Immunology sheet # 7 - Yasmin Hijazi

IMMUNOLOGY

DR. Hassan
Lect. # 7
Yoom el. Taljeh ^_^

This lecture is somehow "m3jo2a" (ma t5afo ) so est3anet b.$eet mn last year to make this sheet as clear as possiple ..

so 9a79e7o w let's go ^_*

Colonal selection theory:
In the B-cell:
Antigen selects the B-cell that actually fits the epitope that is on the antigen and then it activates of the B-cell

In the T-cell:
during development of the B-cell and T-cell
Gene Rearrangement occurs that is responsible for the diversity { these genes are coding for the heavy chains , light chains, α and β chains of the TCR}..
In this re-arrangement, {V, J &D} are arranged in different ways to get the diversity.

Don't forget that during gene rearrangement , mutations may occur also the junctions between (V,J,D) may be not in the right place which will lead to junctional diversity.

We have the ability to produce one thousand million different specificities so we can feel any antigen enters our body.

Now , we'll talk about complement system

complement system:
is group of proteins ( 40-50 proteins) that are present in the body,,
some of them are soluble and others are on the cell surface.
They are activated in a cascade..

Complement system has determined functions :
i. Opsonization … like immunoglobulins
ii. Clearance of immune complex ... immune complex means antigen binds to antibody .. this complex must be removed coz this complex produces problems in the body.
iii. Cell lysis.
iv. Inflammation ... the complement system is v.good mediator of inflammation .
v. solubolization of immune complex to prevent immune percipitaion .

we will go through all these functions during this lecture .


complement nomenclature:
*some of proteins in the complement system are named according to their function they do
ex.
C4b binding protein is called like that coz it binds to C4b.
*Other proteins are known as complement components and they are 9 in number (C1, C2,…, C9)
*Others are known as Factors .. Factor H , Factor B, Factor D… .

Complement activation
Complement system is activated enzymatically.. when complement protien is activated, it splits into 2 pieces big piece bn36eha suffix "b" and small piece bn36eha suffix "a".
For ex.
If we have C3 complement protein and it is activated ,so it splits into big piece called C3b and small on called c3a.

all complement proteins are inactive except one bne7ke 3nha b3deen.

When C3 is activated .. it is written with a dash on top >> (c͞3b) … "so no dash means it is inactive".
The half life of activated complement protein is v.short and is measured in mili seconds then it will be back to inactive form by the help of control protien..The reason upon the short half life is that the complement system is a destructive system and if it is still activated for prolonged period it may destroy the body tissues..


# Complement system is present in the vertebrates and it doesn't present in lower mammalian.


Activation occurs by 2 pathways:
1- classical pathway ( 1st one to be discovered).
2- alternative pathway ( discovered later on).


classical pathway:

- It is activated by an immune complex.

note :
Immune complex is an antigen( molecule or cell attached to an antibody).

- It is pathway for specific immune response coz we have immuneglobulins (antibodies) are necessary for the activation of the classical pathway

Remember :
when we talked about immunoglobulin on the {CH2} domain
There are site for complement activation and other for Fc receptor .. When Immunoglobulin sticks to antigen, the {CH2} domain opens up its site and is ready to receive complement component and this component is 1st complement component which is the C1.


 C1 is made up of 5 different molecules.
One of these molecules is C1q which has 6 polypeptide chains and is composed of globular heads and stem that is collagenous in nature.

also ,There are 2 pairs of identical molecules which are C1r & C1s.

So, C1 actually is made of 5 mol.s.. 1 C1q & 2 C1r & 2C1s.


What is actually happened??
We have a sheep red blood cell as an antigen binds to an antibody which its paratope sticks to epitope of the antigen forming immne complex .. The Fc fragments receive the the globular heads of the C1.. 2 globular heads attach to 2 binding sites..These sites are {CH2} domain in immunoglobulin .


Once this happens,
A conformational change in C1 occurs and it will activate (C͞͞1r) which in turn
activates (C͞1s). { notice the dashes}







the next protein component is to be activated is C4 by C͞1s which splites C4 enzimatically into big piece C4b and small piece C4a.







C4a will release into the solution,, C4b is still sticking to the surface of the immune complex..


