2. Give an overview of the role of the major
histocompatibility complex in immune response
Describe the structure and function of class I and class II
MHC molecules
Discuss the nature of polymorphism in class I and class II
MHC molecules
2
What you should know after completion of this lesson:
3. Outline
6.0 Introduction to MHC
6.1 MHC Molecules and Gene
6.2 Cellular Distribution of MHC Molecules
6.3 Regulation of MHC Expression
6.4 MHC and Immune Responsiveness
6.5 MHC and Disease Susceptibility
3
4. Introduction
MHC is a tightly linked cluster of genes whose product
plays an important role in IC recognition and in
discrimination between self and non-self
Genes in the MHC were first identified as being important
genes in rejection of transplanted tissues
They are also called Human leukocyte antigens (HLA)
Genes within the MHC are highly polymorphic in that
there are large numbers of alleles for each gene, and they
are polygenic in that there are a number of different MHC
genes
4
5. Introduction to MHC . . .
The MHC participates in the development of both
humoral and cell mediated immune responses.
While B cells may react with antigens alone, most T
cells recognize antigen only when it is combined with
an MHC molecule.
A peptide must associate with a given MHC of an
individual, otherwise no immune response can occur.
That is one level of control.
5
6. Located on human chromosome 6
MHC genes encode 3 classes of molecules
6
7. MHC genes encode 3 classes of molecules
A. Class I MHC genes: Encode glycoproteins
Major function of Class I gene products is presenting peptide
antigens to TC (CD8) cells.
Class I antigens are produced by the A, B, and C subloci genes
B. Class II MHC genes: Encode glycoproteins
They present processed antigenic peptides to TH (CD4)
cells.
Class II antigens are produced by the DP, DQ, and DR
subloci genes 7
8. C. Class III MHC genes
Encode various secreted proteins that involve in
Inflammation, and
Immune functions, including components of the
complement system such as C4, C2, and inflammatory
cytokines, including tumor necrosis factor (TNF)
Have no role in antigen presentation
MHC-II molecule cont….
8
9. Structure of Class I MHC
• Two polypeptide chains, a long α
chain and a short β (β2
microglobulin)
• Four regions
• Cytoplasmic region containing
sites for phosporylation and
binding to cytoskeletal
elements
• Transmembrane region
containing hydrophobic amino
acids
9
10. Structure of Class I MHC
• Four regions
• A highly conserved α3
domain to which CD8 binds
• A highly polymorphic
peptide binding region
formed from the α1 and α2
domains
• Β2-microglobulin helps
stabilize the conformation
10
11. Structure of Class II MHC
Two polypeptide chains α and β
of roughly equal length
Four regions
• Cytoplasmic region
containing sites for
phosporylation and binding
to cytoskeletal elements
11
12. Structure of Class II MHC
Four regions
• Transmembrane region
containing hydrophobic
amino acids
• A highly conserved α2, and a
β2 contains sequence for
CD4 binding
• A highly polymorphic peptide
binding region formed from
the α1 and β1 domains
12
16. MHC Class I polymorphism
Locus
Number of alleles
(allotypes)
HLA - A 218
HLA - B 439
HLA - C 96
There are also HLA - E, HLA
- F and HLA - G
Relatively few alleles
16
17. Cellular distribution of MHC
A. Class I MHC genes
Expressed on the surface of nearly all nucleated cells; but
vary in concentration on different cell types
The highest levels of class I molecules are expressed by
lymphocytes.
In contrast, fibroblasts, muscle cells, liver hepatocytes, and
neural cells express very low levels of class I MHC molecules.
The low level on liver cells may contribute to the considerable success of
liver transplants by reducing the likelihood of graft recognition by Tc of
the recipient
17
18. MHC molecule cont….
B. Class II MHC genes
Encode glycoproteins
Expressed primarily on APC(macrophages, DC and
B cells)
The expression level is different across these cells
18
19. Expression of MHC. . .
Class I MHC
Class II MHC
RBCs
APCs
Nucleated cells
19
20. Regulation of MHC Expression
The expression of MHC molecules is regulated by
Cytokines
Interferons (alpha, beta, and gamma) and TNF
increase expression of class I MHC molecules on
cells.
Interferon gamma (IFN-γ) induce the expression II
MHC genes on many cells, including non-APC
20
21. Regulation of MHC Expression cont….
Other cytokines influence MHC expression only in certain
cell types; for example,
IL-4 increases expression of class II molecules by
resting B cells.
