immunology MHC 123 2022.ppt PowerPoint presentation

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1. Antigen Presenting cell
and
Major Histocompatibility Complex

2. Antigen-presenting cells 
(APCs) are a heterogeneous
group of immune
 
cells 
that mediate the cellular immune
response by processing and
 
presenting 
antigens for
recognition by certain lymphocytes such as T
 
cells.
APCs include dendritic
 
cells, macrophages, and B
 
cells
An
 
antigen-presenting 
cell (
APC) is an immune cell that
detects, engulfs, and informs the adaptive immune
response about an infection.

4. How a macrophage presents antigen to T cells (lymphocytes) :

5. Antibodies (from B cells) can recognize
antigen alone
T-cell receptors, however, can only
recognize antigen that has been processed
and presented by Major Histocompatibility
Complex (MHC)
Involves:
-Antigen processing
-Antigen presentation
Major Histocompatibility Complex (MHC)

8. Inheritance of MHC
MHC coded by cluster of genes
○Rejection of foreign tissue is due to immune
response against cell surface molecules,
histocompatibility antigens

9. Inheritance of MHC
Collection of genes
chromosome 6 in humans (Human Leukocyte Antigen
HLA complex)
chromosome 17 in mice (H-2 complex)
○Class I MHC genes
-Encode glycoproteins expressed on all nucleated cells
○Class II MHC genes
-Encode glycoproteins expressed on antigen-presenting cells
(macrophages, B cells, dendritic cells)
○Class III MHC genes
-Encode various products involved in complement and
inflammation

10. Inheritance of MHC
Many different alleles exist at each locus
among the population
○Each set of alleles is called a haplotype
○Genes of MHC lie close together so crossing
over during meiosis occurs infrequently
○Individual inherits one haplotype from mom,
one from dad
-Many in the population are heterozygous
-Alleles are codominant so expressed
simultaneously

11. Class I MHC
Alpha α chain
○Transmembrane
○Encoded by A, B, and C
regions in human MHC
complex
Beta β
2-microglobulin
○Encoded by highly
conserved gene on
different chromosome

12. Class II Molecule
α
1 and α
2 chain
○Transmembrane
β
1 and β
2 chain
○transmembrane

13. Figure b shows top
View of peptide cleft

14. Every individual expresses small
number of different Class I and Class II
○Limited number of MHC must be able to
present enormous array of different antigens
○MHC does not display specificity of Antibodies
○MHC is “promiscuous” 

15. Generation of B-cell receptors
(antibodies) and T-cell receptors is
dynamic, changing over-time
○Gene rearrangement
In contrast, MHC molecules are fixed in
the genes
○Differences in population due to large number
of alleles
-In humans, ~370 A alleles, 660 B alleles, 190 C
alleles

16. Location of genes
Human
Class I MHC are red
○Telomeric end of HLA
complex
Class II MHC are blue
○Centromeric end of
HLA complex

17. MHC Restriction
CD8+ T
c cells are MHC Class I restricted
Can only recognize antigen presented by
MHC Class I molecules
Therefore, cells with MHC Class I are called
“taget cells”, killed by cytotoxic T cells
CD4+ T
H
cells are MHC Class II restricted
Cells with MHC Class II are called antigen-
presenting cells (APCs)

18. Antigen Presenting Pathways
Endogenous Pathway (Cystolic)
○Endogenous antigens – produced in cell, in
infected cell
○Antigens presented on MHC Class I to T
c
cells
Exogenous Pathway
○Exogenous antigen – taken in by endocytosis
by antigen-presenting cells and presented to
T
H cells by MHC Class II

19. Endogenous Pathway
○Inside the cell there is a constant pool of proteins and
their fragments that are no longer needed
-Intracellular proteins are broken down by proteasome
-The cell monitors these fragments and sometimes presents
these on Class I MHC
-If the fragments are foreign (such as viral fragments from an
infection), a T cell will recognize it in the MHC and destroy
the cell
-The Endogenous Pathway is how these fragments find their
way onto the Class I MHC

22. Exogenous Pathway
○Antigen-presenting cells (APCs) phagocytize
foreign antigen, process it and present it on
Class II MHC
○Takes 1-3 hours to be presented on the cell’s
Class II MHC