The complement system consists of a series of heat-labile serum proteins that are activated in turn through cascade reactions, usually initiated by a trigger. Complement activation results in the disruption of cell membranes and destruction of cells or microorganisms. The name "complement" was derived from experiments by Jules Bordet showing that heated serum was still capable of agglutinating bacteria through heat-labile components. There are over 30 distinct serum and membrane proteins that make up the complement system, synthesized mainly by liver cells. Complement facilitates antigen clearance and inflammatory responses through complement-coated pathogen lysis and chemotaxis.

1. COMPLEMENT SYSTEM NOTES
Department of MLT, SMAS, Galgotias University.
A. Vamsi Kumar. Asst Prof
Definition of Complement System:
The complement system consists of a series of heat-labile serum proteins that are activated in turn. The
complements exist as soluble inactive precursors which once activated, a complement component may then
act as an enzyme. Enzymatic chain reactions of this type are known as cascade reactions and usually require
a “trigger” to initiate the reaction chain.
Complement is a chain of enzymes whose activation eventually results in the disruption of cell membranes
and the destruction of cells or invading microorganisms. Complement is an essential part of the body
defense system (Fig. 7.1).
History of Complement System:
The name “complement system” is derived from experiments performed by Jules Bordet.
Experiments:

2. Conclusion:
1. Antibody (Ab) activity did not decay. Therefore Abs are heat stable.
2. Heated serum is capable of agglutinating bacteria.
3. Therefore serum with some heat labile components are capable of agglutinating bacteria. These
components—are called as complement by J. Bordet.
Biological Functions of Complement System:
Complements perform different biological functions like:
1. Cytolysis:
2. Opsonization:
3. Activation of inflammation:

3. 4. Solubilization and phagocytic clearance and immune complex:
Mechanism of actions are as follows:
Components of Complement System:
The complement system is made up of a number (mostly 30) of distinct serum (blood plasma) and
membrane proteins which mostly assist the humoral branch of the immune system. As after initial
activation, the various complement components interact sequentially to generate reaction products that
facilitate antigen clearance and inflammatory response.
Different pathways of complement finally generate a macro-molecular membrane- attack complex (MAC)
which helps to lyse a variety of cells, bacteria and viruses.

4. The complement products amplify the initial antigen-antibody reaction and convert that reaction into a more
effective defense mechanism. Continuous proteolytic cleavage and activation of successive complement
proteins lead to the covalent bonding or fixing of complement fragments to the pathogen surface. Each
precursor of complement is cleaved into two major fragments- named as larger fragment (designated as ‘b’)
and smaller fragment (designated as ‘a’).
The major or larger ‘b’ fragment has two biologically active sites—one binds to cell membranes to the target
cell towards the site of activation and the other for enzymatic cleavage of the next complement component.
The smaller ‘a’ fragments diffuse from the site and play a role in initiating a localized inflammatory
response (chemotactic activity).
1. The proteins and glycoproteins composing the complement system are synthesized largely by liver
hepatocytes, some by blood monocytes, tissue macrophages and epithelial cells of the gastro-intestinal and
genitourinary tracts.
2. The proteins that form the complement system are labelled numerically with the prefix C (e.g., C1 –C9).
3. Some complement components are designated by letter symbols (e.g., factor B, D, P) or by trivial names
(e.g., homologous factor).
4. There are at least 19 of these components; they are all serum proteins and together they make up about
10% globulin fraction of serum.
5. The molecular weights of the complement components vary between 24 kDa for factor D and 460 kDa for
C19.
6. Serum concentration in humans varies between 20 μg/ml of C2 and 1300 μg/ml of C3.
7. Complement components are synthesized at various sites like C2, C3, C4, C5; B, D, P and I are from
macrophages, C3, C6, C8 and B from liver (Table 7.1 and 7.2).
Reference: http://www.biologydiscussion.com/immunology/complement-system/complement-system-definition-functions-and-components/61928