What term is used to describe small regions of a microbe recognized by the immune system?

The term that describes small regions of a microbe recognized by the immune system is "epitope." Epitopes are specific sites on an antigen that are recognized by antibodies or T-cell receptors, allowing the immune system to identify and respond to the pathogen. Each epitope may induce a specific immune response, and multiple epitopes can exist on a single pathogen, contributing to the immune system's ability to recognize and attack a wide variety of infectious agents.

Antigens, while related, refer to larger molecules or structures that can provoke an immune response, which include one or more epitopes. Cytokines are signaling molecules that facilitate communication between immune cells, while receptors are proteins on the surface of immune cells that bind to specific ligands, such as antigens or cytokines, to initiate an immune response. The distinct role of an epitope is crucial in the context of immunology, as it represents the precise portion of an antigen that the immune system targets.