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Flashcards in B cell mediated immunity 2/ complement cascade Deck (75):

What mark potential pathogens for destruction? Is it reversible or irreversible and why?

opsonizing antibodies 

 antibody binding is a reversible reaction


How is a pathogen permanently marked for destruction and removal?

activation of complement cascade


What is the complement cascade? How is it activated?

eries of enzymatic cleavages of complement proteins that ultimately leads to covalent binding of opsonins to the surface of the foreign material 

• the complement system is activated by evidence of infection


complement cascade utilizes three different strategies for identifying and eliminating microorganisms. Name them 

  1. classical pathway
  2. alternative pathway
  3. lectin pathway


When is the classical pathway used?

triggered by antibody bound to the surface of the microbe

this is an example of the acquired immune response being bridged to the innate immune system


When is the alternative pathway used?

triggered by direct recognition of certain microbial structures 

this is a truly innate immune mechanism


When is the lectin pathway used?

 triggered by a plasma protein called mannose-binding lectin which binds to terminal mannose residues on microbial glycoproteins and glycolipids 

is also a truly innate immune mechanism


activation of the complement system results in a series of enzymatic reactions. Name them

  1. proteolytic cleavage/activation of complement proteins 
  2. covalent binding of complement fragments to the surface of the pathogen


T/F the complement cascade permanently “tags” microbes for destruction



Complement proteins are synthesized in the liver in what form and called what?

inactive form 

called zymogens


Describe how complement proteins are activated and what occurs after activation

 activated by proteolytic cleavage (highly specific cleavage) generally into 2 pieces

  1. the larger fragment is typically the enzymatically active component
  2. the smaller fragment often has an inflammatory effect (anaphylatoxins)



What are anaphylatoxins? what are the most common?

anaphylatoxins are complement fragments (produced during complement activation) that recruit fluid and inflammatory cells to sites of antigen deposition.

C3a, C4a, and C5a


Which anaphylatoxins induce mast cell degranulation?

C3a and C5a


What is an important characteristic of C5a?

C5a is a powerful and important chemotactictic factor for neutrophils


C3a, C4a and C5a can each have what effect on tissues?

can activate vascular epithelium resulting in leakage of
fluids from the vascular into the surrounding tissues (swelling)


the complement cascade involves a series of sequential cleavage events, and one of the most important endpoints of these pathways is what?

 the covalent deposition of complement component C3b on the surface of the pathogen;



What is the role of C3b?

C3b serves as a potent opsonin, facilitating uptake and destruction of the pathogen


all 3 of the complement pathways are dependent on the formation of what? why is this important?

C3 convertase

C3 convertase cleaves C3 into C3a (anaphylatoxin) and C3b (covalently binds to microbe surface)


What is the complement component of the lectin pathway? what does it bind to?

the complement component mannose binding protein (MBP) binds to terminal mannose residues
of bacterial surface glycoproteins and glycolipids


once MBP is bound to the pathogen surface, what occurs?

mannan-binding lectin-associated serum proteases
(MASP-1 and MASP-2) interact with MBP and become activated;

once activated, these enzymes can cleave complement component C4 (to produce C4b and C4a) and C2 to produce (C2a and C2b)


T/F after C4 and C2 are cleaved the pathway is identical to the classical pathway



When is the classical pathway activated? What Ab isotypes are associated?

activation of the complement cascade is initiated when complement component C1 binds to the Fc region of Ab molecule(s) (primarily IgM, IgG1, IgG3) that is bound to the surface of a pathogen; 

only one copy of IgM is required, while at least 2 copies of IgG are required


C1 is a complex of 3 proteins. name and describe them

  1. C1q: a large 18-polypeptide molecule; binds to determinants in the Fc region of antibody molecules =>  can only bind to Ab that are bound to their specific antigenic determinant
  2. C1r: inactive serine protease; when C1q binds to Ab, C1r cleaves itself; activated C1r cleaves C1s
  3. C1s: inactive serine protease; is activated upon cleavage by C1r; activated C1s binds to and cleaves complement components C4 and C2


cleavage of C4 and C2 leads to production of what? give the steps

C3 convertase

  1. C4 and C2 are cleaved into C4a and C4b / C2b and C2a, respectively;
  2. cleavage exposes reactive thioester bonds in C4b and C2a
  3. reactive thioester bonds react with microbial surface structures
  4. covalently bind to the microbe and associate to form active C3 convertase (C4b,2a)


