Consequences of Complement Activation – Effector Functions

The MAC and other complement by-products formed during activation of various pathways augments the immune response in many ways as follows. These are known as effector functions of complement products.

1. Cell lysis by MAC

MAC damages cell membrane by making pores or channels in it and allowing the free passage of various ions and water in to the cell. This ultimately leads to cell death. Bacteria, enveloped viruses, irreversibly damaged cells, cancerous cells etc are killed by this mechanism commonly referred to as complement mediated cell lysis.

Gram positive bacteria, which are protected by their thick peptidoglycan layer, bacteria with a capsule or slime layer around their cell wall, or non-enveloped viruses are less susceptible to lysis.

2. Inflammatory Response

C3a, C4a and C5a are called anaphylotoxins. They bind to receptors on mast cells and induce its degranulation leaing to release of histamine and other pharmacologically active mediators of inflammation. They cause vasoconstriction and increase in vascular permeability. Along with the C5b67, they also induce the migration of neutrophils and monocytes to the site of complement activation. This leads to an inflammatory response which is a local protective response.

3. Opsonization

C3b and C4b act as major opsonins that coat the immune complexes and particulate antigen. Phagotytic cells express receptors (CR1, CR3 and CR4) for complement components (C3b, C4b etc.) and are able to bind the complement coated antigen and enhance its phagocytosis. C5a augments this process by enhancing the CR1 expression on phagocytes by 10 folds.

4. Viral Neutralization.

Complement plays a very important role in viral neutralization in many ways:

  • Most viral particles bind to their serum antibody and form particulate immune complex which stimulates the classical pathway. Many viruses are also capable of activating the alternative and lactin pathway in the absence of antibody. So, the MAC is able to destroy the viruses.
  • C3b helps in the formation of viral aggregates by acting as opsonin and thus decrease the net number of infective viral particles. This effect is enhanced in the presence of serum antibody.
  • Complement products also coat the viral particles. This coating neutralizes the viral infectivity by blocking its attachment to target cell and enhancing its phagocytosis by macrophages through complement.
  • Complement cytolyse most enveloped viruses causing fragmentation and disintegration.

5. Solublization of Immune Complex

C3b plays an important role in removing immune complex from the blood. its binding to complexes facilitates their binding to CR1 on RBCs. Though CR1 are present in higher number is granulocytes than the RBCs, but because of much larger number of RBCs than granulocytes in blood, RBCs account for the 90% of the total CR1 in blood. Immune complexes bound to the RBCs are taken to liver and spleen where they are phagocytoed after seperation from RBCs.

Complement System

The term “complement” was coined by Paul Ehrlich to describe the activity in serum, which could “complement” the ability of specific antibody to cause lysis of bacteria. Complement historically refers to fresh serum capable of lysing antibody-coated cells.

Complement system is composed of over 20 different proteins produced by hepatocytes,macrophages and intestinal epithelial cells. Fibroblasts and intestinal epithelial cells make C1, while the liver makes C3, C6, and C9. They are present in the circulation as inactive molecules.

Though some components are resistant to heat, heating serum at 56oC for 30 minutes destroys complement’s activity and thus they are not able to kill the microbes or pathogens present in the blood. Thus complement system are heat labile.  Serum complement levels, especially C3, often drop during infection as complement is activated faster than it is produced.

Several complement proteins are zymogens (proenzymes). Which means they are an inactive substance which is converted into an enzyme when activated by another enzyme. They get activated when infected with pathogen. When activated, they become proteases that cut peptide bonds in other complement proteins to activate them by the process called proteolysis. Proteolysis removes an inhibitory fragment exposing an active site. Complement proteins work in a cascade, where the binding of one protein promotes the binding of the next protein in the cascade.

The compliment components are named in various ways e.g. by numerals (C1- C9), by alphabetical symbols (factor D) and by common trivial names (homologus restriction factor). Complement components named by numerals are numbered in the order in which they were discovered. During activation, some complement components are cleaved into two unequal fragments. The larger part of the molecule is designated “b” while the smaller fragment “a”.

Small fragment,known as Anaphylotoxins, may diffuse away into the micro-environment to produce other effects (they act as chemotactic agents) while in most cases it is the “b” fragment that participates in the cascade reaction sequence or in other words binds to the surface of the cell to be lysed (the fragments of C2 are an exception to this rule: C2a binds to the membrane while C2b is freed into serum or tissue spaces).

Inactivated fragments are indicated by a small “i”. Enzymatically active forms are symbolized by a bar over the letter or number.

Activation of complement results in the production of several biologically active molecules such as MAC, which contribute to nonspecific immunity and inflammation. Complement is not antigen-specific and it is activated immediately in the presence of pathogen, so it is considered part of innate immunity. Since antibody also activates some complement proteins, complement activation is also part of humoral immunity. Their activation proceeds via different pathways in a cascade fashion leading to lysis.

A polymeric membrane attack complex (MAC) is formed following the activation of complement system. MACs breakdown the cell membrane leading to “pore” formation. Movement of ions and fluid into cell through these pores leads to cell lysis. Complement products opsonize the antigen which is then easily phagocytosed. Immune complexes also require complement products for their removal from the body. Also byproducts of complements, cascade induce inflammation which again augments the overall cellular response in the affected area. Beside all these, complements play important role in viral neutralization in several ways.

Complement proteins can be quantified directly by ELISA, and complement activity can be measured by the complement fixation test.