign substance that induces an immune response is accurate and reflects its critical role in immunology.