What is the function of an antibody?

The function of an antibody is to bind to antigens. Antibodies are specialized proteins produced by the immune system in response to foreign substances, known as antigens, which may include pathogens like bacteria and viruses or any other molecules that provoke an immune response. By binding to antigens, antibodies can neutralize threats directly or mark them for destruction by other components of the immune system. This binding process is crucial because it helps to identify and eliminate foreign invaders, thereby playing a vital role in the body's defense mechanisms.

In contrast, the other choices outline functions that do not align with the role of antibodies. Enzymes indeed require substrates, but antibodies do not function in this capacity. Antibodies also do not generate energy; that function is primarily associated with metabolic processes involving nutrients and cells like mitochondria. While some antibodies can be engineered to carry drugs, this is a specific application and not a general function of antibodies. Overall, their primary role in the immune system is the identification and binding to antigens to help protect the body against disease.