We all know there are three pathways to activate complement: classical pathway, lectin pathway and alternative pathway. What is the advantage of having multiple pathways?
Although the initiating event of each of the three pathways of Complement (Clasical, lectin and alternative) activation is diferent, they converge in that all of them can cooperates with both the innate and the adaptative immune system to eliminated blood and tissues pathogens.
I think the major advantage is that when a individual has a deficient in a complement component of ones these pathways the others can function to eliminate dangerous antigens or immunogens. However, if this complement component deficience is present in two pathways, patients can have the most severe clinical manifestation, suffering frequent severe bacterial infections.
Besides, these three pathway can function together to carry out a number of basic functions including:
Lysis of cells, bacteria, and viruses
Opsonization, wich promotes phagocytosis of particulate antigens
Binding to specific complement receptor on cel of the immune system, triggering activation of immune responses such as inflamation and secretion of immunoregulatory molecules that simply or alter specific immune responses
Immune clearance, which removes immune complexes from the circulation and deposites them in the spleen and liver
There can be many reasons for this, but generally redundancy in biological systems is very important in that it makes the system more robust. If one pathway fails, due to mutation or some environmental factors, the other(s) can compensate and the function of the pathway remains. In some cases it can also be used to tune the degree of activity or speed of activation.