Page 119 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

C) Alternative pathway

• The alternative pathway of the complement system is an innate component

of the immune system's natural defense against infections.

• This pathway is initiated by:

* Bacterial endotoxin, polysaccharide capsule

• Antibodies are not required to initiate activation of this pathway

• The pathway is triggered when the C3b protein directly binds the microbe.
* It starts at C3 then C5, C6, C7, C8, C9

* The complement component. C1, C4, C2 are bypassed
Steps

• Alternative complement pathway is triggered by spontaneous C3 hydrolysis
directly to form C3a and C3b.

• C3b is then capable of covalently binding to a pathogenic membrane surface.

• Upon binding with a cellular membrane, C3b is bound by factor B to form C3bB

• This complex in the presence of factor D will be cleaved into Ba and Bb

• Bb will remain covalently bonded to C3b to form C3bBb which is the alternative

pathway C3-convertase.

• C3-convertase which is "hooked" onto the surface of the pathogen, will catalyze

the hydrolysis of C3 in the blood into C3a and C3b

• C3b combines with C3bBb to form C3bBbC3b complex that is the alternative C5

convertase.

• C5 convertase will cleave C5 into C5a and C5b

• C5b with C6, C7, C8, and C9 (C5b6789) complex to form the membrane attack

complex

General Microbiology & Immunology (PM 401) theoretical handbook Page 108