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	File:Droga_alternatywna.png\|Alternative complement pathway		File:Droga_alternatywna.png\|Alternative complement pathway
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			==Alternative Complement Pathway==

			The '''alternative complement pathway''' is one of the three pathways that activate the [[complement system]], which is a crucial part of the [[immune system]]. Unlike the [[classical complement pathway]], which is activated by [[antigen-antibody complexes]], the alternative pathway can be activated on microbial surfaces in the absence of antibodies. This pathway plays a vital role in the innate immune response by enhancing the ability of antibodies and phagocytic cells to clear pathogens and damaged cells, promote inflammation, and attack the pathogen's cell membrane.

			===Activation===

			The alternative complement pathway is continuously active at a low level due to the spontaneous hydrolysis of the thioester bond in the complement component [[C3 (complement)\|C3]], a process known as "tickover." This hydrolysis generates C3(H2O), which can bind to factor B, allowing it to be cleaved by factor D into Ba and Bb. The Bb fragment remains bound to C3(H2O), forming the fluid-phase C3 convertase, C3(H2O)Bb.

			When C3b is deposited on a pathogen surface, it can bind factor B, which is then cleaved by factor D to form the surface-bound C3 convertase, C3bBb. This convertase is stabilized by properdin, a positive regulator of the alternative pathway, and can cleave additional C3 molecules into C3a and C3b, amplifying the response.

			===Regulation===

			The alternative pathway is tightly regulated to prevent damage to host tissues. Regulatory proteins such as [[factor H]], [[factor I]], and [[decay-accelerating factor]] (DAF) ensure that the complement system is activated only on pathogen surfaces. Factor H competes with factor B for binding to C3b and accelerates the decay of the C3 convertase. Factor I cleaves C3b into inactive fragments in the presence of cofactors like factor H and membrane cofactor protein (MCP).

			===Functions===

			The alternative complement pathway contributes to the immune response in several ways:

			* '''Opsonization''': C3b acts as an opsonin, marking pathogens for phagocytosis by [[macrophages]] and [[neutrophils]].
			* '''Cell lysis''': The formation of the [[membrane attack complex]] (MAC) leads to the lysis of pathogen cells.
			* '''Inflammation''': The anaphylatoxins C3a and C5a, generated during complement activation, promote inflammation by recruiting immune cells to the site of infection and increasing vascular permeability.

			==Related Pages==

			* [[Complement system]]
			* [[Classical complement pathway]]
			* [[Lectin pathway]]
			* [[Membrane attack complex]]
			* [[Opsonization]]

			{{Immunology}}

			[[Category:Immunology]]
			[[Category:Complement system]]

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<gallery>
File:Complement_pathway.svg|Diagram of the complement pathway
File:Droga_alternatywna.png|Alternative complement pathway
</gallery>

Alternative complement pathway - Revision history

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