soluble mediators and the complement system - block b

Question 1

soluble blood components

Answer 1

complement

antimicrobial proteins

lactoferrin - binds iron

bactericidal/permeability - increasing protein

defensins (a-defensins and b-defensins) - disrupt membranes

often present in an inactive form and only activated when an immune
response is taking place

Question 2

Defensins

Answer 2

Family of human antimicrobial peptides

Divided into 2 classes: a-defensins and b-defensins

Amphipathic: surface has both hydrophobic and hydrophilic regions

Constitutively secreted at mucosal surfaces = protection

a-defensins – secreted by Paneth cells (small intestine)

b-defensins – produced by broad range of epithelial cells

Defensins are also produced by neutrophils

Question 3

Cathelicidins

Answer 3

Natural antimicrobial peptides

Constitutively produced in neutrophils and macrophages

Produced in response to infection by keratinocytes and
epithelial cells in the lungs and intestine

Synthesized as precursor proteins – proteolytically
cleaved into two peptides

In neutrophils – propeptides are stored in secondary
granules. Cleaved by neutrophil elastase from primary
granules following fusion with phagosomes.

Only one cathelicidin gene identified in humans –
(CAMP – cathelicidin antimicrobial peptide).

C-terminal fragment (LL-37) can bind and neutralise LPS (lipopolysaccharides)

Question 4

Lysozome

Answer 4

Proteolytic enzyme contained inside of
lysosomes (membrane-bound organelle)

Lysozyme degrades the peptidoglycan layer
of Gram-positive bacteria

Bacteria burst open under their own internal
pressure

Lysozyme is inactive against most Gramnegative bacteria - cannot penetrate the outer membrane to reach peptidoglycan

Question 5

The complement system

Answer 5

Identified in 1895 by Jules Bordet, as heat sensitive components of the blood
that possessed non-specific antimicrobial activity

Complex biochemical cascade - important component of both the innate and
adaptive immune response

Integrated system of approximately 30 soluble fluid phase and membrane
proteins

The plasma components are part of proteolytic cascades and function to protect
the host from microbes, remove debris and promote cell survival

Proteases (enzymes that break down proteins) that act in a controlled manner

Question 6

Primary roles

Answer 6

Induces acute inflammation

Opsonises bacteria for
phagocytosis

Bactericidal by inducing osmotic
lysis (MAC) (not in all bacteria)

Facilitates removal of
dead/apoptotic cells

Question 7

Functional proteins

Answer 7

“C” associated with
Classical and MBL
pathways

Complement proteins are
cleaved into one large
(usually “b”) and one
small (usually “a”) peptide.

Components of the
“alternative pathway” are
capital B and D etc.

Question 8

Lectin, Classical, and alternative pathways diagram

Question 9

Classical pathway

Answer 9

C1q recognizes; gram-negative bacteria, some viruses, damaged cells
(mitochondria/chromatin products) and altered proteins or antibodies (IgM & IgG) bound
to antigens

Induces the formation of the classical
pathway C3 convertase (C4b2b)

C1q associates with two proteases
(C1r & C1s)

Binding leads to C1r and C1s
activation

C1r and C1s then cleave C2/4
producing C3 convertase

This triggers the rest of the cascade

Question 10

Lectin pathway

Answer 10

MBL/Lectin pathway is homologous to the classical pathway

A soluble PRRs, Mannose-binding lectin (MBL) or Ficolins, binds to specific sugars that can
be found on many bacteria, fungi and some viruses

MBL secreted by hepatocytes

MBL is similar to C1q (collectins)

This activates MBL-associated
serine proteases (MASPs 1/2) (Like
C1r C1s), which then cleaves
C4/C2 to form C3 Convertase

Question 11

Alternative pathway

Answer 11

The alternative pathway uses
proteins that are structurally and
functionally homologous to those
of the classical pathway

Can be initiated when a
spontaneously activated
complement component binds to
the surface of a pathogen

Can ultimately act as an
amplification loop for all three
complement pathways

Despite its name, the alternative
pathway might account for up to
80–90% of total complement
activation

Question 12

Membrane Attack Complex (MAC)

Answer 12

Assembly of the terminal components
of complement forms a membrane attack complex resulting in a pore in
the lipid bilayer membrane that
destroys membrane integrity.

Cleavage of C5 by a C5 convertase to
release C5b

C5b binds C6, and the C5b,6 complex
then binds C7

Conformational change: exposure of a
hydrophobic site
on C7, which inserts into the lipid bilayer

C8 and C9 bind to the complex, insert into
the lipid bilayer

Leads to polymerization of 10 to 16
molecules of C9 into a pore-forming structure:
Membrane-Attack Complex

Question 13

Functions of other complement components

Answer 13

C3a and C5a (“fly away”) = Anaphylatoxins
small peptides with phlogogenic activity. (phlogogenic = producing inflammation

C3a induce mast cell degranulation and stimulates extravasation of leukocytes from
the blood stream

C5a can activate mast cells and neutrophils and can act as a potent chemoattractant

C5b = Strong inducer of the MAC

C3b/C4b - can also to immune complexes (aggregates of antibodies bound to antigen)
and lead to their clearance

C3b = Opsonin

“tag” – binds to foreign microorganisms or cells making them more susceptible to
phagocytosis

Question 14

regulation table