Antibody function and complement system Flashcards
What are the different ways antobody immunity is called?
-Antibody-mediated immunity is often called ‘ humoral ’.
T cell mediated immunity is ‘cellular.’
A reminder: Not all of a protein antigen binds specifically to an antibody; the part that actually interacts is usually 10 to 20 amino acids long, and is, as you know, called an epitope or antigenic determinant.
How many antigens can an antibody bind?
- The basic structure of antibodies, 2 L and 2 H chains, is rotationally symmetrical, so each antibody can bind two identical antigenic determinants; we say that such an antibody is divalent.
- A complete IgM molecule (which is pentameric), would be predicted to be decavalent (can bind to 10 antigen).
How is this proven? This has been demonstrated experimentally: If we prepare soluble isolated antigenic determinants referred to as haptens and mix them with the appropriate antibody, some will be bound.
Difference between agglutination and precipitation:
Most real-life antigens have multiple, usually different, antigenic determinants or epitopes (a bacterium or a cell may have hundreds). These antigens are therefore multivalent. If we mix such antigens and antibody in solution, not only do the epitopes bind the appropriate antibody’s binding sites but the antibody, being at least divalent, has a very good chance of cross-linking two antigens; a lattice will begin to grow (the lattice is commonly called an immune complex ).
The large immune complexes that are formed at or near equivalence (What does this means?–> ratios of antigen to antibody are optimal) tend to become insoluble and fall out of solution or suspension.
–>When the antigen is a molecule, the phenomenon is called precipitation.
–> when it’s a cell o r cell-sized particle, it is called agglutination. Agglutination is more easily visible than precipitation, and so, for the same amount of antibody, an agglutination test is more sensitive.
What is the Quantitative Precipitin Test?
What is immunodiffusion?
Where in the well was the equivalent point reached? What does this means?
Describe how IgM is better than IgG at activating the complement system:
- This i s the oldest antibody phylogenetically.
- It is also the first seen in blood after immunization (sensitive tests reveal an increase by day 2). You would think it is the best possible antibody, being decavalent, but in practice its shape rarely allows more than two of its binding sites to interact with antigenic determinants.
- What does make it outstanding is its great capacity to activate complement system;why? Since two adjacent Fcs are needed to get the complement cascade started, and IgM always has five adjacent. It can be as much as 500 times more efficient than IgG at activating complement.
- Why, then, even bother to have IgG? One reason is that IgM, being so large, has trouble getting from the blood into the tissues. It is also viscous in solution; if we had only IgM at the same molarity as we have IgG we would hardly be able to pump our blood.
Can it activate phagocytes?
NO, Why?
There are no useful IgM receptors on phagocytes.
-IgM is the only antibody to be made by the fetus. We make a little secretory IgM, which perhaps helps out people who lack secretory IgA (quite a common condition, see Immunodeficiency ).
IgG can activate phagocytes:
It is the most abundant immunoglobulin in the blood.
► It is the only class that passes the human placenta from mother to fetus (active transport required).
- It comes up in the blood a little later than IgM after primary immunization, but the antibody levels go higher and last longer.
- ► The plasma half-life of IgG is about 3 weeks. If antigens (pathogens) get into the blood stream IgG antibodies are very important; phagocytic cells have receptors for the Fc of bound IgG, ► and so IgG is opsonizing ; vital for clearance of most extracellular bacteria.
► It takes two IgGs close together to activate the first component of complement, so this will only happen if the density of epitopes on the antigen is high enough for th at to occur (remember, binding a single epitope should be enough to allow an IgM molecule to activate complement).
IgA in mucous membranes:
Via what pathway can IgA activate the complement system?
This antibody is preferentially made by plasma cells in lymphoid tissues near mucous membranes.
Two IgA’s are assembled into a dimer by the addition of a J chain while in the plasma cell, and then secreted into the interstitial space. Adjacent epithelial cells have receptors for IgA which binds to them, is taken up and moved through the epithelial cell to the luminal (mucous membrane) side.
- There the IgA is exocytosed, still bound to the receptor, which is now called Secretory Component. *Secretory Component protects the IgA from digestion in the gut, and makes it work well as our first line of immunological defense against invading organisms.
- There is some monomer and dimer IgA in the plasma, where it can bind pathogens and activate complement (by the alternative pathway).
IgD as a B cell receptor
Althoug h there is some IgD in the plasma, it is believed that the only important role for IgD is as a B cell receptor.
