Immunity PDF Notes - Alexandria University

Immunity Microbiology department Faculty of veterinary medicine Alexandria University  Complement Complement : Series of serum proteins act as enzyme cascade produced by liver. Characters of Complement : 1-Present in all body fluid except CSF & Urine 2-It present in in...

Immunity Microbiology department Faculty of veterinary medicine Alexandria University  Complement Complement : Series of serum proteins act as enzyme cascade produced by liver. Characters of Complement : 1-Present in all body fluid except CSF & Urine 2-It present in inactive form while after activation ( make lysis to Ag ) 3-Number is more than 23 while the most important complements protein from C1 to C9 4- It is highly concentrated in G pig serum also complement is suitable for in vitro experiments relating to complement activation.  Complement 5-Thermolabile and inactivated at 56 c for 30 m or by few hours at room temperature. 6- It designated by capital litter C followed by Arabic number e.g. C1/C2 or designated as capital litter e.g. B,D,P 7- Each protein consist of 2 subunits after its activation ( a&b) and in most cases the smaller fragment is designated as a While the larger is b except C2a is the larger than C2b Larger fragment bind to functional complex while small fragment diffuse to circulation  3 Complement pathways Microbe-associated molecular patterns (MAMPs) Adaptive Innate Co-stimulation of B cells Note : Microbe-associated molecular patterns (MAMPs) molecules associated with the microbes, but not present in the host. Such as Lipopolysaccharide, peptidoglycan and mannose-rich glycans.  1.Classic Pathway Proteins of classic pathway : C1 to C9 C1 is consists of 3 proteins subunits C1q, C1 r ,C1s C1 q is the recognition subunit of classic pathway ( binding of C1 to Ab occur through it ) C1r& C1s has esterase activity. Classic pathway begin by binding of C1 to FC part of Ab Abs that able to fix complement are IgM/IgG.  IgM is Pentamer IgM Is more efficient than Ig G In fixation of complement:  Initiate the complement cascade a single C1q molecule must bind to at least two immunoglobulin Fc portions.Because IgM is a pentamer (5 Fc portions) a single molecule can be IgG is Monomer sufficient , whereas 2 molecules of IgG required to bind to C1q. Fab Fc Note : Ca and Mg are important for activation of classic pathway:  The a activation of C1 Subunits need one molecule of Ca  So in presence of EDTA ( Chelating agent of Ca ) no classic pathway activation Mg ions help in activation of C2 and making of C3 convertase  Classic Pathway Binding of C1q on FC part of Ab activate C1r activate) C1s ( esterase activity ) C1s Cleavage of Cleavage of C4 C2 C4a & C4b C2a & C2b C4b 2a  Classic Pathway C4b2a complex called ( C3 Convertase) C3 C3 convertase C3a & C3b Diffuse to circulation( inflammation mediator) bind to C4b2a forming C4b2a3b complex that called ( C5 convertase)  Classic Pathway C5 convertase cleaves C5 to C5a & C5b C5a: Diffuse to circulation ( Inflammation mediator ) C5b:Deposit on the surface of Ag lead to activation and deposition of C6 , C7, C8 , C9 forming membrane attack complex ( MAC) MAC form pore on cell membrane of Ag causing lysis of cell N.B C9 Similar to perforin that produced by NK cells and CD8 cytotoxic cells.  2.Alternative Pathway 2.Alternative pathway := properdin pathway Component of alternative pathway : Properdin system: 1- Factor D: Serum protein cleave B factor to Ba & Bb 2- Factor B : Serum protein similar to C2 fragment 3- Properdin : provide stability to C3 convertase Other component : C3 & C5 to C9 No C 1/4/2  2.Alternative Pathway Alternative pathway :  Activated directly by Ag e.g. bacterial component of cell membrane as LPS and endotoxins  Not need Ab, so it work in innate immunity  In normal plasma C3 in presence of proteolytic enzyme cleaves to C3a and C3b which bind to membrane of Ag  While C3b still present in a little amount, then factor B bind to C3b forming (C3b B) factor D activate factor B by cleavage it to Ba & Bb form C3bBb. Diffuse to circulation  2.Alternative Pathway  C3bBb called C3 convertase that convert more C3 TO C3a and C3b But this C3 convertase is still unstable and has short half life so when combine to properdin to become more stable.  