Lecture 3 - Antigens, Antigen Recogn, MHC

a substance which is able to induce an immune (adaptive) response

ALL immunogens are antigens, but not all vice versa
ex: food antigens

1. Age
2. Overall health
3. Route of innoculation
4. Dose
5. Genetic composition determines whether an immune response can be made or not

- older individuals - decreased response to antigenic stimulation
- neonates - immune system not fully developed

- malnutrition
- fatigue
- stress

• Intravenous-carried by blood to spleen
• Intradermal
• Subcutaneous - goes to local lymph nodes
• Oral- can lead to oral tolerance,Ag delivered via GI tract are ignored by the adaptive immunity

• The larger the amount of immunogen exposed to, the greater the immune response
• Very large doses can induce tolerance

determines whether an immune response can be made or not - MHC system genes

MHC system genes

a system of genes that code for all cell surface molecules that play an important role in antigen recognition

1. immunogenicity
2. BCR and TCR react to specific proteins but to different levels of protein structures
3. Pure nucleic acids or lipids are not immunogenic by themselves unless bound to specific carriers:

ability of an antigen to stimulate a host response depends on:
1. macromolecular size
2. foreignness
3. chemical composition and molecular complexity
4. ability to be processed and presented within MHC molecules

The immunogen MUST be at least 10000 D in size to be recognized by the immune system, the most effective immunogens are > 100000 D, the larger the immunogen the more potent it will be

The more taxonomically different, the more successful it is as an immunogen. For example, a plant antigen is more immunogenic for an animal than an antigen from the same animal, substances should be recognized as non-self

• Proteins are stronger than polysaccharides, lipids or nucleic acids. Proteins are more complex (amino acid sequence, secondary, tertiary and quaternary structures)
• Carbohydrates most often occur in the form of glycolipids or glycoproteins (e.g., ABO blood group antigens), not repeating units. Polysaccharides (repeating units) are less immunogen than proteins.

• T cells (TCR) recognize “peptide” (hence primary structure or amino acid sequence) of a protein molecule only when that peptide is held (cradled) within MHC molecules
• BCR is activated by binding to amino acids on the exterior of a protein (recognize tertiary or quaternary structure of a protein antigen)

Polysaccharides are less immunogenic than proteins because they cannot activate T cells (T cells can not recognize polysaccharides), however, B cells can recognize some polysaccharides without help from T cells (memory not generated)

- A, B, H antigens from ABH blood group system are glycolipid
- Rh antigens are protein
- P system antigens are CHO, MNSs are glycoprotein
- Kidd, Duffy, Kell, and Lutheran are protein, …………

• For instance, in systemic lupus erythematosus (SLE), the anti-DNA antibodies are basically specific to DNA-protein (nucleoprotein) complex
NOT DNA itself

determinant site, or epitope

N-acetyl-D-galactosamine (aka immunodominant sugar) in the whole structure of the Ag

• Epitopes are molecular shapes or configurations that are recognized by B or T lymphocytes.
• Large molecules may have numerous epitopes, but each epitope can trigger a specific Ab or activate a specific T cell.

• Sequential
• Example: amino acids following one another on a single chain

• Has folding of one or more chains
• Allows amino acids from different segments to come into close proximity

BOTH linear and conformational

linear epitopes only

nonimmunogenic materials (too small to be recognized by B cells), however, when combined with a larger carrier molecule, they can create new antigenic determinants and trigger an immune response

NO! they are an antigen, but not an immunogen

Haptens by themselves cannot initiate precipitation or agglutination reactions due to small size and because cross-linking of more than one antibody molecule does not occur

• Penicillin is a hapten but when bound to normal proteins in the body can cause dangerous allergic reaction
• Catechols in the poisonous plant Ivy (in nature), if bound to dermal proteins they will cause allergic dermatitis

substances delivered simultaneosly with antigen for the purpose of potentiating or enhacing the immune response by:
1. prolonging
2. stimulating
3. increasing
4. accelerating

Prolonging the existence of immunogen in the area (by preventing antigen from diffusing away from a site of inoculation)

Stimulating innate immune receptors such as TLRs which sense the danger

Increasing the number of macrophages and dendritic cells
(APCs) involved in antigen processing

Accelerating the immune response, including duration of protection and it also decreases need for boosters (helpful in immunizations)

Autoantigens are those antigens that belong to the host. These do not
evoke an immune response under normal circumstances

Alloantigens are from other members of the host’s species

Heteroantigens -from other species-plants, animals, microorganisms

Heterophile antigens -heteroantigens that exist in unrelated plants or
animals but are either identical or closely related in structure so that an
antibody against either antigen will cross-react with the other.

1. Human blood group A and B antigens which are related to bacterial poly
saccharides
2. Reaction of a heterophile antibody in the serum of patients with infectious
mononucleosis (IM) with sheep red blood cells (SRBCs): principle of Paul-Bunnell test.

reaction of the heterophile Ab with horse or bovine RBCs

Major histocompatibility complex
• In humans called HLA (human leukocyte antigen) following discovery an antibody response to circulating WBCs
• Now named MHC (Major Histocompatibility Complex) because they determine whether a tissue transplant is accepted by the (compatible with the) recipient or rejected, thus extremely important in organ transplantation

Are found on all nucleated cells (Class I) in the body, and they play a pivotal
role in the development of both humoral and cellular immunity.

