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.
- 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.
Biological functions of complement