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Immunology Exam 3 study guide
Chapters 5 & 6
Terms in this set (19)
BCR (B cell receptor) and TCR (T cell receptor), how are they created and where in the body are they created?
- Created in the bone marrow by B-cells
- BCR's go through somatic recombination: junctional diversity, RNA splicing for IgM and IgD, upon antigen recognition it undergoes RNA splicing to get a secreted Ig, Isotype switching, and somatic hypermutation
-Created in thymus by T cells
- TCR's are made through somatic recombination and after antigen recognition the genes encoding TCR remain unchanged
Structure of immunoglobulins and the T cell receptor (TCR):
Resembles one Fab arm of an immunoglobulin
- alpha chain = Ig light chain ; beta chain = Ig heavy chain
- 2 variable regions and 2 constant regions
- 1 antigen binding site
- membrane bound (no secreted form)
The T cell receptor (TCR) type of antigen vs Ig (BCR) type of antigen:
TCR antigen: binds to pathogen protein derivatives (peptides) that are resented by a MHC (glycoprotein) molecule on a human cell
BCR antigen: an epitope such as a protein, carbs, and lipids that could be on bacteria, viruses, parasites, or soluble protein toxins
For expression of the TCR at the cell surface what is needed?
TCR complex that is made of CD3 proteins:
- CD3 proteins on chromosome 11: CD3y, CD3 sigma, CD3 epsilon
- c/s chain on chromosome 1
- In total there are 8 polypeptides
Function of a T cell receptor (TCR):
To bind to a specific antigen presented by MHC molecule
Function of the TCR Complex:
Transmits signals to the cell's interior after antigen has been recognized by the alpha:beta heterodimer
What is the CD3 complex?
CD3y, CD3sigma, CD3epsilon
T Cell receptor antigen-binding sites: How many per TCR and number of CDR's per receptor?
1 antigen-binding site on each TCR that is located in the variable region of the alpha:beta heterodimer
- there are 3 CDR's in Valpha and 3 CDR's in Vbeta (6 in total)
T Cell receptor antigen-binding sites: T cell receptor diversity and where are the most of the differences concentrated?
TCR's are all the same on a single cell
- T cells get their diversity from gene rearrangement in the V region as well as junctional diversity (additon of P and N nucleotides)
Severe Combined Immunodeficiency Syndrome (SCID)
when you have nonfunctional RAG gene and expression of B and T cells is not possible
- leads to death during infancy due to common infections
there is a mutation in the genes encoding for the RAG protein giving it partial enzymatic activity
- causes red rashes on infant face and shoulders and is known as rapidly fatal immunodeficiency
Bare Lymphocyte Syndrome
MHC class I deficiency - the TAP protien is nonfunctional so there is no supply of peptides to the endoplasmic reticulum
- patients have less than 1% of normal MHC class I and have poor CD8 T cell responses to viruses. They suffer chromic respiratory infections
seroconversion - HIV-1 infected person begins to make detectable antibodies to the virus. The onset of overt symptoms of AIDs occurs years after seroconversion
- The rate of AIDs decreases with extent of -HLA heterozygosity as compared to individuals who are polymorphic for all HLA classes I and II loci to homozygous for 1,2, or 3.
- Homozygous MHC has 3 presenting molecules
- Hetero : 6 presenting molecules
Organization and rearrangement of the y and sigma TCR genes:
In 1-5% of T cells, y and sigma genes can be expressed instead of alpha:beta heterdimer:
- y TCR resembles alpha TCR on chromosome 7-2 Cy
- sigma TCR resembles beta TCR in between Valpha and Jalpha on chromosome 14-1 Csigma
What is unique about y and sigma T-cells?
They are not restricted by MHC molecules to have a delivery of a peptide
- The ysigma T-cell function is not conserved and there are major differences btwn species
- Bc they have fewer V gene segments they might produce less diversity but sigma TCR compensates by increasing juctional diversit by inserting more than one D gene segment which can have P and N nucleotides added between them as well as in between VD and DJ.
- This also increases potential numbers of recombination
MHC class I molecules and MHC class II molecules, what do they do? What do they activate? Which cells can present with these?
MHC molecules are antigen cells and they present a peptide to a TCR
MHC class I:
- expressed on almost all nucleated cells (not in erythrocytes)
- present intracellular peptides to CD8 (cytotoxic) T cells to undergo apoptosis
MHC class II:
- can be used by dendritic cells, pathogens, B cells, and macrophages
- present extracellular peptides to CD4 (T helper) T cells to aid in immune responses (macrophages, phagocytosis, and secreting cytokines, and B cells making plasma cells)
Structure of MHC class I and II molecules; beta2 microglobulin:
MHC I: 3 domain alpha chain (encoded by loci MHC genes) that is transmembrane and one beta2 microglobulin that is noncovalently complexed to it (not encoded by MHC loci genes)
MHC II: transmembrane alpha chain and a beta chain that each contribute to the peptide binding site and one Ig-like supporting domain.
Both chains encoded by MHC loci genes.
Peptide (length) that can bind in MHC class I and II molecules:
MHC I: 8-10 mostly
MHC II: 13-25
Peptide binding groove of MHC class I and MHC class II molecule:
MHC I: the peptide is grasped by pockets at either end of the peptide groove. Peptide may also have hydrophobic or basic residues at the carboxyl terminus complementary to pocket present in MHC I binding groove
MHC II: two ends of the peptide are not pinned down into pockets at each end. They extend out at each end of the groove
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