Cells of the immune system & antigen recognition

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Cells of the Immune system & antigen recognition

Ovierview
The immune systems has developed to protect the host from pathogenes & other foreign substance
their are sites in which pathogens can resiad, extracellularly & intracelluarly

E.g., extracellularly

• Streptococcus pneumoniae
• Pneumocystis carinni
• Trypanosoma brucei

they are exist outside the cells

E.g., intracelluarly

• Viruses
• Mycobacterium tuberculosis
• plasmodium falciparum

they are pathogens that exist inside our cells

the immune system deals with pathogens that exist intracullarly in different ways from those that exist extracullarly

How dose the immune system deals with these different types of pathogenes

A- Extraceullar pathgoenes
antibodies are the primary defense aginst extracullar pathognes by different ways
= Nutralization =
so for example a bacterial toxins bound to a cell, so if we have an antibody against that bacterial toxins , the antibody will bind to the toxins & block its action resulting in nutraliztion

= Opsonization =
antibody on the surface of extraceullar bacteria can bind to Fc receptor on phagocytic cells & taking mor effectively by phagocytic cells

= complement activation =
activation of complement can result in lysis

B-Intraceullar pathogens
Once the pathognes inside the cells the antibody will not be effective. so it has there another way to deal with these pathogens & this by cell-meidated immunity

pathogenes inside the cells can exist in one of two places

they can exist in the cytosol for example the viruses & some bacteria , the other side where the pathogens can exist in the vesicles for example some bactria & parasites & the way in which the immune systme deals with these two different types of intracullar pathogens is different

in the case of pathogens into the cytosol, the response is mediated by group of cells called cytotoxic T cells (CD8) cells
* if we have a virus that infect a cell & its going into the cytoplasm of the cells & as its replicated there is some of viral-protien that will present on the surface of the cells & then the cytotxic will recognize these & kill the cells

in case of the intraceullar pathogens in vesicles, the response is different , here the immunity is mediated by different T cells, the helper T cells & whats heapning in this case (for example in mycobacterium tuberculosis) & whats happened that when the cells harpor these pathognes for example when tuberculosis infects the macrophages the bacterium inhibits phagolysome fusion that making the macrophages unable to kill the bacteria
as some of the antigne express some of bacteria antigen the T helper cells its get activated & it secretes a series of cytokines & then those cytokines (mostly interferon gamma) the macrophages get activated, now the lysome infuse & kill the bacteria

So into the cytosol it will be a cytotoxic response & in the vesicles it will be a helper response

Notes
When we get infected by a pathogens we can run both the antibody & cell-mediated immunity for example in cases of viruses , once the virus enter the cells we need the cytotoxic cells to kill the virus but that dose not mean that antibody dose not have a role , they are very important in subsequent exposure

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Cells of the Immune System
All cells of the immune system originate from a hematopoietic stem cell in the bone marrow, which give rise to the major lineage one for myeliod cells & one for lymphocyte

APC (Antigen Presenting Cells
these cells that form a link between the innate immnue system & the adaptive immune system so these cells play a role into the innate immune system & in the adaptive cells (by activation of helper T cells

APCs express a molecules called Class II MHC, & they include , Dendritic cells , macrophages , B cells

Lymphocytes

its includes two main cell types

• B cells
• T cells

The T cells includes two major subclasses, the cytotoxic cells (Tc) & the helper cells (Th

The helper T cells includes two subtypes, the T helper 1 & the T helper 2

So from the above table to distinguishes the B cells from the T cells, the marker used is called the CD3 (because CD3 is present on all T cells but not on B cells
CD4 is used to identify the T helper cells, whearse the CD8 used to identify the T cytotoxic cells

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The most important features of the adaptive immune systems is its specificty in response. the specificty of the immune response reside in the antigen receptors on T & B cells

T & B cells have similarity & differences, for example they differe in the valnce, BCR Valnce is 2 if you remmber we said the B cell receptor have two identical heavy chain & two identical light chain so it can bind 2 epitopes, in contrast to TCR which is monovalent, it can bind one epitope

So each B & T cells has a receptor that is unique for a particular antigenic determintes on an antigen

Principles of clonal selection hypothesis
* Each lymphocyte bears a single type of receptor with a unique specificity

* The interaction between a foreign molecule & lymphocyte receptor capable of binding that molecule with a high affinity leads to lymphocyte activation

* The differentiated effector cells derived from an activated lymphocyte will bear receptors of an identical specificity to those of the parental cell from which that lymphocyte derived

* Lymphocytes bearing receptors for self molecules are deleted at an early stage in lymphoid cell development & are therefore absent from the repertoire of mature lymphocyte

Lymphocyte Recirculation
Chances for a successful encounter an antigen are enhanced by circulating lymphocyte through lymph nodes
Lymphocyte enter the secondary lymphoid organs (spleen & lymph nodes) via High Endothelial Venules , then antigen is transported to the lymph nodes via antigen presenting cells, after activation the lymphocytes leave the lymph nodes & travel to the tissues

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برافوا

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