Innate immune system - Complement system

Question 1

Which cells produce the complement system?

Answer 1

Produced by a variety of cells including:
 hepatocytes,
 macrophages
 gut epithelial cells

Are activated under particular conditions to generate products that
mediate various effector functions of complement

Question 2

What is the complement system?

Answer 2

• Heat-labile component of plasma.
• Augments the opsonization of bacteria by antibodies.
• ‘Complements’ the antibacterial activity of the antibody.
• The complement system is an integral part of the innate immune response

Serum and cell surface proteins that interact with one another and
with other molecules of the immune system in a highly regulated
manner to generate products that function to eliminate microbes.

Question 3

What is the end result of this complement activation or complement
fixation cascade?

Answer 3

 Stimulation of phagocytes to clear foreign and damaged material
 Inflammation to attract additional phagocytes
 Activation of the cell-killing membrane attack complex.

Question 4

What are the proteins that usually get involved?

Answer 4

1. Factor B
2. Factor D
3. C1 to 9

These proteins catalyze a series of enzymatic reactions that form the final
products of the complement system.

Question 5

What are the pathways of complement activation?

Answer 5

1. Classical pathway
2. Lectin pathway
3. Alternate pathway

• *Classical is antibody-dependent
• Lectin and Alternate are antibody independent

All the pathways lead to activation of C3 and generation of C5 convertase. which activates C5. Leading to the LYTIC ATTACK PATHWAY.

Question 6

Describe the CLASSICAL PATHWAY

Answer 6

The classical pathway begins when antibody binds to a cell
surface and ends with lysis of the cell.
• The proteins are designated C1 - C9.
• C1 is followed in succession by C4,C2,C3 and C5, with
numerical sanity restored from C6 through C9

Question 7

Describe the LECTIN PATHWAY

Answer 7

The lectin pathway is triggered by mannose-binding lectin
(MBL), which recognizes terminal mannose residues on
microbial glycoproteins and glycolipids

Question 8

Describe the ALTERNATE PATHWAY

Answer 8

The alternative pathway can be initiated when a
spontaneously activated complement component binds to the
surface of a pathogen.
• Some proteins in this pathway, are called factors, followed by
a letter eg. factor B.

Question 9

Several complement proteins are cleaved during activation of
the system, and the fragments are designated with lowercase
suffixes. e.g?

Answer 9

C3 is cleaved to be C3a and C3b

Question 10

C3b is responsible for what?

Answer 10

It is an opsin and is responsible for Opsonization. leads to increased phagocytosis.

Question 11

C3a is responsible for what?

Answer 11

It is a chemoattractant. It is responsible for chemotaxis and inflammation

Question 12

In detail explain the CLASSICAL PATHWAY

Answer 12

• Formation of antigen-antibody complex (immune complex).
• The antibody (IgM/IgG) binds to an antigen.
• Conformational changes in the Fc portion of the antibody which exposes a binding site for C1 protein.

• Activated C1qrs –> cleaves C4 into C4a
and C4b.
• C4a is released into the microenvironment.
• C4b attaches to the target surface near
C1q.
• Activated C1qrs also cleaves C2 into C2a and C2b.
• C2a binds to the membrane in association with C4b
• C2b is released into the microenvironment.

NOW WE HAVE A COMPLEX OF C2a AND C4b = Which is C3 convertase.

And then C3 convertase cleaves C3 into C3a AND C3b

(C3a goes int iccroenvironment?)

BUT we end up with C4bC2aC3b = which is the C5 CONVERTASE

Question 13

In detail explain the MANNOSE BINDING LECTIN PATHWAY

Answer 13

Similar to the classical pathway.
• Circulating mannose-binding lectin (MBL) binds to bacterial surfaces with
mannose containing polysaccharides.

Mannose Binding Lectin (MBL) binds to CHO (carbohydrate) residues
• MASP-1 and MASP-2 bind to MBL.
• MASP : MBL-associated serine proteases.

The resulting C4bC2a complex is a C3 convertase, which
cleaves C3 into C3a and C3b.
• C3b binds to the membrane in association with C4b and C2a.
• The resulting C4bC2aC3b is a C5 convertase.
• The biological activities and the regulatory proteins of the
lectin pathway are the same as those of the classical pathway.

Question 14

What are the Microorganisms inducing MBLectin pathway?

Answer 14

 bacteria- Salmonella, Listeria &
Neisseria strains
 fungi
 viruses including HIV-1

Question 15

In detail explain the ALTERNATIVE PATHWAY

Answer 15

In serum there is low level spontaneous hydrolysis of C3.
• Serum C3 contains an unstable thioester bond that undergoes slow
spontaneous hydrolysis to yield C3a and C3b.

• C3b binds the surface of foreign cell and then binds factor B, which becomes susceptible to factor D.
• Factor D –> bound factor B into Ba and Bb forming C3bBb (stabilized by another
properdin ).

