Immunology sheet # 10 - Karmel Qasem

بسم الله الرحمن الرحيم

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http://www.mediafire.com/?wbyoi8l92mi6i3a
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Hey all … I want to say that I have written this sheet without having handout –since it is not available yet – so I did my best to include everything said by doctor…



In the last lecture we discussed the generation of immune response, the mechanism of antigen recognition and the activation of T and B cells.

Now this take place centrally to generate the effector mechanism which can be cellular (in form of cells) or humoral (in form of antibodies) where as the immune response is always characterize by having cellular response.

Those cells that act effector mechanism are studied under the topic "cell mediated immunity" which is more than one type. It is either: delayed type hypersensitivity, cytotoxicity, antibody dependent cell mediated cytotoxicity (ATCC) and phagocytic cell (macrophages).

** delayed type hypersensitivity; is a reaction develops (48-72) hours after introduction of the antigen, the prototype of this reaction is (ppd) test [[(Purified Protein Derivative) is a diagnostic tool for tuberculosis]]. So in this case the antigen injected intradermally in the forearm, after (48-72) hour a reaction is characterized by localized erythema (due to blood supply) and induction (infiltration of these cells) develops at the site of injection.

So what we measure is the infiltration (induction) and there is a certain criteria determine whether it is +ve or –ve reaction.
A positive test depending on the dose usually is characterized by induction of 5-10 mm. There are many tests can be used other than ppd.

It is usually mediated be CD4 th1, whereas Th2 mediate the antibody production.
Macrophages and monocyte are recruited and activating killing both; infected and normal cells … why? Because the innate immunity doesn't differentiate between self and non-self , unlike the immune repose which has this ability.

Macrophages are activated because the release of many types of substance , if you remember when we said phosphorelation of ITAM create signal that will cause the activation of transcription of many factors some of which are cytokines…activated macrophages are killing machinery , once they activated they release very important mediator among those , ( reactive oxygen intermediate) which can stimulate the production of nitric oxide. So many intermediates are produced which result in killing.

IL-1 and TNF will attack plymorphonuclear cells to the site, and the activated macrophages will remove cellular debris and even normal cells.
So this amplification of immune response, that’s mean the immune response is generated and this will activate the inflammatory response and the inflammation will lead to tissue damage, this damage will be repaired by macrophages.

And those macrophages stimulate repair be secreting platelet-derived growth factor, TG-beta and fibroblast growth factor.



** Cytotoxic T cell CD8 is activated either directly or as a sequence of activation of T-helper lymphocyte.

Cytotoxic T cell CD8 lymphocyte can act with antigen presenting cell MHC class 1 , so that they can recognize a cell and they secrete the cytokine that are producing by T helper cell, - they are capable of producing these mediator- also activation of T helper release IL-2 which is a second signal. So target cell recognition subsequent to pMHC-TCR-CD8 interaction leads to upregultion of fas ligand in which CD8 can cause apoptosis of infected cell. It can achieve killing be 2 mechanism; perforins (create osmotic pressure imbalance) and granzymes ( dip through the membrane creating holes that will allow activation of caspases which are required for apoptosis.
These substances are released upon establishment of contact between CD8 and the target cell such as viruses and tumor cells , they don’t need more than 5 min to release a killing effect .

The 2nd thing we want to talk about is the production of antibodies, in which they will end up in opsonizacion to bridge the antigen with macrophages and that will result in killing.

Always remember; T helpers are central regulator; they are needed in every aspect of immune response.

Plasma cell will start producing the antibodies, they start with Igm then Igg then it may produce (Iga and Ige) depending in the antigen. They are losing all surface-marking (no receptors) … act just like factories to produce antibodies.
There are some cells called memory cells , so in future if the individual exposure to same antigen , they will be ready to mount a response on accelerated rate… here the memory cell have receptors \because if they don’t , the antibodies will not be able to recognize the antigen in future exposure.

Generation of switching signal to convent Igm to Igg is due to interaction between CD40 and CD 154.

