complement System

1. Complement
system
• Roopendra Kumar
• 3rd semester
• Department of microbiology

2. Introduction
The complement system is a part of the immune system that enhances
(complement) the ability of antibodies and phagocytic cells to clear microbes and
damage cells from an organisms promotes inflammation, and attack the pathogens
cell membrane.
Complement began in the 1890s when Jules Bordet at the Institute Pasteur in
Paris.
Paul Ehrlich in Berlin independently carried out similar experiments and coined
the term complement defining it as, the activity of blood serum that completes the
action of antibody.

3. Function of
complement• Lysis of cells,bacteria and viruses.
• Opsonization,which promotes phagocytosis of particular antigens.
• Binding to specific complement receptors on cells of the immune
system, triggering specific cell functions, inflammation, and secretion
of immune regulatory molecules.
• Immune clearance, which removes immune complexes from the
circulation and deposits them in the spleen and liver.

5. Complement
components• Complement components are designated by numerals( C1-C9), by
letter symbols, or by trivial name ( homologous restriction factor).
• Peptide fragments formed by activation of a component are denoted
by smaller letters.
• In most cases,the smaller fragment resulting from cleavage of a
component is designated (a) and the larger fragment(b).
• Ex:-(C3a,C3b)
• Note:- C2 is an exception: C2a is the larger cleavage fragment.
• The larger fragment binds to the target near the site of activation,
and the smaller fragments diffuse from the site and can initiate
localised inflammatory responses by binding to specific receptors.

6. Complement activation
• Antigen-Antibody Reaction(Classical pathway).
• Bacterial Endotoxin( Alternative pathway).
• Lectin pathway( Manose binding lectin).

7. Classical pathway
The classical pathway of
complement system is considered
part of adaptive immune response
since it begins with the formation of
antigen-antibody complexes.They
may be formed when an antibody
binds to antigenic determinants, or
epitopes,situated on viral,fungal,or
bacterial cell membrane.
The initial stage of activation
involves the complement
components C1,C2,C3 and C4
which are present in plasma as

8. Alternative pathway
The alternative pathway of
complement activation is dependent of
antibody-antigen interaction,also
considered to be part of the innate
immune system.
The alternative pathway uses its own
set of C3 and C5 convertase.As we
will see,the alternative pathway C3
convertase is made up of one
molecule of C3b and one molecule
unique to the alternative pathway,Bb.A
second C3b is then added to make the
alternative pathway C5 convertase.

9. Lectin pathway
The lectin binding pathway like the alternative
pathway, does not depend on antibody for its
activation. However,the mechanism is more like
that of the classical pathway,because after
initiation,it proceeds,through the action of C4
and C2,to produce a C5 convertase.
The lectin pathway is activated by the binding of
mannose binding lectin(MBL) to mannose
residues on glycoproteins or carbohydrates on
the surface of microorganisms including certain
Salmonella, Listeria,and Neisseria strains,and
Candida albicans. MBL is an acute phase
protein produced inflammatory response