Answer the following question given this paragraph: 

Complement System The complement system is a series of proteins constitutively found in the blood plasma. As such, these proteins are not considered part of the early induced immune response, even though they share features with some of the antibacterial proteins of this class. Made in the liver, they have a variety of functions in the innate immune response, using what is known as the “alternate pathway” of complement activation. Additionally, complement functions in the adaptive immune response as well, in what is called the classical pathway. The complement system consists of several proteins that enzymatically alter and fragment later proteins in a series, which is why it is termed cascade. Once activated, the series of reactions is irreversible, and releases fragments that have the following actions: • Bind to the cell membrane of the pathogen that activates it, labeling it for phagocytosis (opsonization) • Diffuse away from the pathogen and act as chemotactic agents to attract phagocytic cells to the site of inflammation • Form damaging pores in the plasma membrane of the pathogen Figure 21.13 shows the classical pathway, which requires antibodies of the adaptive immune response. The alternate pathway does not require an antibody to become activated.


Q: The complement system is a series of proteins constitutively found in the what?


A:
blood plasma