Page 117 - MNU microbiology theoritical 2025
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General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025
2- Activation stage
• C1s to generate an active serine protease that:
A- Cleaves C4 into two fragments C4a and C4b.
➢ C4b fragment binds to adjacent proteins and carbohydrates on the surface of the
antigen.
➢ C4a fragment is released into the microenvironment.
B- Cleaves C2 into two fragments C2a and C2b.
➢ C2a binds to adjacent proteins and carbohydrates on the surface
of the antigen in association with C4b and
➢ C2b is released into the microenvironment.
• The resulting C4bC2a complex is a C3 convertase, which cleaves
C3 into C3a and C3b
➢ C3a: like a chemical attractant serves as a signal to attract other
phagocytes, and causes mast cell degranulation
➢ C3b can also bind directly to cells making them susceptible to
phagocytosis )opsonization).
➢ C3b combines with C4b2a to form C4bC2aC3b complex that is a C5 convertase.
• C5 convertase catalyzes the cleavage of C5 component into C5a and C5b.
➢ C5b fragment binds to the antigen surface.
➢ C5a floats away.
3) Membrane attack stage
• C5b fragment reacts with other complement
components including C6, C7, C8 and C9 to
form a membrane attack complex (MAC).
This structure forms a hole in the cell
membrane and causes cells to lyse
General Microbiology & Immunology (PM 401) theoretical handbook Page 106