****How is C3 convertase production regulated? What prevents the C4b and C2a from binding to “self” cellular components marking them for destruction by phagocytes?****

  • the thioester bonds are readily hydrolyzed;
  • therefore, C4b and C2a are only able to bind to structures immediately adjacent to the bound antibody molecule.
  • majority of the C4b and C2a thioester bonds are hydrolyzed before these components even make contact with the microbe.
  • C3b deposition on host cells occurs, but host cells have complement control proteins that rapidly deactivate C3b that has been deposited on them. 
  • C4a is an anaphylatoxin that has vasoactivation activity.
  • C2b is an inert protein fragment that has no known role.


C4b,2a cleaves C3 to produce a large fragment (C3b) and a small fragment (C3a). Give the function of each

 C3b has an exposed thioester bond that allows it to covalently bind to the pathogen surface (or to the C3 convertase itself);

C3a fragment is an anaphylatoxin that acts on vascular tissue to increase vascular permeability;

C3a can also trigger mast cell degranulation


phagocytes have complement receptors (CR1) that bind to C3b, and this binding encourages what?

remove the foreign material via phagocytosis


T/F C3 convertase of the alternative pathway can amplify the amount of C3 deposition that results from the classical pathway



formation of this second C3 convertase (from alternative pathway) involves what from the alternative pathway?

factors B and D


in the formation of the 2nd C3 convertase, define the roles of Factors B and D

  • factor B binds to C3b molecules that have bound to the microbial surface; 
  • when bound to C3b, factor B becomes susceptible to cleavage by factorD 
  • plasma protease factor D cleaves factor B into a small Ba fragment and a large Bb fragment  
  • the resulting product is C3b,Bb; this complex is the active C3 convertase of the alternative pathway


factor B deficiency results in what?

total lack of alternative complement activation

 reduced C3b deposition following either lectin or classical pathway activation


the C3b,Bb complex is analogous in structure and function to the C4b,2a complex. so what is its function?

 it cleaves C3 into C3a and C3b, and exposes the thioester bond of C3b


What provides the dramatic amplification of C3b deposition on the microbial surface?

 cleavage/activation of C3 by C3b,Bb because it is an accelerating reaction


the alternative pathway of complement activation is a purely innate immune response that is triggered when?

directly by constituents of bacterial surfaces


C3 is always spontaneously cleaved (at a very low rate) to produce C3b and C3a. Where do each fragment go?

the C3b is covalently deposited on the nearest adjacent surface. 

C3a is an anaphylatoxin


Knowing that C3b will bind to the nearest adjacent surface, what will occur if there is a pathogen present?

When a pathogen is present and is bound by C3b, the complement cascade will be initiated because the pathogen has no complement control mechanisms


Describe the process once the C3b is bound to the pathogen

  1. once C3b is bound to a pathogen, it serves as a ligand for factor B binding;
  2. once factor B is bound to C3b, it becomes susceptible to cleavage by factor D,
  3. which cleaves factor B into a Bb and a Ba fragment;
  4. the Bb fragment stays complexed to C3b, 
  5. resulting C3b,Bb is the enzymatically active C3 convertase of the alternative pathway (alternative C3 convertase)


 interaction of complement receptors with complement components bound to a microbial surface has what effect?

facilitates uptake and destruction of the microbe


what is expressed on antigen-presenting cells (macrophage/monocytes,
polymorphonuclear cells, B cells, and follicular dendritic cells)? What does it bind to?

complement receptor 1 (CR1)

it binds to C3b and C4b.


 What is the role of C3b wrt macs and neutrophils?

 C3b serves as an opsonin for macrophages and neutrophils, which recognize C3b via their CR1 


C3b / CR1 interaction is insufficient to initiate phagocytosis/respiratory burst, but does have what actions?

acts synergistically with IgG/Fc receptor interaction or IFN-y signaling to initiate these processes


***Where is complement receptor 2 (CR2): expressed? What does it bind to? What is it a component of? ***

on B cells and follicular dendritic cells 

binds to
C3d, C3dg, iC3b, and Epstein-Barr virus;

CR2 is a component of the B cell co-receptor


C3b deposited on a pathogen surface can be further cleaved to produce fragments iC3b, C3d, or C3dg. Why is this important wrt to different complement receptors?