Once secreted by plasma cells, antibodies can mediate a variety of antigen specific responses;
1. formation of immune complexes with soluble or insoluble antigens
2. activation of the complement pathway ( classical one )
3. ADCC ( Nk cells and eosinophils)
4. sensitization of mast cell IGE ( in hypersensitivity reaction)
5. fetal protection * maternal igG)
6. protection of mucosal surface ( igA)

if you don’t have memory cell , problem will arise because the body will deal with the antigen every time in same manner where as memory cell are responsible for increasing the production of immune response at accelerated rate. This long –lasting ability to re call an immune response is called memory, a hallmark of the adaptive immune system in 1st exposure, it will be as low response (10-14) days, whereas in 2nd exposure it will take only few days.

The initial activation of antigen naive T cells increases their numbers by more than 1000 fold.
i.e. you have 10 to power 5 naive cell for specific antigen (TCR) , other exposure to antigen will increase the number of cells that carry the receptors up to 10 power 7 or 8 .. Meaning it will be greater in 10-100 folds.

More than 95% of activating cell will die , that if we don’t do this , the newer T cell that produced against new antigen will find no place ,,,, so there is a mechanism of tolerance called ( activated cell death ) will result in space for new T cells.

- The initial humoral response can be divided into four phases;
1. Lag: recognition and activation of antigen.
2. log; production of antibodies in accelerating rate
3. plateau
4. decline ( their consternation will drop ,,, and this depending on their half-life of the Ig)

- Antibodies production and increase in titer takes place during the log phase to reach a plateau in 10-14 days with a lag period of 5-7 days in primary responses.

- Low affinity Igm predominates in the primary immune response in addition to low titer and low duration

- High affinity of Igg and longer duration are present in secondary response

- secondary B cell responses have a greatly reduced lag phase , an acclerated log phase and a plateau in which titers can reach 1000 to 1000000 fold greater than those of primary response .

- Duration of plateau varies depending on many factors.














Note; the figure above show us that there is a sharp increase in the antibody consternation on the 2nd exposure of the antigen, and the plateau continues for long period of time. There is also Igm in 2nd exposure but it is amount will be less and for short period of time it will present.



There is a question asked by student; what do we mean when we say" duration"??

Answer; duration of resistant- the duration that you detect the antidotes amount… duration is function of consternation and half-life. Example if the antibodies detect after 20-30 year after exposure, you can imagine the titer that produced in secondary immune response.

In responding to a secondary antigenic insult, B cells undergo hyper mutation of their variable region, this lead to what is so called " affinity maturation '' which is characterized of secondary response because the re- exposure to antigen will select the mutants of better activity (high affinity) =]

Balancing lymphocyte activation and control achieved by 2 ways:
1. activation effector T cell in which they mediate normal reaction against pathogens and inflammatory disease

2. Tolerance or regulatory T cells in which they mediate no response to self and controlled response to pathogens.

If 1>2 then we get damaging to host tissue
If 2>1 lead to sever immune mediated damage

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Immunological tolerance

The mechanism which act to prevent damage to the host are either :
1. T regulatory cell ( down-regulation)
2. Tolerance : as the name indicate is form of unresponsive( no acting)

i.e. if we take 100 individuals and vaccinate them , only 45-50% of them will response , injected for 2nd time we will get 80% , 3rd time we will get up to 98% but beyond this cant been achieved.

- Exposure to an antigen may or may not lead to an immune response
- Different forms of the same antigen can cause different outcomes (such as soluble antigen is not immunogenic as particulate one) ALSO different consternation will cause different outcomes.

i.e., if u inject the antigen intradermally this will lead to immune response whereas injected it intravenously you will not get immune response, this is refer to something called " rate of elimination" means: we need the antigen to expose to immune system for sufficient period of time and the antigen that was give IV will eliminate fast in contrary to intradermally which eliminate slower, giving the immune system sufficient time to induce response. That’s why we give adjuvant in order to decrease the rate of elimination and get good immune response.

