Untitled - D Ank Unlimited

alternative complement pathway 34 alternative complement pathway
complement activation in which the early components C1,
C2, and C4 are not required. It involves protein properdin
factor D, properdin factor B, and C3b, leading to C3
activation and continuing to C9 in a manner identical to
that which takes place in the activation of complement by
the classical pathway. Substances such as endotoxin, human
immunoglobulin A (IgA), microbial polysaccharides, and
other agents may activate complement by the alternative
pathway. The C3bB complex forms as C3b combines with
factor B. Factor D splits factor B in the complex to yield
the Bb active fragment that remains linked to C3b and Ba,
which is inactive and is split off. C3bBb, the alternative
C2
Activation unit
Classic Pathway
FACTOR I
Trypsin
Antigen-antibody
complex
C1q
Recognition unit
C3a
C4
C4b, 2a
C3
C4a, 2a3b
C3
C3b
C3bi
C5a
C5
Activator
C3b
Mg Factor B
++
C3bB
Properdin (P) Factor D
C3bBb(P)
C3
C5b + C6, C7, C8, C9
Alternate Pathway
C3a
C3b 2 Bb(P)
Initiation convertase
Activation unit
Membrane attack complex
formation (lytic unit)
pathway C3 convertase, splits C3 into C3b and C3a,
thereby producing more C3bBb, which represents a positive
feedback loop. Factor I, when accompanied by factor H,
splits the heavy chain of C3b to yield C3bi, which is unable
to anchor Bb, thereby inhibiting the alternative pathway.
Properdin and C3 nephritic factor stabilize C3bBb. C3 convertase
stabilized by properdin activates complement’s late
components, resulting in opsonization, chemotaxis of leukocytes,
enhanced permeability, and cytolysis. Properdin,
IgA, IgG, lipopolysaccharide, and snake venom can initiate
the alternate pathway. Trypsin-related enzymes can activate
both pathways.
Microbial
polysaccharide
C3bBb
C3bB
FACTOR H
C3c C3d
C3b regulation
Alternative complement pathway.
Properdin
FACTOR D
FACTOR B
Stabilization