Which of the following correctly defines an antibody?

An antibody is best defined as a protein that specifically recognizes and binds to an antigen. Antibodies are produced by B cells in response to the presence of foreign substances, known as antigens, such as viruses, bacteria, or toxins. Each antibody has a unique structure that allows it to attach to a specific antigen, effectively targeting the invader for destruction or neutralization by the immune system. This specificity is crucial for the adaptive immune response, as it helps ensure that the body can effectively combat a wide variety of pathogens.

In relation to the other options, while substances that induce an immune response are important, they do not define an antibody specifically. Moreover, some cells play a role in the immune response by engulfing pathogens, but this describes the function of phagocytes rather than antibodies. Finally, the function of presenting antigens to T cells is primarily carried out by antigen-presenting cells, such as dendritic cells and macrophages, rather than antibodies themselves. Understanding these distinctions is key to grasping the nuanced roles different components play within the immune system.