The Complement System: A Decisive System in Body Defence.

  

The Complement System: A Decisive System in Body Defence.

This article will discuss the complement system's role in the body's defense mechanisms, including its site of origin and mechanism of action.

Keywords:

cellular immunity|humoral|Mannose-binding lectin (MBL)|Histamine|Body defense system|WBC|Classical Pathway|Lectin Pathway|Properdin or Alternative Pathway

Introduction

The complement system is crucial in the body's defense against invading pathogens and tumor cells.

The complement system is a system of plasma enzymes. The liver synthesizes enzymes of the complement system. It comprises over 30 enzymes circulating in the blood and is responsible for cell killing by humoral and cellular immunity.

These enzymes are inactive and become active when stimulated by an antigen-antibody complex or other pathways.

When in active form, they work in a sequence of cascade reactions to remove pathogens, kill pathogens, initiate and promote inflammation, and activate other immunological cells.

Main types of complements

There are nine named complement enzymes in the complement system, and their names are C1, C2, C3, C4, C5, C6, C7, C8, and C9. Complement C1 has three subunits C1q, C1r, and C1s.

Activating one complement of this system triggers cascade reactions that activate other system complements.

The complement system gets activated by three pathways-

1. Classical Pathway-Antibody antigen complex binds with C1 and activates C1. Activated C1 triggers a sequence of reactions that activates C3.

2. Lectin Pathway:

Mannose-binding lectin (MBL)binds with mannose residues on the surface of the bacterial wall and stimulates the complement system.

3. Properdin or Alternative pathway-

Polysaccharides on invading microbes' bacterial cell walls, tumor cells interact with Properdin and initiate the complement system. Spontaneous hydrolysis of C3 forms active C3 that activates the complement cascade. and C5.

When active, the complement system causes invading microorganisms and tumor cells to lysis.

Functions of the complement system:

1. Opsonization

Complements, especially C3b, coat the surface of pathogens, enabling efficient and prompt phagocytosis by phagocytic cells.

Opsonization is a process of coating the surface of pathogens with complement enzymes.

2. Cell lysis:

Complements C5b, C6, C7, C8, and C9 form a membrane attack complex (MAC) that penetrates the cell membrane and leads to cell death.

3. Inflammation

Active C3 (C3a)and C5(C5a) cause histamine release from granulocytes, mast cells, and platelets. Histamine is a potent vasodilator. Blood vessels dilate under the influence of histamine, increasing capillary permeability, so leucocytes and other cells come to the antigen-antibody complex site, causing inflammation.

4. Enhancement of antibody-dependent cell-mediated cytotoxicity

The complement system enhances antibody-dependent cell-mediated (ADCC) so that immune cells, for example, natural killer cells, destroy target cells.

Mechanisms of lysis are

1. The active complement from C5 to C9 causes perforation in the cell membrane of invading microorganisms and tumor cells. Ions enter the cell and cause its death.

2. Active C3 (C3a) and C5(C5a) release histamine from granulocytes, mast cells, and platelets. Histamine is a potent vasodilator. Blood vessels dilate under the influence of histamine, increasing capillary permeability, so leucocytes and other cells come to the antigen-antibody complex site.

3. Active C3of the system performs two functions-

It causes opsonization and phagocytosis of bacteria.

It activates other complement enzymes.

4. Active C5, C6, and C7 attracts WBCs to antigen-antibody reaction site.

Applied:

Deficiencies in the complement system increase susceptibility to infections.

Uncontrolled function and inappropriate activation of the complement system can cause autoimmune diseases, chronic inflammation, and tissue damage.

Take home:

The complement system is critical for host defense by opsonizing pathogens and other mechanisms. Understanding its mechanisms of function provides insight into the development of diseases.