IgE and immediate hypersensitivity (allergy):
IgE is important in resistance to what?
It stimulates eosinophils to release what?
Because its Fc end binds to a corresponding receptor on mast cells and basophils, forming a trigger for these histamine-loaded cells, this antibody is the cause of immediate hypersensitivity or allergy ( see Type I Immunopathology).
Why does it exist? It is important for resistance to parasites, where it triggers the mast cells to release not only histamine, but eosinophil chemotactic factor. Eosinophils are uniquely effective at killing parasites.
The Classical pathway to activating the complement system:
What two antibodies can activate the classical pathway?
What part of the complement system binds to the Fc regions and initiantes the cascade of rxns to activate it?
What is the oder of activation of the complement proteins?
What is one inhibitor of the complement system?
The Lectin pathway of activating the complement system:
Mediated by mannose-binding protein (MBP or MBL, a lectin)
Of what immunity is this pathway also part of?
What do lectins bind?
What structures that MBP binds are contained only in CHOs of bacteria but not humans?
What proteases associate with MBP when it binds mannose?
Part of innate immunity
The lectin pathway is mediated by mannose-binding protein, MBP or MBL, a lectin. Lectins are proteins that bind (usually foreign) carbohydrates.
-MBP binds certain mannose – containing structures found in carbohydrates of bacteria but not humans.
MBP is functionally similar to C1q (analogous) in the classical complement pathway, so the lectin pathway goes MBP-4 -2 -3 -5 -6 -7 -8-9.
- There are several alleles of MBP in humans, and Caucasians have an allele that results in low levels of serum MBP; about 8% have very low levels. These people have marginal immunity — they may be fine except when the immune system is stressed (in infancy, in old age, in the presence of anything that compromises the immune system).
- Associating with MBP when it binds mannose are some serine proteases, the MASPs, which activate C2 and C4 and get the cascade rolling.
The Alternative pathway to activate complement system:
**** Can be activated even without antibody****
This pathway is considered part of what immune system?
In this pathway who provides the surface for the binding of C3b so the C5 proteins can be activated?
What compound can activte C5?
It can accelerate the activation of the complement systems by the________ pathway?
The alternative pathway is activated by certain cell wall structures of microorganisms such as:
-dextrans, levans, zymosan, and endotoxin
►a bacterium might activate Complement this way even in the absence of antibody.
-This pathway is considered part of the innate immune system.
Complement is activated by a cascade that involves C3, which is always breaking down at a low rate to C3a and C3b, which are usually rapidly degraded. So if C3b could be stabilized, C5 could be activated. The cell wall structures provide a surface for the binding of of what factors?
C3b and factor B, properdin (P), and factor D
Forming what? A stable C3bDbC3b complex (trimer of Db and two C3b units) forms which can activate C5 (and thus 6-7-8 -9).
The alternative pathway seems to be a more primitive, early, less- specific sort of defense, since it can work even without waiting for antibody to be made. Recently it’s been shown to accelerate the activation of C by the classical pathway, and it can make things worse in autoimmunity.
What are the 4 actions of the complement system?
1- Lytic action
What proteins form a lesion on the target cell membrane that makes that cell loose its ability to regulatw osmotic pressure?
What bacteria commonly requires this elimination pathway due to its ability to resiste phagocytosis?
Activating C3 and C5 is a vital part of the complement-mediated inflammatory mechanism, responsible for 3 of the 4 actions listed below.
The fourth is the formation of the membrane attack (lytic) complex or MAC : C5, activated by any of the three pathways described, but very strongly by the classical pathway, activates C6, C7, C8, and C9. –**C8 and C9 form a lesion on the target cell membrane which, on electron microscopy, looks like a hole, which in fact it is; the cell loses its ability to regulate its osmotic pressure and lyses or pops.
What does lytic means?
It is lytic if the membrane attack complex is activated.
*was *Neisseria (gonorrhea, meningitis) require lysis because they resist death by phagocytosis. This electron micrograph shows MAC holes in a complement-treated bacterium.
2- What does opsonizing means?
What product of the Complement system has the ability to opsonize?
What antibody is opsonizing?
One split product of activated C3, namely C3b, adheres to membranes.
- -Phagocytic cells* (PMN, macrophages) have C3b receptors, and so can get a firm grip on an antigen if it is opsonized with C3b.
- IgG is also opsonizing, because phagocytes have receptors for its Fc end called FcR (there are several different FcR).
- There are no FcR for IgM, but the complement it activates is, of course, opsonizing.
