What is defined as a substance that stimulates an immune response because it is recognized as foreign?

An antigen is defined as a substance that stimulates an immune response because it is recognized as foreign by the immune system. Antigens are typically found on the surface of pathogens, such as bacteria and viruses, as well as other foreign substances like allergens. When the immune system encounters an antigen, it activates specific immune cells that recognize and respond to the foreign substance, leading to the production of antibodies and other immune responses aimed at eliminating the threat.

The immune system's ability to distinguish between self and non-self is vital for the protection of the body against diseases. Antigens can induce the production of antibodies, which are specialized proteins that can specifically bind to these foreign molecules and help neutralize or eliminate them. This process is a critical aspect of the adaptive immune response, providing long-lasting immunity and memory in future encounters with the same antigen.

In contrast, antibodies are the proteins produced by the immune system in response to antigens, while pathogens are the agents that can cause disease (like bacteria and viruses). An anticodon, on the other hand, is a sequence of three nucleotides in tRNA that pairs with a complementary codon in mRNA during protein synthesis and is not directly involved in the immune response.