Editing Humoral immunity (section)

==Complement system==
{{main|Complement system}}
The complement system is a [[biochemical cascade]] of the [[innate immune system]] that helps clear pathogens from an organism. It is derived from many small blood plasma proteins that work together to disrupt the target cell's [[plasma membrane]] leading to [[cytolysis]] of the cell. The complement system consists of more than 35 soluble and cell-bound proteins, 12 of which are directly involved in the complement pathways.<ref name=Janeway5/> The complement system is involved in the activities of both innate immunity and acquired immunity.

Activation of this system leads to cytolysis, [[chemotaxis]], [[opsonization]], immune clearance, and [[inflammation]], as well as the marking of pathogens for phagocytosis. The proteins account for 5% of the [[blood serum|serum]] [[globulin]] fraction. Most of these proteins circulate as [[zymogen]]s, which are inactive until [[proteolytic cleavage]].<ref name="Janeway5" />

Three biochemical pathways activate the complement system: the [[classical complement pathway]], the [[alternate complement pathway]], and the [[mannose-binding lectin pathway]].<ref>{{cite journal | vauthors = Carroll MC | title = Complement and humoral immunity | journal = Vaccine | volume = 26 | issue = 8 | pages = I28–I33 | date = December 2008 | pmid = 19388161 | pmc = 4018718 | doi = 10.1016/j.vaccine.2008.11.022 }}</ref> These processes differ only in the process of activating [[C3-convertase|C3 convertase]],<ref>{{Cite book |title=Immunobiology: The Immune System in Health and Disease |vauthors=Janeway Jr CA, Travers P, Walport M, Shlomchik MJ |date=November 21, 2001 |publisher=Garland Science |edition=5th |location=New York |chapter=The complement system and innate immunity |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK27100/ |via=www.ncbi.nlm.nih.gov}}</ref> which is the initial step of complement activation, and the subsequent process are eventually the same. 

The classical pathway is initiated through exposure to free-floating antigen-bound antibodies. This leads to enzymatic cleavage of smaller complement subunits which synthesize to form the C3 convertase.
[[File:B_cell_activation.svg|thumb|461x461px|[[B cell]] activation is a large part of the humoral immune response.]]
This differs from the mannose-binding lectin pathway, which is initiated by bacterial carbohydrate motifs, such as mannose, found on the surface of bacterium. After the binding process, the same subunit cleavage and synthesis occurs as in the classical pathway. The alternate complement pathway completely diverges from the previous pathways, as this pathway spontaneously initiates in the presence of hydrolyzed C3, which then recruits other subunits which can be cleaved to form C3 convertase.  In all three pathways, once C3 convertase is synthesized, complements are cleaved into subunits which either form a structure called the membrane attack complex (MAC) on the bacterial cell wall to destroy the bacteria <ref>{{Cite journal |last1=Mathern |first1=Douglas R. |last2=Heeger |first2=Peter S. |date=September 2015 |title=Molecules Great and Small: The Complement System |journal=Clinical Journal of the American Society of Nephrology |language=en |volume=10 |issue=9 |pages=1636–1650 |doi=10.2215/CJN.06230614 |issn=1555-9041 |pmc=4559511 |pmid=25568220}}</ref> or act as cytokines and chemokines, amplifying the immune response.