Final answer:
The incorrect statement about the complement system is (option B) that it only appears in the blood during a response to a pathogen. In fact, complement proteins are continually present in the blood and participate in innate and adaptive immune responses.
Step-by-step explanation:
The statement that is incorrect about complement is: B. only appear in the blood during a response to a pathogen. The complement system is composed of a group of proteins that are indeed constantly present in the blood, not just during an immune response. It includes at least 20 blood proteins and activates through cascading reactions involving several pathways, including the classical pathway and alternative pathway. These pathways lead to the destruction of pathogens by coating them with complement proteins. The complement system is not composed of antibodies synthesized in response to an antigen; rather, it is a part of both the innate and adaptive immune systems.