HOSA Dental Terminology Practice 2025 - Free Dental Terminology Practice Questions and Study Guide

An antibody is defined as a specific substance produced by an animal's immune system in response to the presence of an antigen, which is typically a foreign substance such as a virus, bacteria, or toxin that the body recognizes as a threat. Antibodies are specialized proteins that bind to specific antigens, neutralizing them or marking them for destruction by other immune cells. This is an essential part of the adaptive immune response, allowing the body to remember and effectively fight off previously encountered pathogens.

In contrast, the other options relate to different concepts. Harmful substances that destroy microorganisms do not specifically define antibodies, as antibodies function more to identify and neutralize rather than eliminate. A type of white blood cell is related to the immune response but is not an antibody itself; instead, certain white blood cells produce antibodies. Lastly, a viral agent causing illness refers to the pathogens that antibodies help defend against, but it does not describe what an antibody is. Thus, option A accurately captures the defining role and function of antibodies in the biological context.