• C3bBb, a C3 convertase, will continue to generate more C3b.
• Some of the C3b generated by the C3 convertase associates with the C3bBh
complex to from a C3bBbC3b complex.
• This is the C5 convertase of the alternative pathway

Question 16

Now, describe the MEMBRANE ATTACK PATHWAY

Answer 16

C5 convertase from the classical (C4b2a3b), lectin (C4b2a3b) or alternative (C3bBb3b) pathway –> activation of the late components of the
complement system –> **cytocidal membrane attack ** complex.
• C5 convertase cleaves C5 –> C5a & C5b

• C5b + C6 & C7 and inserts into the membrane.
• The C5b6C7 complex = IS THE membrane attack complex (MAC).

• Subsequently C8 binds, followed by several molecules of C9.
• The C9 molecules form a pore in the membrane through which the cellular
contents leak and lysis occurs.
• The cell cannot maintain its osmotic stability and the lysis occurs by an
influx of water and loss of electrolytes
• More effective in GNB than in GPB as MAC formation is easier in the outer
membrane in Gram negatives whereas it is difficult in the rigid thick layer
of peptidoglycan in Gram positives.

Question 17

The —— form a pore in the membrane through which the cellular
contents leak and lysis occurs.

Answer 17

The C9 molecules form a pore in the membrane through which the cellular
contents leak and lysis occurs.

Question 18

Why is complement system more effective in Gram negative than in gram positive bacteria?

Answer 18

More effective in GNB than in GPB as MAC formation is easier in the outer
membrane in Gram negatives whereas it is difficult in the rigid thick layer
of peptidoglycan in Gram positives.

Question 19

C5 convertase from the classical is called the?

Answer 19

(C4b2a3b)

Question 20

C5 convertase from the lectin pathway is called?

Answer 20

(C4b2a3b)

Question 21

C5 convertase from the alternative pathway is called?

Answer 21

(C3bBb3b)

Question 22

What are essentially the functions of the complement system?

Answer 22

1. Opsonization (C3b)
2. Chemotaxis (C5b6C7)
3. Anaphylatoxins (bronchospasms) (C3a)
4. Recruitment and activation of neutrophilia (C5a)
5. Increased vascular permeability (C2a, C5a)
6. Cell lysis (MAC)
7. Mast cell degranulation (C3a)

Question 23

1. Opsonization and phagocytosis

Answer 23

• C3b, bound to immune complex or coated on the surface of

pathogen, activates phagocytic cells

Question 24

2. Cell lysis

Answer 24

• Membrane attack complex formed by C5b6789 components

ruptures the microbial cell surface which kills the cell.

Question 25

3. Chemotaxis

Answer 25

• Complement fragments attract neutrophils and macrophages
to the area where the antigen is present.
• These cell surfaces have receptors for complements, like C5a, C3a.

Question 26

4. Activation of mast cells and basophils and enhancement of inflammation

Answer 26

• C5a, C4a, and C3a induce acute inflammation by activating mast cells and
neutrophils.
• Bind to mast cells –>degranulation –> release of vasoactive mediators such
as histamine.
• Anaphylatoxins- mast cell reactions they trigger are characteristic of
anaphylaxis.
• Binding to specific complement receptors –> trigger : specific cell
functions, inflammation, and secretion of immunoregulatory molecules

Question 27

5. Production of antibodies

Answer 27

• B cells have receptor for C3b.
• When C3b binds to B-cell, it secretes more antibodies.
• Thus C3b is also an antibody producing amplifiers which
converts it into an effective defense mechanism to destroy
invading microorganism.

Question 28

6. Immune clearance

Answer 28

• The complement system removes immune complexes from
the circulation and deposits them in the spleen and liver.
• Thus it acts as anti-inflammatory function.
• Complement proteins promote the solubilization of these
complexes and their clearance by phagocytes

Question 29

REGULATION OF COMPLEMENT

Answer 29

• Potential to be extremely damaging to host tissues
• Regulatory mechanisms are required to restrict the
complement pathway.
• Various plasma and cell membrane proteins regulate
complement activation by inhibiting different steps in the
cascade.
• The membrane of most mammalian cells has a high level of
sialic acid, which contributes to the inactivation of
complements.

Question 30

During regulation of the complement system, C1 inhibitor binds to active C1r: C1s causing it to dissociate from?

Answer 30

C1q, limiting the ability of C1s

Question 31

Clinical indications for possible complement deficiencies include:

Answer 31

 recurrent mild or serious bacterial infections
 autoimmune disease
 renal disease
 vasculitis
 age-related macular degeneration.

Question 32

Complement-related disease

Answer 32

1. Diseases associated with complements can be due to the deficiencies in any of the protein components or in regulatory components.
2. A family history increase the suspicion of an inherited complement deficiency
   • Autosomal co-dominant conditions