MIIM - Ocular Defences: The Immune System II - Week 4

Question 1

What antibody is the principle specific protective mechanism operating at the conjunctival surface?

Answer 1

sIgA (secretory IgA)

Question 2

Describe in 6 steps how IgA is transported across the mucosal epithelium in eyes.

Answer 2

• sIgA dimers are secreted into the interstitial space by plasma cells.
• They migrate to the lacrimal glands.
• They bind to membrane receptors on the surface of acinar epithelial cells.
• The sIgA-receptor complex is endocytosed, and bound to a transport vesicle.
• It fuses to the plasma membrane at the luminal surface.
• The dimer is cleaved from the receptor and released into the acinar lumen.

Question 3

How are sIgA dimers protected from cleavage once in the acinar lumen?

Answer 3

When the dimer is cleaved from the receptor, there remains a secretory component from the receptor, which protects against proteolytic enzymes.

Question 4

Briefly describe the three layers of the tear flim.

Answer 4

On the corneal epithelium is the mucin layer which has antibodies anchored by their Fc regions to the epithelium.
On top of this is the aqueous layer.
On top of this is the lipid layer.

Question 5

Can intracellular organisms be eliminated by B-cells?

Answer 5

B-cells produce antibodies, and these are too large to enter cells, so B-cells alone cannot eliminate intracellular organisms.

Question 6

How do T cells know whether a self cell has a pathogen in it?

Answer 6

Proteins within the cell are catabolised into peptides, and become complexed to MHC molecules, and transported to the cell surface.
T cells have receptors which can detect these peptides.

Question 7

What kind of epitopes can T cell receptors recognise, and how does this relate to antigen presenting cells (or dendritic cells)?

Answer 7

They can only recognise short linear epitopes, and thus there is a need for them to be degraded into small peptides. This is one of the functions of antigen presenting cells.

Question 8

What are CD8+ and CD4+ cells?

Answer 8

CD4+ cells are helper T cells

CD8+ cells are cytotoxic T cells

Question 9

What is the function of cytotoxic T cells?

Answer 9

To bind and kill tissue cells infected with intracellular pathogens and neoplastic cells where they have foreign peptides on their MHC markers.

Question 10

Name the four functions of helper T cells.

Answer 10

To help B cells produce antibodies
Activate cells of the innate immune system
Aid in the production of cytotoxic T cells
Regulate unwanted responses

Question 11

Briefly describe how the CD nomenclature was made.

Answer 11

Certain molecules are found only on a subset of leukocytes.

Molecular markers are given a cluster of differentiation, CD for short.

Question 12

Describe how B-cells are activated compared to how T-cells are activated.

Answer 12

B-cells - free antigens drain to a lymph node and activate B-cells
T-cells - antigen presenting cells internalise an antigen/pathogen and migrate to a lymph node to activate T-cells.

Question 13

Describe the two kinds of MHC markers, and on what cells each type is found.

Answer 13

MHC Class II - found only on antigen presenting cells

MHC Class I - found on almost all nucleated cells

Question 14

True or false

MHC markers expressed on the cell surface always contain a peptide.

Answer 14

True

Question 15

What gene is responsible for MHC markers, what chromosome, and what expression type?

Answer 15

HLA gene, found on the short arm of chromosome 6. They are co-dominantly expressed.

Question 16

What two things must a T cell recognise if it is to be activated?

Answer 16

It must recognise both the peptide and the MHC marker it is attached to.

Question 17

Do all MHC markers bind the same peptides?

Answer 17

No, each one binds different groups of peptides.

Question 18

There are many different polymorphisms for MHC markers. What four things do they affect?

Answer 18

The ability to generate an adaptive response
The resistance/susceptibility to an infectious diseaseThe resistance/susceptibility to an allergic reaction
The resistance/susceptibility to an autoimmune disease

Question 19

Name three kinds of antigen presenting cells.

Answer 19

Dendritic cells
Macrophages
B cells

Question 20

Do antigen presenting cells have both MHC class I and II markers?

Answer 20

Yesd

Question 21

Describe in three steps how antigens are taken up and presented by antigen presenting cells.

Answer 21

Exogenous antigens are taken up into endosomes by the APCs
Some are present free in the cytosol called endogenous antigens
The antigen is degraded internally
They are displayed as peptides to T cells on either class I or Class II markers

Question 22

Which T cell recognises antigens found on class I markers? What about class II?

Answer 22

Class I - CD8 cytotoxic T cells

Class II - CD4 helper T cells

Question 23

What determines which class of MHC marker an antigen will be presented?
Describe for each class.

Answer 23

The location of the antigen within the APC
Endogenous end up on class I
Exogenous end up on class II

Question 24

Describe superantigen binding. What does it result in?

Answer 24

They do not require processing to activate T cells, and can bind directly to class II chains.
Results in the activation f a large number of T cell activation.