C3bBb then convert more C3 To C3a/ C3b forming C3bBb3b which act as C5 Convertase convert more C5 to C5a and C5b  C5b deposit on surface of Ag then activate and deposit C6 to C9 forming MAC  3.Lectin Pathway 3-Lectin pathway : Lectin Pathway Components : MBP ( mannose binding protein ) = MBL ( mannose binding lectin) It’s a serum protein that bind to mannose ( monosaccharide) that found on surface of bacteria and fungi. MBP when bound to bacterial surface activate cleavage of MASP ( mannose binding protein associated serine proteases.) MASP cleave C4 and C2 then complete as classic pathway All component of classic pathway present except C1 NO need of Ab to be activated  Difference between complement pathways items Classic Alternative lectin Role in immunity Specific immunity Innate immunity Innate immunity Activation Require Ag –Ab not need Ab Not need of Ab complex for Require Ag only But activated by fixation of C1 Activation done by mannose on Mos products such surface of MOS as endotoxins Complement C1 /4/2/3/5/6/7/89 No need for C1/4/2 No need for C1 pathway It needs factor B while need MBL & components Factor D MASP Properdin C3 /5/6/7/8 9 C4/2/3/5/6/7/8/ 9 Pathway ……………Discuss…… …………Discuss…… ………Discuss………… ….  =Properdin pathway IgM/IgG The 3 pathways come together to form a “common final pathway”. Biological effects of complement activation =Function of complement : 1- lysis of the Pathogen 2- Opsonization of pathogen by C3b /C4b 3- Triggering the inflammation C3a and C5a 4- Neutralization of virus and toxin by C3b 5- Clearance of immune complex from the circulation 6- Enhancement of Ab production  1. Lysis of pathogens Activation of Complements mediate lysis of MOS through MAC formation The 3 pathways come together to Membrane attack complex (MAC) form a “common final pathway”. 1. Lysis of pathogens Bacterium MAC punches “holes” in membrane lead to lysis of Mo 2. Opsonization C3b or C4b make opsonization of pathogen and help in phagocytosis C R C C3b R Complement receptors = CR 3- Triggering the inflammation: C3a, C4a and C5a are chemoattractant ( C5a is more potent) that attract the neutrophils to the site of infection. They act as anaphylatoxin : bind to receptor on mast cell causing degranulation to mast cell and releasing of histamine Causing increase capillary permeability, smooth muscle contraction the main character inflammation. 4. Neutralization of virus and toxin by C3b 5. Enhancement of antibody production (activation of B cells): Enhancement of Ab production: C3b bind to complement receptors( CR) on B cells leading to B cell proliferation and class switching. NOTE : Class switching is the process whereby an activated B cell changes its antibody production from IgM to either IgA, IgG, or IgE depending on the functional requirements. This switch improves the ability of antibodies to remove the pathogen that induces the humoral immune response 6. Removal of harmful immune complexes from the body Deposition of C3b on immune complexes surface facilitates their clearance from circulation by phagocytic cells in liver and spleen. If excess immune complexes are not removed from circulation, they will be deposited in blood vessels wall leading to inflammatory reaction and damage Cleared by macrophages in of the surrounding tissue. liver and spleen  Regulation of complement activation Regulation of complement activation : To prevent the abnormal effect of complement the activation of complement is regulated by: 1- Proteins on the cell surface : The presence of high level of sialic acid in the cell surface contributes to rapid inactivation of C3b molecules on host cells Other control proteins : CD59 also called ‘(protectin) regulates complement activation cascade at the final step, by inhibiting formation of membrane attack complex (MAC) 2- Serum proteins: C1 Inhibitor: bind to C1 and inhibit its activation (classical pathway). factor H and factor I : inactivate C3b  Sequelae of complement activation Sequelae of complement deficiencies 1. Decreased circulating complement C3 concentration lead to Recurrent infections and sepsis due to Defect in opsonization and killing of pathogens by MAC. 2. Diminished B cell responses 3. Immune complex diseases such as autoimmune disease