The lectin pathway is part of the complement system, which is part of the innate immune system. It acts as the first line of defense against microbes entering the body. In the lectin pathway, MBL attaches to mannose on microbes. This activates the complement system by breaking down C4 and C2 proteins to form a C3 convertase, which then breaks down C3 into C3b and C3a. C3b opsonizes the microbe while C3a causes inflammation. C3b then binds to the microbe and C3 convertase to form a C5 convertase, continuing the complement cascade and ultimately forming a membrane attack complex that causes cell lysis of the micro

1. How Complement Works
The Lectin Pathway

2. Stage 1
• Microbes enter the
body through wounds,
respiratory means,
digestion, etc.
Microbes
Person

3. Stage 2
• Microbes that have
entered the body
stimulate the innate
immune system (in all
age groups)
• This acts as the first
line of defence
Microbes
C3
Macrophage
Natural Killer
Cells
Neutrophils
MBL
C3b
C3a

4. Stage 3
• Complement is
activated and
comprises of two
pathways in the
innate immune
system
– The alternative
pathway
– The lectin pathway
• The classical pathway
is part of the adaptive
immune system
Microbes
C3
MBL
C3b
C3a
C3
C3b
C3a
C3
C3b
C3a

5. • In the lectin
pathways, the MBL
attaches to the
mannose on the
microbe
MBL
Stage 4
Microbe
Mannose

6. Stage 5
a) The MASP2 on the
MBL breaks down
the C4 protein into
C4b and C4a. C4a
leads to
inflammation and
cell recruitment
b) The protein C2 is
also broken down
into its ‘b’ (major)
and ‘a’ (minor)
components
C4
C4b
C4a
a)
b) C2
C2a
C2b

7. Stage 6
• The ‘b’ components of
the C4 and C2
proteins bind together
to the microbe to form
a C3 convertase
MBL
Microbe
C4b2b
C3 convertase

8. Stage 7
• The protein C3 is broken
into its ‘b’ and ‘a’
components by the C3
convertase
• The C3a leads to
inflammation and cell
recruitment
• The C3b leads to
opsonisation and removal
of immune complexes
• It is thought that this
process happens
spontaneously and thus
the C3 convertase would
catalyse the breakdown
C3
C3b
C3a

9. Stage 8
• The C3b protein binds
to the microbe and
the C3 convertase
molecule to make a
C5 convertase
Microbe
C3b
C3b C4b2b3b
C5 convertase

10. Stage 9
• C5 is broken down
into C5b and C5a
• The C5a protein leads
to inflammation and
cell recruitment
C5
C5b
C5a

11. Stage 10
• C6 binds to C5b and
accepts C7, C7binds
to the lipid membrane
and C8 then initiates
binding and
polymerization of C9
• C6, C5b, C7 and C8
are all components of
MAC (membrane
attack complex)
C5b
C7
C6
C8
Plasma Membrane

12. Stage 11
• C9 is another
component of MAC
• It binds to C5b, C6,
C7 and C8, and
polymerizes to form
membrane pores
• MAC leads to cell
lysis and death of the
microbe through
osmotic lysis
C5b
C7
C6
C8
Plasma Membrane
C9
MAC