Biological functions of complement

July 24, 2018

Complement shows various biological functions which are listed here below:

1. Immune adherence and opsonization

This involves the covalent binding of complement proteins to the surface of microbial membranes.
This opsonization or coating by complement proteins promotes adherence of the opsonized microbial components to the cell membranes of phagocytic cells.
Phagocytic cells (macrophages, neutrophils) have receptors for C3b.
The complement protein C3b is a potent opsonin.

2. Chemotaxis

Fragments C5a and C5b67 complex are chemotactic factors.
They attract leucocytes at the site of infection or at some distance away, with the result of raising the level of functioning of the leucocytes in immune defense.
C3a is a potent leucocyte chemo attractant and also an activator of the respiratory burst.

3. Anaphylatoxin effect

Fragments C3a, C4a and C5a can produce anaphylatoxic reaction by degranulation of mast cells with the release of histamine and other mediators.
They cause increased vascular permeability and smooth muscle contraction.

4. Lysis of the target cell membrane (Cytolysis)

This involves a collection of complement proteins associating upon the surface of a microbial membrane to form a membrane attack complex (MAC).
This leads to the formation of membrane pores and ultimately, microbial cell lysis.

5. Hypersensitivity reactions

Complement participates in Type II hypersensitivity reaction (e.g. incompatible blood transfusion) and in Type III hypersensitivity reactions (e.g. Arthus reaction and serum sickness).

6. Endotoxic shock

Endotoxins can efficiently activate the alternative pathway of the complement cascade.
Excessive C3 activation leads to tissue damage by disseminated intravascular coagulation (DIC).
Schwartz-mann reaction produced in rabbits by intravenous injection of endotoxin is a good model of excessive C3 activation.
In endotoxic shock with Gram negative septicemia or dengue hemorrhagic fever, endotoxin becomes coated with C3b and sticks to platelets by immune adherence.
Complex of C567 generated in the cascade reaction causes lysis of platelets with release of clotting factors.
In general, complement as a whole plays a major role as an effector mechanism for the host to eliminate foreign substances, including infectious disease agents with the help of antibodies.
This is largely accomplished by the released products such as C3a, C3b and C5a.