OSMT (Ontario Society of Medical Technologists) Practice Exam

An antigen is defined as a foreign substance that enters the body and triggers an immune response. These substances can be proteins, polysaccharides, or other molecules found on the surfaces of pathogens such as bacteria, viruses, or parasites. When antigens are recognized by the immune system, they stimulate the production of antibodies, which are specific to those antigens and help the body mount an effective immune response to eliminate the foreign invader.

The other choices mention different biological components: a cell that engulfs foreign debris refers to phagocytes, which are part of the immune system but are not themselves antigens. A small self protein does not fit the definition of an antigen, as antigens are typically foreign to the body. A flagellate that is motile describes a type of organism but does not pertain to the concept of an antigen. Therefore, identifying an antigen correctly as a foreign substance in the body is essential for understanding how the immune system recognizes and responds to potential threats.