CR1 cannot bind to these fragments


binding of CR2 to iC3b, C3d, or C3dg acts as what type of signal? what is the result of the signal?

synergizing signal when the B cell receptor is bound to its specific ligand;

leads to B cell activation


What enables FDCs to retain immune complexes for long-term stimulation of B cells?

CR2 on follicular dendritic cells can also bind to iC3b, C3d, or C3dg-tagged antigens, enabling them to retain immune complexes for long-term stimulation of B cells


complement receptors 3 and 4 (CR3 and CR4): are expressed on what? and bind to what? Why are these receptors important?

 on macrophages and neutrophils

bind to iC3b

  • augment activities of Fc receptors and CR1 in activating phagocytosis
  • unlike CR1/C3b binding, CR3/iC3b binding is sufficient to activate phagocytosis



complement receptors


high-affinity antibodies can bind to soluble protein antigens forming complexes that are too small to be phagocytosed (because they do not contain enough IgG molecules to form a stable interaction with Fc receptors). How can these be removed from circulation?

immune complexes can initiate the classical complement cascade resulting in C3b deposition on the immune complex


****What is a very important function of erythrocytes (RBCs) wrt immune complexes?****

  • erythrocytes bear CR1 receptors;
  • the vast majority of immune complexes in the circulation are bound to erythrocytes;
  • when erythrocytes circulate thru the spleen and liver, tissue macrophages remove and degrade the immune complexes by binding to complement component C3b via their CR1 receptors 


 T/F since blood is filtered in the glomeruli to form urine, blood pressure in these structures is very high, and deposition of immune complexes likely occurs constantly at low levels, even under normal conditions



What are specialized epithelial cells (that express CR1) covering the capillaries within kidney glomeruli?



What is the function of podocytes?

these cells help to remove and dispose of immune complexes that cross the basement membranes of capillaries


What is an immune complex disease that can result in damage to and eventual failure of the kidney? What does the kidney damage a result from?

systemic lupus erythmatosis (SLE)

kidney damage is the result of high levels of immune complexes that results in massive deposition of immune complexes on the renal podocytes;

the podocytes are overwhelmed, and chronic inflammation damages the kidney glomeruli (glomerulonephritis); 


the membrane attack complex is formed by what complement components? What is this complexes role?

 (C5b, C6, C7, C8, and C9);

this complex can cause direct lysis of either prokaryotic or eukaryotic cells


Describe the 5 steps (general) of the membrane attack complex

  1. formation of C5 convertase
  2. cleavage of C5; C5b fragment initiates formation of the membrane attack complex
  3. C5b binds successively to C6 and C7
  4. a single molecule of C8 binds to the complex
  5. 10-16 molecules of C9 bind to the complex each adding a hydrophobic site within the C9 monomers is exposed and inserts into the membrane


What is the C5 convertase of the classical pathway? alternative?

the C5 convertase of the classical pathway is C4b,2a,3b;

the C5 convertase of the alternate pathway is C3b2,Bb


Describe the cleavage of C5. Name the roles of the fragments

C5 binds to the C3b component of either of the C5 convertases and is cleaved to create C5a (anaphylatoxin) and C5b fragments

 C5b fragment initiates formation of the membrane attack complex

C5a fragment is a vasoactivator and is an important chemokine for neutrophils


What complement component is structurally similar to perforin molecules (protein in lytic granules of CTLs)?



deficiency of any of the membrane attack complex components (C6-C9) only seems to result in susceptibility to what pathogen?

 susceptibility to Neisseria sp.


deficiency of C5 can lead to quite serious problems. Especially with neutrophils so name it. What disease does it also cause during infancy?

C5a has important vasoactivating properties and it is an important neutrophil chemotractant.

Deficiency of C5 has been implicated in a disease that is typically diagnosed during infancy called Leiner Disease. Symptoms include severe seborrheic dermatitis, severe diarrhea, recurrent local and systemic infection, central nervous system problems, and failure to thrive


during complement activation, cleavage of C3 and C5 results in formation of C3a and C5a, respectively; What type of properties do they have? why?

 these products have immunomodulatory properties; because these molecules can induce anaphylaxis under extreme circumstances, they are designated anaphylatoxins


What are the common shared roles of C3a and C5a(most potent/stable)?

