An Overview of the Body's Complement Pathways

The complement pathways are a group of three biochemical pathways that contribute in body defense. There are three complement pathways: the classical complement pathway, the alternative complement pathway, and the lectin pathway. While the three pathways differ in the way they are activated, once activated they all produce the same benificial complement proteins. Basically the complement proteins are a series of serum proteins that when activated participate in four important body defense functions. These include:

1. inflammation. Inflammation is the means by which body defense cells and defense chemicals leave the blood and enter the tissue around an injured or infected site. Complement proteins known as C5a, C3a, and C4a lead to vasodilation, increased capillary permeability, and the expression of the adhesion molecules on leukocytes and the vascular endothelium that enable them to adhere to the inner wall of the capillaries, pass between the endothelial cells, and enter the surrounding tissue. Vasodilation also enables a variety of defense chemicals in the plasma of the blood to enter the tissue.These defense chemicals include antibodies and complement proteins. C5a also causes neutrophils to release proteases and toxic oxygen radicals for extracellular killing of microbes.

2. phagocyte chemotaxis. Complement proteins C3a and C4a are chemoattractants for leukocytes. Chemotaxis enables the phagocytes to move toward the infected area in order to remove microorganisms.

3. opsonization (enhanced attachment). The complement proteins C3b and C4b are known as opsonins because they bind microbes to phagocytes. One portion of the molecule binds to microbial proteins while the other portion binds to receptors on phagocytes. In this way, microbes can be engulfed by phagocytes more effectively.

4. lysis of biological membranes. A series of complement proteins known as the membrane attack complex or MAC put pores in cellular membranes resulting in lysis. This is used to kill such things as gram-negative bacteria and virus-infected cells.

These processes will be discussed in greater detail in Units 2 and 3.