The complement cascade of proteolytic factors involved in cellular lysis can be initiated by several different factors, including antibody-dependent and antibody-independent recognition of infectious organisms. In the lectin-induced complement cascade, carbohydrates on the surface of microbial cells activate the complement cascade by binding to mannan-binding lectin (also called the mannan-binding protein, Mbl/Mbp). Mbp is an acute phase serum protein whose expression is induced by microbial infection. The binding of Mbl to microbial ligands activates the Mbl associated serine proteases Masp1 and Masp2, triggering the cleavage of C2 and C4 to create C4bC2a, a C3 convertase that cleaves large numbers of C3. Masp1 and Masp2 are similar to the C1 protease in the classical complement pathway. Once formed the C3 convertase cleaves and activates the remaining complement factors leading ultimately to formation of a pore in the bacterial membrane by the membrane attack complex (MAC) that lyses the bacterial cell. The lectin-induced pathway also appears to play an important role of the activation of phagocytotic cells by infection. Although the initiating event activating the complement cascade is distinct in the lectin-induced pathway, from the C3 convertase onward the lectin induced complement pathway is the same as the classical complement pathway. Since antibodies are not required in the lectin-induced pathway, this aspect of the immune response is part of the innate immune response. The importance of this pathway to the immune response has been demonstrated by the identification of children and adults with little or no Mbl who lacked normal phagocytotic responses and are highly susceptible to infection.
