Which of the following substances is recognized as non-self and can trigger an immune response?

The correct answer is an antigen. Antigens are defined as substances that the immune system recognizes as foreign or non-self. This recognition is crucial for the immune response, as antigens can stimulate the production of antibodies and activate various immune cells to combat perceived threats. Antigens can be derived from a variety of sources, including pathogens such as bacteria and viruses, or even non-infectious substances like pollen or food proteins.

While pathogens are indeed a category of antigens, not all antigens are classified as pathogens, making the term "antigen" the broader and more accurate choice to describe a substance recognized by the immune system. Allergens are a specific type of antigen that can trigger allergic reactions but not all antigens are allergens, thus limiting their applicability. Antibodies are the immune system’s response to antigens and do not trigger an immune response themselves; instead, they help to neutralize or eliminate the antigens.