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Figure 1: The complement system: the classical complement pathway involves a sequentially acting multistep cascade in which the complement components C1q, C1s, C1r, C4, C2, and C3 play important roles. C1r and C1s, the two serine protease proenzymes, along with C1q constitute C1, the first component of the classical pathway [25]. The activation of the C1 complex (C1q + C1s–C1r–C1r–C1s) subsequently activates the complement through the cleavage of C4 and C2 to yield the central molecule C3 convertase that cleaves C3, leading to the activation of the C2–C9 components and thus the formation of the terminal membrane attack complex (MAC) [26]. The MAC binds to cell membranes and facilitates cell lysis. The alternative pathway is initiated by low-level activation of C3 via its hydrolysis (C3b) and activated factor B. The activated C3b binds factor B that is cleaved by factor D to form C3 convertase. The main difference between classical and alternative pathway is that the initiation of alternative pathways is not dependent on the presence of immune complexes. The lectin pathway is activated following the recognition and binding of pathogen-associated molecular patterns (PAMPs) by mannose-binding lectin (MBL) [27]. The binding of MBL to repetitive carbohydrate patterns on pathogen surfaces has the potential to activate the lectin pathway through the MBL-associated serine protease (MASP), designated as MASP-2, that in turn leads to the activation of complement components C4, C2, and C3. The association of MBL-MASP complex of the lectin pathway is analogous to C1 complex of the classical complement pathway.