C4b binds to the next component which is C2 .
Again, C2 is split by C1s into 2 pieces:
1- C2b goes out to solution
2- C2a stays bound to complex

Till now, we have, on the activated surface
of the immune complex, C4b sticks to the C2a.

(when we say (C4b,2a), this means that C4b binds to C2a).



Now , C4b,2a activates C3 and it splits into:
1-C3a goes out to solution.
2-C3b stays bound to Complex.


So we have C4b,2a,3b on activating surface.

Notice the arrangement of the complement proteins in the classical pathway..
C1>>C4>>C2>>C3 .. this arrangement is according to the discovery of complement proteins..


C4b, 2a complex is called C3 convertase of the classical
Pathway coz it acts on C3 and convert it into C3a &C3b and belongs to
classical pathway of the activation of the complement.



C4b ,2a, 3b complex is called C5 convertase of the
classical pathway coz it acts on C5 and convert it into 2 pieces
and belongs to classical pathway of the activation of the
complement



Diagram indicates classical pathway

































Let's start with
(Alternative pathway ):

As we said be4 that the components of complement system are inactive .. actually small part of C3 which is in our body shows slow conversion into active C3 due to enzymatic activity or hydrolysis ..

now C3 is active and it is plit into C3b and C3a.

before C3b is inactivated, it stickes to foreign body and deposits on the foreign surface..so, C3b will be protected from inactivation by control protein then it becomes stable and fixed.










When C3b becomes stable, it binds to another protein of complement system which is known as Factor B .. " dr. Said that they used nomenclature {Factor} to make difference btwn
Alternative and Classical pathways"


** To know the meaning of tick- over, suppose that someone
switched on a car and left it. We say that the engine is ticking over,
that means the engine is rounding but the car doesn’t move.



by the time C3b binds to Factor B,
Factor D breaks down Factor B into 2 pieces:
1- Ba
2- Bb

Note:
Factor D is the only complement protein is always
active (t7t el 6alab da2man).



Ba is released in to the environment, while Bb sticks to the complex and we get C3b and Bb on the foreign surface.

Hl2 bs shwyet trkeeez ;)
The complex C3b,Bb activates C3 and splits it into C3a & C3b
and as we know that C3a goes out to the solution
and C3b sticks to the complex .. So on the activating surface ,
we have C3b,Bb,3b { 2 molecules of C3b and 1 molecule Bb).

** C3b,Bb complex is called as alternative pathway C3 convertase
coz it converts C3 and belongs to the alternative pathway.



C4b, 2a complex >>>> C3 convertase of the classical pathway

C3b,Bb complex >>>>> C3 convertase of alternative pathway



Now ,
The complex C3b,Bb,3b will act on the C5 and split it into two pieces:
C5a and C5b
so complex C3b,Bb,3b is called as C5 convertase of the
alternative pathway.




What does Amplification loop mean??!!
C3 convertase splits C3 into C3a &C3b.. dr. said that not all (C3b)s bind to the
C3b,Bb complex .. some of them bind to the complex while the others may stick to another
activating surface and start another alternative pathway..they will bind to more factor B and this will be split by factor D and produce more convertases on the activating surface..this process is known as amplification loop.

So, once alternative pathway is activated, it starts to amplify ( just like snow ball, when it rolls on a snowy steep, it gets bigger and bigger ).























(6awlo balkom el mo7a'9ara 58 mins so صبر جميل وبالله المستعان ))))))



Question was asked by hadeel  :
Does alternative pathway interfere with classical pathway ??!!
dr. answered: yes, The C3b which came from the classical pathway can also move
the alternative pathway.

Note:
- C2 in classical pathway and Factor B in alternative pathway are equivalents and their genes are usually found on the same chromosome.
- C3b in alternative pathway (2ltanyeh) is equivalent with C4b . (??!!!)
- C2a is an active enzyme.
- C4b isn't an active enzyme.
- Bb is an active enzyme.



Diagram indicates alternative pathway

































" elmo7a'9ara mesh nawyeh to5lo9 :'( "


Hll2,
we notice that in each pathway there's an enzyme act on C5
these enzymes are (C5 convertase of the
alternative pathway & C5 convertase of the classical pathway)..
these enzymes split c5 into C5a & C5b
C5b likes lipid membranes so it sticks to the plasma membrane.




