Regulation of the Properdin Pathway Positive surtace Regulation Negative regulation R开开开R开 99W9 C3a C3 Factor1 Factar Mg2+Blocks access to C3t -Factor H dissociation of C3bBb Factor D-一 Ba roperdin Stabilies C3+b C33 Activation C5 and promotes formation of MAC Alternative or Properdin Pathway The alternative or properdin pathway can be initiated with the complete absence of antibodies.This pathway can be activated by nonimmune activators for example repeating polysaccharide unitsor the LPS found on the cel was of some microbes This alternative pathway differs from the classical pathway in that it has substitutes for the early acting Cl,C4,and C2 components.Therefore,it can directly initiate the reactions of the late-acting C3-C9 components.It also differs in that it provides an immediate line of defense that does not require immunological memory. This pathway is characterized by a two-phase system in which six proteins participate Initiation is the first phase,particle-bound C3b acts like the Cl recognition complex The second phase is called amplification,driven by a positive feedback loop involving bound C3b,Factor B,and Factor D. Initiation Phase.C3 is a two-chain molecule,which upon cleavage,a highly reactive it might bind.If the acceptor surfaces are chemically suitable,interaction withAlternative or Properdin Pathway The alternative or properdin pathway can be initiated with the complete absence of antibodies. This pathway can be activated by nonimmune activators, for example, repeating polysaccharide units or the LPS found on the cell walls of some microbes. This alternative pathway differs from the classical pathway in that it has substitutes for the early acting C1, C4, and C2 components. Therefore, it can directly initiate the reactions of the late-acting C3-C9 components. It also differs in that it provides an immediate line of defense that does not require immunological memory. This pathway is characterized by a two-phase system in which six proteins participate. Initiation is the first phase, particle-bound C3b acts like the C1 recognition complex. The second phase is called amplification, driven by a positive feedback loop involving bound C3b, Factor B, and Factor D. Initiation Phase. C3 is a two-chain molecule, which upon cleavage, a highly reactive thiol-ester group is exposed on the C3b fragment. This reactive group could be attacked by water, by the surface of an organism, or by foreign substances to which it might bind. If the acceptor surfaces are chemically suitable, interaction with