topic 4

Question 1

Hematopoesis of lymphocytes (basic)

Answer 1

pluripotent hematopoetic stem cell=common lymphoid progenitor=b cell, t cell=before they reach periphery they have specific antigen receptors

A single progenitor cell gives rise to many different lymphocytes, then some are eliminated b/c they are self reactive and what’s left are the mature naive lymphocytes

Question 2

Compare and contrast TCR and BCR

Answer 2

They have a similar structure with a constant region and a variable region. The main structural difference is that BCRs have two antigen binding sites and TCRs just one.

They differ greatly in how they function and how they engage antigens

Question 3

BCR structure

Answer 3

An antibody is simply a BCR that isn’t connected. Immunoglobulin is synonymous with antibody.

They have 2 antigen binding sites both with a heavy chain and a light chain. Both chains have a variable region and a constant region. The heavy chain is longer. The part that binds the membrane is the Fc region and the part that binds the antigen in the Fab region.

Question 4

Hypervariable regions

Answer 4

Within the variable regions, there are 3 regions CDR1,2,3 (or complement determining region) that vary even more.

Question 5

What do Fab and Fc regions determine?

Answer 5

The Fab region is involved in antigen recognition.

The Fc region dictates its isotype and thus biological activity (it tells other cells, NK/macrophage, how to respond)

Question 6

What are the five different kinds of isotypes? What are they like? What is their function?

Answer 6

IgA-relatively common, dimer, mucosal immunity

IgD-trace, no immunity, wont’ talk about it

IgE=rare, monomer, won’t talk much about it

IgG-high conc. monomer, opsonization, complement activation, immunity in blood, etc.

IgM-Pentamer, slighly common, complement activation, naive b cell antigen receptor.

Question 7

Some functions of antibodies

Answer 7

Bacterial toxins-cells with receptors grab the toxins, antibodies then neutralize them, and they’re then ingested by macrophages

Whole bacteria in EC space-They are opsonized or surrounded by antibodies and then taken up by macrophages

Bacteria in plasma-The complement system is activated, opsonization by antibodies, then lysis and ingestion.

Question 8

What does Rituximab do and how does it work?

Answer 8

Like an antibody. Bind CD20 on B cells (in cancer) as if B cell were antigen, then with Fc, it binds NK cells to destroy the B cells.

Question 9

Antibody/Antigen binding factors

Answer 9

Depends on sequence of amino acids of antigen but also on size and shape of the antigen epitope.

Question 10

Forces involved in antigen antibody binding

Answer 10

electrostatic, hydrogen, VDW, hydrophobic

Question 11

What regions determine Ab specificity

Answer 11

In the variable region, at the very tip, there are loops which are the CDR1-3 regions. They have hypervariabiliity.

Question 12

TCR structure

Answer 12

TCR protein resembles one Fab region of a BCR. Heavy chains are equivalent to Beta chains and light to Alpha chains. It also has CDR1-3 regions.

Question 13

How do TCR engage antigens?

Answer 13

TCR don’t engage antigens directly; Rather, a peptide (9-12aa long) epitope must be located on the surface of another cell in an MHC. It depends only on sequence not shape. CDR-3 is in the middle and binds with the peptide and CDR1/2 are on the periphery and bind with the MHC.

Question 14

What delivers specificity to BCR?

Answer 14

The germline DNA contains various L, V, J, and C genes in the light chain and various L,V,D,J,and C in the heavy chain.

There is somatic recombination. then in the light chain one V meets up with one J or in the heavy, one D meets up with one J then the DJ meets up with one V.

Transcription occurs. These regions are spliced together and translation occurs.

Question 15

What delivers specificity to TCR

Answer 15

The same process occurs, with Beta being like the heavy chain and alpha like the light chain.

Question 16

Development of lymphocytes and antigen receptor formation

Answer 16

The common lymphoid progenitor forms Pro B/T cells.

They express either a B chain or a heavy chain. If they don’t express a viable one, they die.

This pre B/T cell with one chain of a receptor then produces the other chain to form a complete receptor. If they can’t do this, they die. At this point, they are an immature B/T cell.

If a B cell recognizes self in any way, it is negatively selected.

If a T cell doesn’t recognize anything at all, or if it recognizes self too strongly, it is destroyed. For T cells, a weak antigen recognition is needed b/c it must recognize MHC but not kill self cells.

If these cells are positively selected, they are then mature T/B cells.

Question 17

How to create hyper variable regions and imprecise coding joint?

Answer 17

With just the great amount of different V,D,J gene segments, you can get 10^6 different kinds of receptors, but with the creation of hypervariable regions, you can get 10^11 or 10^16 for heavy chains

Recombination Signal sequences (RSS) are either 23 or 12 base pairs. They are located at the ends of V, D, or J. They recruit RAG1/2 to create hairpin and take out the area in between them. Then, other enzymes open things up, and randomly add nucleotides on the N-end. This creates an imprecise coding joint. This is what creates the hyper-variable region CDR-3 by random N-nucleotide addition rather than nucleotides from the genome.

Question 18

TCR Hyper-variable regions binding MHC

Answer 18

CDR3 is the hypervariable region created by the random nt addition. It is located in the middle of the antigen binding site and binds with the peptide. CDR1/2 are on the periphery and bind with the MHC.

Question 19

What is allelic exclusion?

Answer 19

There are two types of light chains, lambda and kappa. There is one type of heavy chain.

In a cell, once a functional (non-self reactive, etc.) chain is expressed, it inhibits the expression or rearrangement of the other cell. Therefore, the heavy chain can be either from the mother or father. If the maternal lambda works, the other small chains are inhibited. If not, another one is tried, but in the either, only one light chain can be used,either from the father or mother and either the lambda or kappa.

Heavy chain gets a couple chances to get it right, light chain gets more.

Question 20

Negative and positive selection in t cell development

Answer 20

A stem cell forms a double negative (no CD4/CD8) pro-T cell.

It goes through a similar process as the b cell first finding a beta chain that is functional when paired with a pre-alpha chain forming a pre-TCR. If the pre-TCR is functional (beta selection), it is paired with an alpha chain.

Once a beta and alpha chain are paired that work, it becomes a double positive (both CD4/CD8) immature T cell.

If the beta/alpha combo doesn’t work, it is not positively selected and is eventually destroyed.

If the beta/alpha combo shows a weak recognition for class II MHC + peptide, it becomes a mature CD4+ T cell and is thus positively selected

If the beta/alpha combo shows a weak recognition for class I MHC + peptide, it becomes a mature CD8+ T cell and is thus positively selected.

If the beta/alpha combo shows a strong recognition for class I or II MHC, it is negatively selected and destroyed.