Answer: Cytokines
Step-by-step explanation:
The immune response can be defined as the response that is generated by the body in order to protect the body from diseases and cure the diseases.
The immune cells release antibodies that binds to the antigen and kills it. The harmful microorganisms are killed by the immune response of the body that is generated by the immune system.
The cytokines are the chemicals that move in the blood and ills the pathogens. These are not cells. So, the correct answer is Option D.