  • both induce smooth muscle contraction, mast cell/basophil degranulation, and have vasoactive effects on local blood vessels (increase vascular permeability and blood flow)
  •  increases extravasion of Ab, complement proteins, phagocytic cells, and lymphocytes into inflammatory tissues
  • increased fluid in tissues increases lymph flow to 2 ̊ lymphoid tissues which facilitates initiation of acquired immune responses


What anaphylatoxin acts directly on neutrophils and monocytes? What does it cause? What effect does it have toward complement receptors?


 increases their adherence to blood vessel walls and acts as a chemattractant;

upregulates phagocytosis  

upregulates CR1 and CR3 expression


wrt to the complement cascade, why must C3 convertase be tightly regulated?

since amplification of the complement cascade is dependent on a positive feedback loop in which C3 convertases produce more C3 convertase, without regulation this cascade would run out of control and could severely damage host tissues


there are two types of proteins that regulate the complement cascade by disrupting enzymes at key stages of the pathway. Name them

1) plasma (soluble) regulatory proteins

2) cell-surface regulatory proteins


What binds to activated C1r:C1s causing them to dissociate from C1q? What does this result in?

C1 inhibitor (C1INH)

this limits the time C1 is enzymatically active


What are 2 important characteristics of the C1 inhibitor (C1INH)?

  1. controls spontaneous activation of C1 in plasma that constantly occurs at background levels
  2. inhibits a plasma protease that is involved in production of bradykinin, which has effects similar to C2 kinin 


What is a disease suffered by patients that are
deficient in C1INH production? What is the effect? How is it treated?

hereditary angioneurotic edema (HANE)

  •  chronic spontaneous complement activation leads to excess of cleaved C4a and C2b fragments;
  • C2b is further cleaved to form C2 kinin which causes fluid induced swelling (edema);

treatment: symptoms of HANE can be effectively treated by C1INH replacement therapy


What is the action of C4-binding protein (C4BP)?

  • binds to C4b (of the C3 convertase-C4b,C2a) and displaces the C2b fragment, deactivating the C3 convertase 
  • renders the C4b fragment susceptible to cleavage/inactivation by the plasma protease factor I


Describe the role of Factor H

factor H: binds to C3b making is susceptible to cleavage/inactivation by factor I, with the formation of iC3b


What happens with factor I deficiency? How does it create deficient responses wrt bacteria?

factor I deficiency leads to depletion of C3 as the convertase activity of C3b,Bb is amplified without control; in addition, this deficiency prevents the formation of iC3b which is the ligand for CR3 (which is the receptor thru which macrophages and neutrophils are activated in the absence of antibody);

iC3b is important for opsonization of bacteria during innate responses;

factor I deficient patients are more susceptible to ear infections and abscesses caused by pyogenic bacteria


Name the plasma (soluble) regulatory proteins

C1 inhibitor (C1INH)

C4-binding protein (C4BP)

factor H

Factor I


Where are cell-surface regulatory proteins expressed? What is their role?

these regulatory proteins are expressed on the surface of host cells and help to prevent damage of self tissues following activation of the complement cascade


Name the 4 cell-surface regulatory proteins and their associated function

•• decay-accelerating factor (DAF): binds C4b and C3b components of the C3 convertases, causing their dissociation and inactivation
•• membrane co-factor protein (MCP): binds to C4b and C3b, making them susceptible to cleavage/inactivation by factor I
•• complement receptor 1 (CR1): CR1 expressed by host cells can bind to C4b and/or C3b (either on the cell or an adjacent bacterium) making them susceptible to cleavage/inactivation by factor I
•• CD59 (protectin): binds to C5b,6,7,8 complex; prevents polymerization of C9 in the membrane, thus prevents pore formation 


How are CD59 and DAF linked to the plasma membrane?What happens to patients that lack production of the phosphoinositol glycolipid tail are deficient in what? What do they suffer from? and suffer from a disease known as paroxymal nocturnal hemoglobinuria; this disease is characterized by episodes of intravascular red blood cell lysis by complement

via a phosphoinositol glycolipid tail;

 deficient in CD59 and DAF and

suffer from a disease known as paroxymal nocturnal hemoglobinuria; this disease is characterized by episodes of intravascular red blood cell lysis by complement