Phagocytosis & Antigen-presentation

Question 1

Explain mechanism of phagocytosis

Answer 1

Bacteria enters & is detected by phagocyte (neutrophil detect pathogen first)
- Pathogen recognition receptors e.g. toll-like receptors on phagocyte bind to PAMP
- Neutrophil signal macrophages to engulf- more efficient
- Macrophages produce cytokines & interferons to stimulate other immune cells that body is being attacked.

Bacterium becomes attached via its receptors
- Phagocytes engulfs & ingests bacterium
- its isolated in membrane, forming the phagosome
- Phagosome fuses w/ lysosome which contains digestive enzymes, H2O2 & reactive oxygen species- dissolve pathogens.
- Hydrogen pump also allows acid to be made in the vesicle

Some material is released by exocytosis to trigger more immune response
- Some antigen is presented to educate T & B cells

Question 2

What is a phagocyte?

Answer 2

A type of white blood cell.

3 main groups are: monocytes & macrophages; granulocytes (e.g. neutrophils, basophils & eosinophils); & dendritic cells.

Question 3

Explain the process of antigen presentation (simplified version)

Answer 3

• The phagocytic cell engulfs pathogen
• It is broken down into small pieces & presented to T-cells.
• small pieces are recognised by T-cell receptors
• If the T-cell sees the pieces of virus as ‘foreign’ it will send a signal to the B-cells
• who will then make an antibody for virus.
• This signal is called T-cell help and is crucial to the production of the correct antibodies.

Question 4

What are the different responses of antigen presentation? What does antigen presentation do?

Answer 4

2 types- response to:
- extracellular infection
- intracellular infection

Antigen presentation mediates the transition from innate to adaptive immune response.

Question 5

Explain the process of antigen presentation for an extracellular infection i.e bacteria

Answer 5

1. Pathogens are internalised in an endosome by phagocytosis- Fuse w/ lysosome.
2. bacterial antigen is degraded by lysosomal enzymes into amino acids & peptides that fit MHC class II are generated.
3. APC also synthesises new MHC class II in the endoplasmic reticulum- It then goes to Golgi body before going to endosome that contains the antigen.
4. In Golgi body, MHC class II molecule is protected from binding to self antigens by molecule called invariant chain
5. In acidic environment of endosome, the protection is removed
6. The MHC class II binding site binds to 1 of the bacterial peptides that have been created. NOTE: it can only bind to peptides!!
7. The endosome containing an occupied MHC class II molecule fuses w/ cell membrane &presents its antigen to TH1 cells (T helper cells; CD4+).

NOTE- look at digram on notes

Question 6

Explain the process of antigen presentation for an intracellular infection I.e viruses

Answer 6

1. The viral antigens are packaged into peptides by proteases, called proteasome
2. The viral peptides are carried into endoplasmic reticulum by molecule called transporter associated w/ antigen presentation (TAP).
3. viral peptides associate w/ MHC class I molecules in the ER
4. MHC class I molecules need the antigen to fold correctly & finish their biosynthesis in the Golgi body
5. MHC class I molecules are displayed on the APC’s surface transported via exocytosis
6. recognised by cytotoxic T cells (CD8+) which bind to the antigens via receptors.