The Complement System And Its Biological Consequences.

1. ANUSHI JAIN
ROLL NO : 12
PAPER 2
MSC I
The Complement
System

2. INTRODUCTION
• The complement system is a group of serum
proteins, which participate in innate as well as
adaptive immune response.
• The complement system is characteristically
activated by the antigen-antibody interaction.
• It helps antibacterial cells and molecules of the
immune system.

3. PATHWAYS
 The complement system can be activated by three
ways :
1. CLASSICAL PATHWAY
2. ALTERNATIVE PATHWAY
3. LECTIN PATHWAY

5. COMPONENTSClassicalPathway
• C1 (C1q,C1r,C1s)
• C2 (C2a,C2b)
• C3(C3a,C3b)
• C4(C4a,C4b)
AlternativePathway
• C3 (C3a,C3b)
• Factor B
(B,Ba,BB)
• Factor D
• Properdin
MembraneAttackComplex
• C5 (C5a,C5b)
• C6
• C7
• C8
• C9

6. BIOLOGICAL CONSEQUENCES
Complement system serves as an important mediator of
the humoral response , It plays a major role in host
defense through destruction of invading
micro0organisms.
 The Membrane-Attack Complex (MAC) mediates cell
lysis.
 The split products or the active forms of the
components of the complement participate in
Inflammatory response , Opsonization of antigen ,
viral neutralization and clearance of the immune
complexes.

7. 1. CELL LYSIS
 The membrane-attack complex (MAC) formed by
complement activation can lyse gram-negative bacteria ,
parasites , viruses , erythrocytes , and nucleated cells.
 Gram-positive bacteria are generally resistant to the
complement mediated lysis because of thick peptidoglycan
layer in their cell wall which prevents the insertion MAC
into their inner membrane.
 The complement system lyses these target cells through
the alternative pathway and lectin (MBL) pathway in
absence of antibodies and through the classical pathway in
the presence of antibodies.

8.  MAC formed forms a pore in the cells and disturbs the
osmotic balance of the cell and ultimately destroys it.
 Lysis of nucleated cells require multiple MACs ,
whereas lysis of RBC requires single MAC.
 If MAC is removed in time ,the cells are capable of
repairing the damage and restoring its osmotic
stability.

11. 2. INFLAMMATION
 Inflammation is the most important
consequence of the complement activation.
 C3a , C4a and C5a act as anaphylatoxins , i.e.
, they cause smooth muscles to contract.
 C5a and C3a act as chemoattractants , i.e. ,
they effect the motility of the cells.

12. 3.OPSONIZATION
 It’s a process in which bacteria and other cells
are altered in such a manner that they are
more readily engulfed by the phagocytes.
 C3b is a major opsonin and the receptors for
C3b are CR1 , CR3 and CR4.

14. 4. VIRAL NEUTRALIZATION
 Viral neutralization means neutralization of viral
infectivity.
 There are various ways by which viruses can be neutralized
:
A. Formation of larger viral aggregates
 Reduces the net number of infectious viral particles.
B. Coating of a antibody and/or complement (Cb3) to
the surface of the viral particle
 Blocking attachment to host cells
 Binding of binding particle to cells possessing CR1
receptor or Fc.
 Lysing most enveloped viruses.

16. 5. CLEARANCE OF IMMUNE COMPLEXES
 Erythrocytes plays an important role in clearance of the
immune complexes.
 The coating of the soluble immune complexes with C3b
facilitates their binding to the CR1.
 This interaction between CR1 and C3b results in adherence
of immune complexes to the erythrocytes.
 Erythrocytes carry these immune complexes to the spleen
and liver ; wherein these immune complexes are removed
from erythrocytes and phagocytosed.
 The importance of clearing immune complexes can be
seen in patients with autoimmune disease Systemic
Lupus Erythematosus.

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