The defense against pathogens such as viruses and bacteria are mediated by the immune system which is in principle divided in two parts.
The adaptive or acquired immune system is continuously evolving throughout life and continuously one of the key elements is the development of specific antibodies.
The other part is the innate immune system which is already in place at birth. The innate system is a non-specific first line of defense comprised of cells and mechanism to defend against infection caused by other organisms. The complement system plays an important part of the innate immune system.
Complement was originally described in the late nineteenth century and the word complement was coined by the famous German physician Paul Ehrlich in his general theory of immunity.
Complement, is something in the blood that "complements" the cells of the immune system. The complement system is a complex system composed of a large number of proteins that acts in a sequential cascade. Many of these proteins are pro-enzymes that require proteolytic cleavage in order to become active.
Since complement acts non-specifically, regulation of the cascade is crucial. This is achieved by a number of regulatory components such as inhibitors.
The complement can act through three different pathways called the classical, lectin and alternative pathway. The activity of each respective pathway is triggered by different mechanisms/components.