[計(jì)算機(jī)]第3章補(bǔ)體系統(tǒng)(編輯修改稿)
2025-04-18 08:53 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 vasion The Fab of IgG or IgM bind to epitopes on an antigen. C1q, C1r, and C1s then assembles on the Fc portion of the antibodies to form C1, the first enzyme of the classical plement pathway. The enzyme C1 is able to cleave C4 into C4a and C4b, as well as C2 into C2a and C2b. initiation I g M , I g G 1 a n d I g G 3C o m p l e m e n t fi x a ti o n b y a n ti b o d yIgG2 C 1 q b i n d i n g s i te sC e r ta i n b a c te r i a , v i r u s e s a n d u e a te c r y s ta l s c a n a c ti v a te c l a s s i c a l p a th w a y w i th o u t A b .M i c ro b eC 2 bC2C 2 aC 4 b1 1 0 k D3 4 k D7 4 k DC 1 qC 1 r C 1 sC a + +p r o te a s ea c ti v i tyC3 Convertase The enzyme C1 is able to cleave C4 into C4a and C4b. The C4b binds to adjacent proteins and carbohydrates on the surface of the antigen. C2 then binds to the C4b and C1 cleaves C2 into C2a and C2b. The C4b2a functions as a C3 convertase that can subsequently cleave hundreds of molecules of C3 into C3a and C3b. C 4 bC 2 aC 3 bC 5 aC 5 aC 5 bC5C 5 bC5 Convertase Much of the C3b binds to adjacent proteins and carbohydrates on the antigen to participate in opsonization while C3a can stimulate inflammatory responses. Some of the C3b binds to C4b2a to form C4b2a3b, a C5 convertase that can cleave C5 into C5a and C5b. C 5 bC6C7C8C9C9C8L YS I SM EM B R A N E A T T A C K C O M PL EX ( M A C )c o n fo r m a ti o n a l c h a n g e i n C 9 p r o te i n s tr u c tu r e H YD R O PH I L I C A M PH I PA T H I CThe Membrane Attack Complex (MAC) Causing Cell Lysis This C5b6789n, or membrane attack plex (MAC), puts pores into lipid bilayer membranes of human cells to which antibodies have bound. This results in cell lysis. MAC can also damage the envelope of enveloped viruses and put pores in the outer membrane and cytoplasmic membrane of gramnegative bacteria causing their lysis. p o r e? The electron micrograph shows holes punched through the cell wall of the bacterium Shigella dysenteriae by the terminal ponents of the plement system. Some of the holes are larger than expected for C9 channels and probably were enlarged later by the action of lysozyme Alternative (innate) pathways ? There is a spontaneous conversion of C3 to C3b. Ordinarily the C3b is quickly inactivated: the C3b binds to inhibitory proteins and sialic acid present on the surface of body39。s own cells, and the process is aborted. ? However, bacteria and other foreign materials that may get into the body lack these proteins and have little or no sialic acid. So the C3b binds a protein called Factor B forming a plex of C3b?Bb. ? C3b?Bb is also a C3 convertase acting on more C3 to form: 1. C3b?Bb?C3b, which is a C5 convertase and can start the a
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