04. Component of CELL MEDIATED IMMUNITY

  Component of CELL MEDIATED IMMUNITY MHC MOLECULES Antigen presenting cells(APC) T helper cells(T H ) and T cytotoxic c...

 

Component of CELL MEDIATED IMMUNITY

1. MHC MOLECULES
2. Antigen presenting cells(APC)
3. T helper cells(T_H ) and T cytotoxic cells(T_C )

 

A. MHC MOLECULES-(Human leucocyte antigen= HLA)

 

1) MHC molecules are found on the surface membranes of cells.

2) There are 2 CLASSES of MHC molecules

 

	CLASS II MHC MOLECULES	CLASS II MHC MOLECULES
Found on:	all nucleated cells of the body	Surface of ANTIGEN PRESENTING CEELS (dendritic cells, macrophages and B cells),sperm
3 types:	HLA-A, HLA-B and HLA-C	HALA-DP,HLA-DQ and HLA-DR
Inherited:	Co-dominantly from parents so each has 2 set of gens on chromosome 6, Also 2set of MHC Class I/II molecules found on the surface of our cells.
polymorphic genes:	large umber of alleles for the MHC Class I & class II genes
	e.g. : HLA-A1, HLA-A2, HLA-A3 ,	e.g. : HLA-DR4, HLA-DR6 (2-12 different DR ,DP ,DQ  molecules )
Composition :	α chain and small polypeptide called β2 microglobulin.	α chain and β chain
Bind to:	Large number of different linear peptides (peptide antigen bind to peptide binding groove ) Then (peptide antigen + MHC Class I/II molecule) interact with the T cell receptor.

 

3) HAPLOTYPE: The set of genes inherited from one parent on one chromosome

4) GENOTYPE: both HAPLOTYPE together (one the each parent )

5) Genetic polymorphism: multiple forms of the same gene in one population

6) Different individuals have different genotypes and express different MHC molecules on their cells.

7) However every cell in a single individual expresses the same MHC molecules.

8) The host cell bind peptide antigens derived from within the cell.

9) If the cell is infected with a pathogen, peptides from the pathogen are also bound to the MHC, molecules.

10) This MHC/peptide complex can interact with a T cell receptor.

11) Then the T cell act to eliminate the intracellular pathogen.

12) The MHC is the most highly polymorphic gene system in the body and there is a diversity of MHC molecules within the population.

13) This ensures that each individual is capable of presenting a wide rang of peptide antigens derived from pathogens to the T cell receptor

 

T helper cells(T_H ) ,T cytotoxic cells(T_C ) and Antigen presenting cells(APC)

 

	T HELPER CELLS (TH cells):	T Cytotoxic cells(TC )
surfaces marker :	CD4 marker	CD8 marker
antigen reorganization	TCR bind to (Antigen + MHC class II molecules) on Antigen presenting cells(APC) Also, the CD4 molecule on the TH cell binds to the MHC Class II molecule.	TCR bind to (Antigen + MHC class I molecules) on Antigen presenting cells(APC) Also, the CD8 molecule on the TC cell binds to the MHC Class I molecule. They can recognize antigen when its present by any nucleated cell of the body
Type of antigen :	Exogenous antigens because recognizes-antigens derived from phagocytosed pathogens	Endogenous antigens Only recognizes antigen derived from protein synthesized in the cytoplasm (virally infected cells )
Functions :	Synthesizing and secreting CYTOKINES. IL-2, IFN-γ and TNF-α by (Th1 cells)→development of delayed type hypersensitivity responses IL-4, IL-5 and IL-10 by (Th2 cells ) promote allergic responses Cytokines like gamma interferon activate macrophages and make them more efficient phagocytes. Cytokines like IL-2 "help" T cells to proliferate and differentiate Cytokines like IL-4  and gamma interferon "help" B cells to become activated and to proliferate and differentiate into plasma cells. Promoted class switching of antibodies from IgM to other isotypes. Cytokines activate NK cells, mast cells, eosinophils. Cytokines like (GM -CSF) stimulate the bone marrow to produce more granulocytes and monocytes/macrophages. Cytokines like gamma interferon  and IL-4  regulate the immune response by activating or suppressing effector cells.	Cytotoxic T cells act by killing all nucleated infected cells (LYSE the infected cells)(CYTOTOXICITY). killing tumour cells and also in mediating graft rejection   The pathogen(virus) will generate foreign peptides in the cytoplasm →associate with MHC Class I molecules. →Expression peptide-MHC Class I complexes at the cell surface leads → recognition by cytotoxic T cells →killing of the infected cell.
		Antigen presenting cells(APC) –(dendritic cells and macrophages)
		These cells take up pathogens intracellularly process pathogen proteins into linear peptide antigens conjugate the antigens with Class II MHC molecules and display them on the surface membranes.