Page 116 - MNU microbiology theoritical 2025

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

A) Classical Pathway

To initiate the classical pathway, a specific antibody must first bind to the

pathogen to form an antibody-antigen complex.

Steps

1) Recognition stage:

• It is activated by antigen-bound antibody molecules

• This activates the first protein in the complement

cascade, the C1 complex

• The C1 complex is a multipart protein complex, and

each component participates in the full activation of the

overall complex.

• Following recruitment and activation of the C1 complex, the remaining classical

pathway complement proteins are recruited and activated in a cascading sequence

- C1 has 3 subunits (C1q, C1r, C1s) held together by calcium

- C1q recognizes the Fc region of the antibody and binds to it.
- C1r & C1s generate enzymatic activity

- Binding of C1q heads to antibody complexed with antigen causes a conformational

change in the (C1r:C1s), which leads to activation of an autocatalytic enzymatic

activity in C1r; the active form of C1r then cleaves its associated C1s to generate an
active serine protease

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General Microbiology & Immunology (PM 401) theoretical handbook Page 105