Final answer:
An immunogen, more commonly known as an antigen, is (5) any substance that can evoke an immune response, usually a protein or large polysaccharide that binds to immune cell receptors and can stimulate the production of antibodies.
Step-by-step explanation:
An immunogen is best defined as any substance that can evoke an immune response. In the context of immunology, immunogens are more specifically known as antigens. Antigens are usually proteins or large polysaccharides that are capable of eliciting an adaptive immune response. When an antigen enters the body, it binds to specific receptors on immune cells such as T cells and B cells. B cells respond to foreign antigens by producing antibodies, which are molecules that match and bind precisely to a specific antigenic determinant or epitope on the antigen.
Some key points to remember are that not every molecule that enters the body is immunogenic, and the body has mechanisms of tolerance to prevent immune responses to harmless or self antigens. Also, the term antigen was initially used to describe molecules that stimulate the production of antibodies, combining the words antibody and generator. Over time, the understanding of antigens has expanded to include their role in stimulating cellular immunity, making them synonymous with immunogens.