Classical complement pathway

The cleaved products attract phagocytes to the site of infection and tags target cells for elimination by phagocytosis.

In addition, the C5 convertase initiates the terminal phase of the complement system, leading to the assembly of the membrane attack complex (MAC).

[1][4] The binding of C1q with pathogen surface or antigen-antibody immune complex leads to conformational changes and the activation of the serine protease C1r.

C3b is very similar to C4 in both structure and function also has a thioester bond that forces it to attach to surface nucleophile of the activator(namely the pathogen or IC).

[7] Because of its role in the innate immune system classical complement has been implicated in a number of pathogen related disorders.

[8] Obesity in turn results in an abnormally high level of complement activation via production of the C1 component of the classical pathway, which can lead to tissue inflammation and eventually insulin resistance, however the exact mechanisms that causes this is yet unknown.

[8] Immunotherapies have been developed to detect and destroy cells infected by the HIV virus via classical complement activation.

This is important for targeting the virus in its intracellular phase because the antibodies specific to the synthetic peptides can trigger the classical complement pathway and induce the death of HIV infected cells.

Therapies that utilize classical complement activation have been shown to be effective in targeting and killing cancer cells and destroying tumors.

When injected into target tissue encourages recruitment of C1q and activates downstream events, eventually leading to the formation of the C5b-9 complex which damages tumor cells, killing them.

[2][16] Among the many functions of C1q, C1q triggers clearance of immune complexes and apoptotic cells by activating the classical pathway and binding directly onto phagocytes.