What is the term for an antigen capable of eliciting an immune response?

The term for an antigen capable of eliciting an immune response is known as an immunogen. Immunogens are specific types of antigens that can provoke an immune response when introduced into the body. They are typically recognized as foreign by the immune system and can stimulate the production of antibodies or activate T-cells.

Membrane proteins, while they can serve as antigens, do not specifically describe the capability to elicit an immune response as a defining characteristic. Carbohydrate antigens may also serve as antigens but not all carbohydrates are capable of inducing an immune response; hence they do not exclusively define immunogenicity. Pathogens, such as bacteria or viruses, often carry immunogens, but the term itself refers to the organisms that can cause disease, rather than the specific ability to evoke an immune response. Thus, immunogen is the most accurate term representing the concept of an antigen that can stimulate an immune response effectively.