A substance that stimulates an adaptive immune response is a(n):

• A. antigen
• B. antibody
• C. complement protein
• D. cytokine

Answer: (A)

Verified step by step guidance

1. Understand the key terms involved in the question: an antigen, antibody, complement protein, and cytokine.
2. Recall that an antigen is any substance that the immune system recognizes as foreign and can stimulate an adaptive immune response.
3. Recognize that an antibody is a protein produced by B cells that specifically binds to an antigen, but it does not stimulate the response; it is part of the response.
4. Know that complement proteins are part of the innate immune system and help destroy pathogens but do not directly stimulate the adaptive immune response.
5. Understand that cytokines are signaling molecules that mediate and regulate immunity, inflammation, and hematopoiesis, but they are not the initial stimulators of the adaptive immune response.