Complement pathway: Three cascades that are easy to follow

Complement pathway is a cascade of biochemical reaction which involve cleavage and attachment of fragments that together result in activation of the membrane attack complex (MAC). The complement system, therefore, consists of three complement pathways each of which requires different protein components. They are:

  • The Classical Pathway
  • Alternative Pathway
  • Lectin Pathway

There is a brief overview of the complement system that outlines the components and key concepts; You can check it out here.

The classical pathway was the first complement pathway to be described and is dependent on the presence of an antibody (IgM and IgG) for it to be functional.  The alternative and the lectin pathways on the other side are antibody independent.

All three pathways result in the activation of C3 and the formation of a C5 convertase which leads to the activation of C5 and the lytic pathway. Each of these pathways is a series of sequential steps that proceed in a cascading manner. Since some of the steps are enzymatic in nature, there is amplification as the pathways proceed.

The three pathways are as illustrated below:

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The Classical Complement Pathway

C1 is a multiunit protein containing 3 different proteins namely protein q, r, and s. it binds to the fc region of the IgG or IgM once the antibodies have interacted with an antigen and not when they are free. If the latter occurs, the compliment will be used up falsely, or it will result in an attack of self. This binding process occurs in the presence of calcium and magnesium ions which is via the C1q and must crosslink at least two antibody molecules of IgG and one molecule of IgM before it is firmly fixed.

The binding of C1q results in activation of C1r which in turn activates C1s and the resulting formation is an activated C1qrs. The activated C1qrs is an enzyme that cleaves C4 into two portions C4a and C4b. C4a then binds to the membrane surface of the antigen. The activated C1qrs also cleaves C2 into C2a and C2b. C2a joins C4a on the membrane surface of the antigen resulting in the C4aC2a complex, known as the C3 convertase which cleaves C3 into C3a and C3b. C3b joins C4bC2a on the membrane surface to form the C4bC2aC3b complex which is the C5 convertase.


Biological activities of the classical pathway

Several products of this process have activities that contribute to host defenses while some may have detrimental effects if produced in an unregulated manner. These products include:

  • C2a- a prokinin cleaved by plasmin which results in edema (swelling)
  • C3a- an anaphylatoxin which can activate basophils and mast cells resulting in allergy or hypersensitivity
  • C3b- an opsonin which enhances phagocytosis
  • C4a- Weak anaphylatoxin than C3a
  • C4b- an opsonin.

If the process, if not regulated, will result in the continued production of C2a, C3b, and C2b. it is therefore kept in check by:

  • C1 inhibitor which dissociates the Cq, Cr, and Cs, therefore, inactivating it
  • C3a inactivator which inactivates C3a
  • Factor H which facilitates the degradation of C3b by factor 1
  • C3 inactivator which inactivates C4a while the C4b is controlled by the C4 binding protein and factor 1
  • C4 binding protein facilitates the degradation of C4b by factor 1 and also preventing the association of C2a and C4b, therefore, blocking the formation of C3 convertase

Lectin Pathway

This pathway is like the classical pathway, it is initiated by the binding of Mannose Binding Lectin to the bacterial surface which results in the association of the two serine proteases MASP1 and MASP2 that are similar to C1r and C1s respectively while MBL is similar to C1q

The formation of MASP1, MASP2 and MBL complex result in activation of more MASPs and subsequent cleavage of C4 to C4a and C4b with the C4b binding to the surface of the foreign antigen that has mannose. The activated MASP 1and MASP 2 also cleaves C2 into C2a and C2b where C2a joins C4b to form a complex that cleaves C3 into C3a and C3b. the C3b joins the C4bC2a complex to form the C5 convertase ( C4bC2aC3b)

Alternative Complement Pathway

In serum, there is spontaneous low-level hydrolysis of C3 into C3a and C3b. if it happens in the absence of an antigen, the C3b is rapidly deactivated by, sialic acid on the surface of most cells, factor H and I thus making sure that the reaction does not continue. When bacteria and other foreign substances are present in the system, the generated C3b binds to factor B resulting in C3bB. When this happens, factor B becomes susceptible to cleavage by factor D to produce Ba and Bb. The latter then joins with C3b to form the C3bBb complex which is a C3 convertase. In the process, more C3 are therefore cleaved. Some of the C3b binds to the C3bBb to for the C5 convertase (C3bBbC3b complex). 

The alternative pathway can be activated by most gram-negative bacteria such as Neisseria and gram-positive bacteria such as Staphylococci, certain viruses, and parasites. The result is their lysis and therefore the alternative pathway of the complement activation provides protection against antigens before the other antigen responses are activated.

Membrane attack pathway

The C5 convertase from the three pathways (C4b2a3b from classical and Lectin pathways, and C3bBbC3b from the alternative pathway) cleaves C5 into C5a and C5b. C5b rapidly associates with C6 and C7 and inserts into the membrane to form the C5b67 complex referred to as the Membrane Attack Complex.

Subsequently, C8 binds followed by several molecules of C9. The C9 molecules form a pore in the membrane through which the cellular contents leak and lysis occur. C5a on the other side is a very strong anaphylatoxin, a neutrophil chemotactic factor, an activator as well as a stimulator for the production of cytokines by the macrophages. Its activity is however regulated by the C3 inactivator.

In some instances, the MAC can dissociate from the membrane and move into the fluid phase resulting in bystander analysis. The damage to bystander cells is prevented by protein S (nitronectin) which binds to the soluble MAC and prevents its binding to other cells.