However, IFN- downregulate the Expression of class II
molecules by B cells
MHC expression is decreased by infection with certain
viruses, including human cytomegalovirus (CMV), hepatitis
B virus (HBV), and adenovirus 12 (Ad12).
21
22. MHC and Disease Susceptibility
Some HLA alleles occur at a much higher frequency in
those suffering from certain diseases than in the general
population.
Diseases associated with particular MHC alleles
Diseases HLA RR
22
24. Outline
7.1 Introduction to Immunoglobulins and antibodies
7.2 Immunoglobulin Basic and Fine Structure
7.3 Antibody-Mediated Effector Functions
7.4 Antibody Classes and Biological Activities
77.5 Antigenic Determinants on Immunoglobulins
7.6 Class switching and generation of antibody diversity
24
25. Antibodies
Molecules that are produced by the body in response to a
foreign substance and are antagonist to it.
They are known as Immunoglobulins when not referring to
their specificity
• Immunoglobulins reside in the globulin fraction of serum, mean the
γ - globulin fraction was identified as containing serum antibodies,
which were called immunoglobulins, to distinguish them from any
other proteins that might be contained in the γ globulin fraction.
25
26. Antibody
Most antigens are complex and contain many different
antigenic determinants,
• Thus, the immune system usually responds by producing
antibodies(homogenous) to several epitopes on the antigen.
This response requires the recruitment of several
clones of B cells.
26
27. Antibody structure
The structure consists of two identical light (L) chains, and two
identical heavy (H) chain
Heavy chain
Contains two regions, a) a region with great sequence variation and
therefore called the variable (V) region.
The remaining part of the protein revealed five basic sequence
patterns, corresponding to five different heavy-chain constant (C)
regions
• Namely (µ, y, a, d, and £ )so the heavy chain determines what class of the
antibody is.
27
28. Antibody structure cont...
Light chain
Has variable and constant chain
There were two light chain types, kappa () and lambda ().
In humans, 60% of the light chains are kappa and 40% are
lambda
NB; it is important to remember that antibodies generally do
not kill or remove pathogens solely by binding to them.
28
30. Antibodies characterstics
A. Genetic Variation in Antibody Heterogeneity
Isotypic variation- variability in the CH and distinguish
immunoglobulin subclasses and subclasses within a species
30
31. 31
Allotypic variation- AAs substitutions on a given molecule
Idiotypic variation- variation of AAs sequence in the antigen
binding region given the molecule its antigenic specificity
Antibodies
35. IgG
Characteristics
150,000 MW
Highest serum conc.
Transported across placenta via Fc
IgG1 and IgG3 bind with high affinity to
Fc receptors on
phagocytic cells and thus mediate
opsonization
Four subclasses (IgG1-4)
Monomeric in form
35
36. IgA
Characteristics
• Dimer, monomer
• Two subtypes (1,2)
• Secretions
• Tears, Saliva, GI tract, Colostrum
• Secretory IgA has Secretory component made by epithelial
cells, that protects it from enzymatic and other chemical
degradation
• It prevents viral and some bacterial attachment to mucosal
epithelial cells
• Dimeric form has J chain - connects
36
37. IgM
Characteristics
Pentameric form
Soluble and cell associated
Primary antibody response
• Role in Lab Diagnostics, RBC
Does not pass placenta
Effective agglutinator
Most efficient C’ fixation
37
B
IgM
IgD
38. IgE
Characteristics
Extra Constant region domain
(C4)
Lowest serum concentration
Binds to Mast cells via Fc -
Allergic reactions
Responsible for immediate
hypersensitive reactions for the
symptoms of hay fever, asthma, hives,
and anaphylactic shock.
Protection - Parasitic infections
38
39. IgD
• Low concentration in serum (<1%)
• Function largely unknown - may act as Antigen
receptor for B cells
• No clinical need to measure - clue to physician
competence & clerical error
39
40. Antibody - Antigen Interactions
• Antibody binding site
• Folded structure - forming cleft
between Heavy and Light chains -
tertiary
• Apple analogy - best fit
• Antigen binding site
• Quite small
40
41. 1. Neutralization of microbes and microbial toxins
A. Blocks microbes binding
41
The biological function of antibodies
42. B. Blocks other cells infection
42
The biological function of antibodies cont…
43. C. Blocks toxins before they find the their receptors
43
The biological function of antibodies cont…
44. 2. Antibody mediated opsonization and phagocytosis
44
The biological function of antibodies cont…
45. 3. Antibody dependent cell mediated cytotoxicity
45
The biological function of antibodies cont…
(A)
A. Killing of the
antibody coated
cells
B. Killing of
helminths