Binds to plasma membrane

once C5b attaches to the cell membrane, it will bind to further components of complements.. it will bind to C6 then C7.. after that there's no enzymatic activity so there's no splitting and the complement components stay intact coz C5b is not enzymatic reactive and the convertases goes away !!!



As soon as C5b attached to C6 and C7 , the complex (C5b,6,7)
becomes hydrophopic then it is implanted in the cell membranes.

Being of C5b,6,7 on cell membrane will attract C8 and C9
so we have C5b,6,7,8,9 which still has ability to incorporate
more than one C9 .. up to 18 molecules of C9.

These molecules are arranged in a circle or donut shape
and make a hole in the plasma membrane.



This Hole in the plasma will cause leakage of the fluids coming from outside the cell to inside it.. so the cell will swell and swell l7ad ma tef23 O_o !!
Hay el process is known as cell lysis which is one of the functions of the complement system

C5b,6,7,8,9 is called as (MAC) (membrane attack complex) because it attacks the membrane producing channel through it then leakage occur so broken and lysis of the cell




Hll2 bedna ne7ke 3n opsonization:
- - C3b & C4b ,which stuck on the activating surface, have specific receptor is called complement receptor 1 (CR1)
- - Complement receptor 1 (CR1) is found on the surface of RBC and phagocytic cells {macrophages and neutrophils}

Any bacteria or immune complex has Cb3 or Cb4 on its surface, it will be eaten (phagocytized) by phagocytic cell as result of binding Cb3,4 to CR1 that what is called opsonization
CR1, which is found on the surface of RBC, attaches to each C3b & C4b find it on its way while RBC is circulating then after attachment ,, RBC goes to spleen ( spleen has macrophages which also have CR1.. the dr. said that CR1s on macrophages have higher affinity to C3b,C4b than those on RBCs) so they will bind to immune complexes and get rid of them
this is known as immune complex clearance

If Immune complexes are still circulating in the body, they may precipitate and obstructe vessels

Immune complex clearance is mediated by RBC.

We said that C3a, C4a and C5a will be released in to the solution,
They are known as anaphylatoxins..C5a is the strongest ,C4a is the weakest
Their functions are :
1. releasing histamin
2. increase the vascular permeability of blood vessels
3. They can also act as chemotactic factor (chemoattractants) .. they attract neutrophils

So, They are inflammation mediators (one of the functions of complement system is inflammation mediators.)


P.I.P: “Prevention of Immune Precipitation”
P.I.P is the main function of the classical pathway
When we have multiple antigens and antibodies, the antibodies
make cross-linkage to the antigens and the immunoglobulins
becomes bigger and bigger and this reaction (the aggregate) will
precipitate ..


This precipitation of immune complex happens due to:
1. The cross-linkage of the adjacent antigens by the same antibody.
2. Fc-Fc interactions .


To prevent precipitatipn, activate classical pathway which
produces C3b and then insert C3b between the Fc fragments and separate them from each other be4 they aggregate and accumulate .

if precipitation occurred,,,



getting rid of precipitated immune complexes is by activating alternative pathway which
produces C3b and then insert C3b between the Fc fragments and separate them from each other in that way we solubilize the immune complexes precipitation this process is called solubilization (one of complement system functions).






*This is v.important in renal immune complex diseases like glomerular nephritis
*Most of the kidney failure cases are due to التهاب الكلى المناعي which is precipitation of immune complexes in small blood vessels in the kidney which will produce inflammation that will lead to destroy blood vesssels so classical pathway prevents these complexes from precipitation and if they precipitate , alternative pathway can solubilize them.



Note 1:


Solubilization: is the function of the alternative pathway.
P.I.P: is the function of the classical pathway.




Note 2 :
Distinguish btwn prevention of precipitation and solubilization of precipitated immune complex.



Yaaaaaaaaaaaaaay … finish 5ern 



At the end,
Sorry for any mistake was written above
Sorry for being late but as u see that the lecture is too long and we had quiz pharma

I hope everything is clear
Corrections are more than welcomed

Best wishes for all of u in yr exams

Done by:
YASMIN HIJAZI