Final answer:
The alternative complement pathway is part of the innate immune system and is found even in primitive animals such as sponges and cnidarians. This system involves a cascade of proteins that protect the organism by lysing pathogens, and it operates in blood plasma, even in the absence of antibodies.
Step-by-step explanation:
The alternative complement pathway is a part of the immune system that defends against pathogens. This pathway can be found even in animals as primitive as sponges (Porifera) and cnidarians like jellies (Cnidaria). These simple multicellular eukaryotes do not have an adaptive immune system with antibodies, but they do possess primitive immune defense mechanisms to protect against disruptions to homeostasis. The alternative complement pathway does not rely on antibodies, unlike the classical pathway. Instead, it involves a cascade of complement proteins that can be activated spontaneously, leading to the lysis of pathogens.
In more complex organisms, the complement system is a series of proteins found in the blood that contribute to the innate and adaptive immune response. This system consists of proteins normally circulate in the blood in a non-functional precursor form and get activated during the immune response.
The alternative complement pathway is not bound to a specific location of activity within cells but occurs in the blood plasma. Therefore, while the complement system in humans and other vertebrates is complex, even the most primitive animals have a simpler form of this pathway to help defend against pathogens.