Final answer:
The complement system consists of three main types of complement activation pathways: the classical, alternative, and lectin pathways. Each pathway involves different key mediators and results in the formation of the membrane-attack complex (MAC). The classical pathway is antibody-dependent, while the alternative and lectin pathways are antibody-independent.
Step-by-step explanation:
The complement system is a part of the immune system that helps to defend against pathogens. It consists of a series of proteins that become activated in the presence of microbes and can opsonize pathogens, induce inflammation, and result in pathogen lysis. There are three main types of complement activation pathways: the classical pathway, the alternative pathway, and the lectin pathway.
The classical pathway is activated when antibodies bind to antigens on the surface of pathogens. The key mediators involved in the classical pathway are the antibodies and complement proteins C1q, C2, and C4. The end result of the classical pathway is the formation of the membrane-attack complex (MAC), which can kill pathogens by disrupting their osmotic balance.
The alternative pathway is activated spontaneously and does not require antibodies. It involves the activation of complement proteins C3 and factors B and D. The key mediators involved in the alternative pathway are complement proteins C3b, C5, C6, C7, C8, and C9. The end result of the alternative pathway is also the formation of the MAC.
The lectin pathway is activated when a pattern-recognition molecule called mannose-binding lectin (MBL) binds to carbohydrates on the surface of pathogens. The key mediators involved in the lectin pathway are MBL, MASP-1, MASP-2, and complement proteins C2 and C4. The end result of the lectin pathway is the same as the classical and alternative pathways, which is the formation of the MAC.