What type of antigen reacts with an antibody that it did not induce?

The correct answer is heterologous antigen. This term refers to an antigen that is not derived from the same individual but can still elicit an immune response upon exposure. Heterologous antigens are recognized by antibodies produced in response to other antigens, meaning that the antibody reacts with the heterologous antigen despite not having induced its production.

Understanding the other types of antigens provides context for why heterologous antigen is the best choice here. Homologous antigens are those that originate from the same species as the antibodies being formed; they directly induce the immune response they correspond with. Autoantigens are components of the body's own tissues that can trigger an autoimmune response, where the body mistakenly attacks its own antigens. Isoantigens refer to antigens that are present in some individuals of the same species but absent in others, often related to blood group antigens, and they can also elicit an immune response.

Thus, heterologous antigen is distinctively identified by the fact that it interacts with antibodies that have been generated by exposure to different antigens, making it a prime example of cross-reactivity in the immune system.