Antigen Processing And Presentation

Front 1

How does the B cell or T cell reocgnize an antigen?

Back 1

When a B cell or a T cell recognizes an antigen, it does not recognize the entire molecule.

Instead, the immunoglobulin receptor of a B cell or the T cell receptor recognizes distinct regions of the antigen known as the antigenic determinats or epitopes

Front 2

What does the immunoglobiin receptor of B cells recognize?

Back 2

It recognizes the epitope of an intact unprocessed antigen
-This receptor can recognize sveral types of antigens including proteins, carbohydrates, lipids or chemicals

Front 3

What does the T cell receptor recognize?

Back 3

It recognizes a processed antigen, where it is presented with an MHC molecule on the surface of an antigen presenting cell
-ONly peptides can be complexed with MHC so T cell receptors can only reocnigze peptide antigens.

Front 4

There are two different systems of processing and presentation of protein into peptide fragments. What are they?

Back 4

1) Deals with proteins that are made in the cytoplasm of a cell
2) Deals with proteins that are taken up by a cell via endocytosis or phagocytosis

Front 5

List the sequence of events for the processing and presentation of a protein made inside the cytosplasm of the cell

Back 5

These proteins include normal intracellular proteins in healthy, uninfected cells and viral proteins in virus-infected cells. These proteins are made by the cellular ribosomes in the cytoplasm of the infected cell
-1) Intraceullar Protein is degraded into peptides by a protease complex called a proteasome
2) Specific transporter proteins called TAP proteins transport peptides from the cytoplasm into the lumen of the ER
3) In lumen of ER peptide bind to MHC class I proteins
4) MHC class 1 peptide complexes are transported to the cell surface where they are presented to T cells

Front 6

List the sequence of events for the processing and presentation of a protein taken up by endocytosis or phagocytosis

Back 6

Includes normal extracellular protein (blood proteins) bacteria, toxins or viruses
-Involves MHC class II
1)Soluble proteins are taken up by endocytosis into intracellular vesicles that contain proteases
2) Proteases degrade protein into peptides
3) Bacteria and viruses are taken up by phagocytosis
4) Phagosomes fuse with lysosomes which contain lysozyme proteases and other bactericial substances which kill the bacteria
5) Proteases degrade the bacterial or viral proteins into peptides
6) Peptides dervied from these proteins remain inside vesicles and are not in the cells cytoplasm
7) MHC class II proteins leave ER in vesicles and fuse with organelles in which antigen processing occurs (endosomes, phagolysosomes) and bind peptides
8) MHC class II peptide complex is tranported to cell surface and presented to T cells
9) MHC class II proteins are expressed on antigen presenting cells such as macrophages, dendritic cells and B cells

Front 7

MHC class II proteins coming from the ER is blocked by a polypetide called ___________

Back 7

invariant chain

Front 8

When is the invariant chain released?

Back 8

In endosomes or lyosomes, it is released and the MHC class II proteins bind peptides

Front 9

List the steps for MHC Class II presentation pathway

Back 9

1) Antigens taken up from extracellular compartment are degraded into peptides in endosomes or lysosomes
2) MC Class II proteins travel from the ER to these sites of antigen proessing
3) Peptide binding site of the MHC class II proteins coming from the ER is protected by a polypetide called the invariant chain
4) In the lysosomes or endosomes, invariant chain is digsted by proteases leaving a peptide called CLIP to be bound to peptide-binding groove
5) CLIP peptide is exchanged for other peptides derived from other digested proteins with the help of a molecule called HLA-DM
6) Peptide/MHC II complexes are transported to the cell surface in vesicles
7) Peptide/MHC II complexes are then displayed on the cell surface and presented to T cells

Front 10

Why do we need 2 types of MHC molecules?

Back 10

-Pathogens that infected cytosol are detected by MHC class I molecules
-Pathogens can also infected infracellular vesicles
-Proteins in intracellular vesicles are not accessible for presentation by MHC class I molecules
-Therefore, they need to use MHC class II molecules

Front 11

Why are MHC so polymorphic?

Back 11

To present as many different types of peptides as possible
-Provides protection against a wide range of pathogens
-If no MHC polymorphism, system could be easily used by pathogens to evade recognition by immune system

Front 12

How are the MHC class I molecules in the ER exposed to eptides from cytoplasmic proteins?

Back 12

The MHC class 1 proteins are prsented the lumen of the ER. TAP proteins transport peptides in the cytosol to the ER and the peptides will bind with the MHC class 1 proteins within the alpha chain of the MHC class I protein

Front 13

HOw does the MHC class II molecule become avaliable to bind Anitegens only when it is in an endosome?

Back 13

When the MHC class II molecule is in the endosome, the invariant chain which occupies the peptide binding site of the MHC class II protein is released, so this allows the MHC class II protein to bind peptides.