The three pathways all generate homologous variants of the protease C3-convertase. The classical complement pathway typically requires antibodies for activation (specific immune response), while the alternative and mannose-binding lectin pathways can be activated by C3 hydrolysis or antigens without the presence of antibodies (non-specific immune response). In all three pathways, a C3-convertase cleaves and activates component C3, creating C3a and C3b and causing a cascade of further cleavage and activation events.  C3b binds to the surface of pathogens leading to greater internalization by phagocytic cells by opsonization. C5a is an important chemotactic protein, helping recruit inflammatory cells.  Both C3a and C5a have anaphylatoxin activity, directly triggering degranulation of mast cells as well as increasing vascular permeability and smooth muscle contraction.  C5b initiates the membrane attack pathway, which results in the membrane attack complex (MAC), consisting of C5b, C6, C7, C8, and polymeric C9. MAC is the cytolytic endproduct of the complement cascade; it forms a transmembrane channel, which causes osmotic lysis of the target cell. Kupffer cells and other macrophage cell types help clear complement-coated pathogens.  As part of the innate immune system, elements of the complement cascade can be found in species earlier than vertebrates; most recently in the protostome horseshoe crab species, putting the origins of the system back further than was previously thought.
The classical pathway is triggered by activation of the C1-complex (which consists of one molecule C1q and two molecules C1r and C1s), either by C1q's binding to antibodies from classes M and G, complexed with antigens, or by its binding C1q to the surface of the pathogen. This binding leads to conformational changes in C1q molecule, which leads to the activation of two C1r (serine protease) molecules. Then they cleave C1s (another serine protease). The C1-complex now binds to and splits C2 and C4, producing C2b (used to be called C2a, but nomenclature has been updated because "a's" are conventionally smaller) and C4b. The inhibition of C1r and C1s is controlled by C1-inhibitor. C4b and C2b bind to form C3-convertase (C4b2b complex: Many older texts, however, use the traditional nomenclature for this, calling it C4b2a). Production of C3-convertase leads to cleavage of C3 into C3a and C3b; the latter joins with C2b and C4b (the C3 convertase) to make C5 convertase.
The alternative pathway is triggered by C3 hydrolysis directly on the surface of a pathogen. It does not rely on a pathogen-binding protein like the other pathways. In the alternative pathway, the protein C3 is produced in the liver, and is then cleaved into C3a and C3b by enzymes in the blood. If there is no pathogen in the blood, the C3a and C3b protein fragments will be deactivated. However, if there is a nearby pathogen, some of the C3b is bound to the plasma membrane of the pathogen. Then, it will bind to factor B. This complex will then be cleaved by factor D into Ba and the alternative pathway C3-convertase, Bb.
The C3bBb complex, which is "hooked" onto the surface of the pathogen, will then act like a "chain saw", catalyzing the hydrolysis of C3 in the blood into C3a and C3b, which positively effects the number of C3bBb hooked onto a pathogen.
After hydrolysis of C3, C3b complexes to become C3bBbC3b, which cleaves C5 into C5a and C5b.  C5a and C3a are known to trigger mast cell degranulation. C5b with C6, C7, C8, and C9 (C5b6789) complex to form the membrane attack complex, also known as MAC, which is inserted into the cell membrane, "punches a hole", and initiates cells lysis.
The lectin pathway is homologous to the classical pathway, but with the opsonin, mannan-binding lectin (MBL) and ficolins, instead of C1q. This pathway is activated by binding mannan-binding lectin to mannose residues on the pathogen surface, which activates the MBL-associated serine proteases, MASP-1, MASP-2, MASP-3, which can then split C4 into C4a and C4b and C2 into C2a and C2b. C4b and C2a then bind together to form C3-convertase, as in the classical pathway. Ficolins are homologous to MBL and function via MASP in a similar way. In invertebrates without an adaptive immune system, ficolins are expanded and their binding specificities diversified to compensate for the lack of pathogen-specific recognition molecules.
Complement system
Adaptive immune system
Complement membrane attack complex
Innate immune system
Endocannabinoid system
Coagulation
Circulatory system
Dynamical system