Which proteins are released by cells in response to pathogens to stimulate the immune response?

The correct choice is interferons. Interferons are a group of signaling proteins that are produced and released by cells in response to the presence of pathogens, particularly viruses. Their primary role is to initiate and enhance the immune response. When a cell is infected by a virus, it can release interferons to signal nearby cells to heighten their antiviral defenses, making them less susceptible to infection. This communication helps coordinate the immune response and can also activate other immune cells that help the body combat the infection.

In contrast, antibodies are proteins produced by B cells that specifically target and neutralize pathogens or mark them for destruction, but they are not released in direct response to pathogens in the same way as interferons. Cytokines are also immune signaling molecules, but they have a broader role, including facilitating communication between immune cells rather than specifically being a defense mechanism against pathogens. Complement proteins are part of the innate immune system and help in opsonization, lysis of pathogens, and inflammation, but they are not released in response to pathogens in the same manner as interferons.