A unique property of the immune system isits ability to discriminate between self and non- self …
Ability to respond to self is inherited but it is regulated at the phenotypic level by induction of tolerance

The lack of self response is the result of teaching or educating lymphocytes not to respond to self antigen, so this is regulated AT PHENOTIPC LEVEL DURING DEVELOPMENT OF IMMUNE SYSTEM ; while immune response being generated certain mechanisms act to a avoid self reactivity , if this reactivity wasn’t eliminated during the development of B or T lymphocyte.

All human being have the genetic make up to response to everything – self and nonself – but at the level of phenotypic we regulated to response only to nonself

 So Tolerance is a state of unresponsiveness that is specific for a particular antigen which is induced by prior exposure to that antigen

Destructive responses are prevented by a variety of mechanisms that operate during the development of the immune system and during the generation of each immune response.

Immune response to self is prevented by different mechanism
• Centrally: related to primary lymphoid tissue ( thymus and bone marrow ) – where development take place there-.
• Peripheral ; in periphery ( lymph node, spleen …ect )

Negative Selection during T cell Development


Thymocyte are derived from bone marrow, they have no receptor on there surface, going toward the thymus then they acquired TCR and CD4 and CD8 so they were double –ve then double +ve then signal +ve and this what so called " negative selection".
Thymocyte that they are double +ve become CD4 or CD8 depending on antigen presentation, they expose to antigen by antigen presenting cell of the thymus e.g. Dentrtic cell so if they receive a signal in conjunction with class 1 they will become CD8 but if they receive a signal in conjunction with class 2 they will become CD4.
And they will leave the medulla as signal +VE ..

Those cell that die , are actually negatively selected , so negative selection in the thymus refers to deletion of a cell that doesn’t recognize on antigen , and the antigen should be present in certain affinity usually intermediate because high/low affinity will also end up in killing.

- Lack of +ve or –ve selection will cause them die, and negative selection will cause death too.
- That’s why the majority of Thymocytes 95% dies in the thymus because the process is stressful for cells.
- so death is contribute to 2 factor :
Negative selection and lack of +ve selection ( if it is not introduce on the antigen )
- So all cells that recognize self antigen in thymus will die , that's mean if the TCR recognize an antigen in the thymus whether it is self or non-self that cell will die … 7okman !!!
- Because the safest mechanism is to eliminate auto reactivity.
Example. If we inject the antigen in the thymus ( although it is not self antigen it will cause death of the cell that recognize it ,,, because the body want to make sure that all cells that recognize antigen during developments will be deleted ..


- Mature T cells are self tolerant, MHC restricted and responsive to foreign antigens.
- The fundamental steps of T cell self tolerance occur in the thymus.
- T cells undergo expansion, rearrangement of TCR genes and begin to express surface markers.
- Thereafter, they undergo positive selection (MHC restriction) and negative selection (deletion of self reactive cells).
- Immature T cell encountering their antigen during development are clonally deleted.
- All T cells are susceptible to the process of negative selection.
- Antigen presenting cells for negative selection are most likely dendritic cells
- The molecular mechanisms responsible for inducing apoptosis are not fully known


Negative Selection during B cell Development
- If the immature cell recognizes an antigen in the bone marrow as self, it will be eliminated.
- The difference here, it will not be always physical elimination, it could be functionally.
- They have a chance to change their receptor by something called "receptor editing" by rearrangement of genes of light chain but now they are for nonself . If they fail to do that, they will be eliminated.
- Receptor editing in B cells is mediated by reactivating their RAG1 and RAG2 genes and expressing a new Ig light chain, thus acquiring a new specificity
- To induce B cell tolerance, the antigen has to be multivalent (signaling requires cross linking), be present at high enough concentration and react with Ig receptor with a high enough affinity.
- B cell tolerance is short-lived.

**In both cases minimal consternation and affinity is required
** The mechanism is very efficient and don’t fail, otherwise auto immunity disease will be very common.

So t cell tolerance is difficultly established but its more durable (persist for long period of time) whereas b cell short lived and easy to established.








Done by: Karmel Qasem
Date of lecture 21-12-2010