Their main function is to bring antigen to the cell surface for recognition by T
cells, because T-cell activation will occur only when antigen is combined
with MHC molecules

encoded by the genes of the most polymorphic gene systems in humans

- each HLA allele varies greatly among different individuals
- The polymorphism of the MHC system is essential to our survival, because MHC molecules play a pivotal role in triggering the immune response to diverse immunogens.

on the short arm of chromosome 6 and are divided into 3 categories/classes

- Class I
- Class II
- Class III

found in three different loci (designated as A, B, and C)

found on the D region (designated as DR, DQ, and DP)

found in a region between class I and II, and codes for C4A, C4B, C2, and B complement proteins

Class I and II gene products are involved in antigen recognition and
influence the repertoire of antigens to which T cells can respond

Class III proteins are secreted proteins that have an immune function, but they are not expressed on cell surfaces.

YES - Within human population, many alternate forms or alleles of MHC
genes exist

Alleles are alternate forms of a gene that code for a slightly different variety of the same product)

because there are so many possible alleles at each location

short arm of chromosome 6

Each person inherits 2 copies of chromosome 6, thus it is possible to have
2 different alleles for each gene unless the person is homozygous for that
allele

These MHC genes are described as being codominant meaning all alleles
that an individual inherits code for products that are expressed on cells

- haplotype
- one haplotype is inherited from each parent, and each haplotype consists
of a package of genes for A, B, C, DR, DP, and DQ

The full genotype would consist of two of each gene at a particular locus

- organ donation
- paternity testing

- ALL nucleated cells, although they differ in the level of expression
- highest expression on lymphocytes

Glycoprotein dimers made up of 2 noncovalently linked polypeptides:
- alpha chain - has 3 domains, alpha1,2, 3. a3 binds to CD8 on T cells
- Beta 2 microglobulin - not polymorphic

the top - in between a1 and a2

a3

primarily on APCs like B lymphs, monocytes, macs, and dendritic cells

DP
DQ
DR

- two noncovalently bound polypeptides (heterodimers) that are encoded by separate genes in the MHC complex
- DR - expressed at highest level

dendritic cells - most effective APCs that have the higest level of Class II

- 2 alpha and 2 beta chains, which are BOTH anchored to the cell membrane
- both ends of the peptide-binding cleft are open, and this allows for capture of longer peptides than is the case for class I molecules

the top - between a1 and b1

b2

they bind peptides within cells and transport them to the plasms membrane, where T cells can recognize them in the phenomenon, known as antigen presentation

degraded peptides within cells are transported to the plasma membrane where T cells can then recognize them

when they are combined with MHC molecules (or cradles by MHC molecules)

- those that attack cells from outside - Bacteria
- those that attack cells from inside - viruses and intracellular bacteria

- they present peptides synthesized within cell to CD3+, CD8+ cytotoxic cell
- example: viral, tumor and parasitiv Ags

- they preent exogenouse proteins taken INTO the cell from outside and degraded (bacteria) to CD3+, CD4+ T helper cells

Endogenous pathway

antigen is endogenous (inclue viral, tumor, or even some intracellular bacterial Ags)

Peptides that associate with the class I molecules are approximately 8-10 amino acids long and are derived from partial digestion of proteins processed
in the cytoplasm

Self peptides ignored by T cells --> (healthy individual)

Display of hundreds of class I molecules complexed to antigen allows CD8 positive T cells to continuously check cell surfaces for the presence of non-self antigens.

- intracellular processing of Ag and Class I MHC interaction
- expressed on surface of cell CD3+8+ cytotoxic T cell recognizes it as foreign via TCR
- cytokines are produced, which release perforins and granzymes to the binded cell ----> apoptosis of cell

CD8+ - cytotoxic T cells ---> force cells to apoptosis

EXOgenous pathway

- Ag (bacteria) taken in from outside through phagocytosis (especially by DC)
- this leads to B cell activation --> plasma cells --> antibodies

MHC II molecules are synthesized in the ER and associated with a protein called the invariant chain (Ii)

DCs are the most potent activators of T cells, and they are excellent at capturing and digesting exogenous antigens such as bacteria

- Class II MHC and Ag peptide expressed on outside of cell and CD4+ T cell (Th) recognizes via TCR
- Th cell will proliferate or induce B cell ---> proliferate into plasma cells --> produce antibodies

• Important in tissue transplant procedures
• Classes I and II can induce graft rejection
• Appear to play a role in development of autoimmune diseases

these are autoimmine diseases

ankylosing spondylitis
- inflammation of the vertebrae of the spine

celiac disease

Type I diabetes

Rheumatoid arthritis
- inflammation of multiple joints

- HLA typing
- serologic identification

Complement and dye are used to determine whether is antigen-antibody recognition

- positive - will take up the dye and stain dark
- negative - will not take up the dye and have no color

Exposure to the following:
1. pregnancy
2. blood transfusions
3. previous transplant

• 30% to 50% of women (3 or more pregnancies) develop HLA antibodies
• antibodies present for weeks to months or possibly many years

• About 50% of patients receiving multiple blood transfusions develop antibodies against HLA molecules
• Leukocyte-reduced blood: decreases chances for patient sensitization to donor HLA antigens

• About 90% of patients develop HLA antibodies within 2 weeks of failed graft (for successful transplantation: ABO compatibility, careful HLA matching)