Immunology

Question 1

does the eye contain many commensal bacteria?

Answer 1

few

Question 2

describe the physical defences of tears

Answer 2

act to flush out bacteria etc

anti-adhesive mucous layer

Question 3

describe the chemical defences of tears

Answer 3

• Lysozyme breaks down bacterial cell walls which are made of peptidoglycan
• Lactoferrin and transferrin have antibacterial properties
• Tear lipids
• Angiogenin has an antimicrobial effect within tear film
• Secretory IgA prevents attachment
• Complement
• IL-6, IL-8 and MIP are antimicrobial proteins that recruit leucocytes

Question 4

describe the cellular defences of tears

Answer 4

have neutrophils, macrophages and conjunctival mast cells

Question 5

what is the principle APC in the external eye

Answer 5

Langerhans cells - rich in class II MHC molecules (produce cytokines)

Question 6

where are Langerhans cells found in the external eye

Answer 6

abundant at the corneo-scleral limbus

few in the peripheral cornea

absent from central third of cornea

Question 7

what is the only part of the eye with lymphatic drainage

Answer 7

conjunctiva

Question 8

describe the immune system in the conjunctiva

Answer 8

• Dendritic cells act as APCs
• MALT tissue: small concentrations of lymphoid tissue in submucosal membrane sites. Macrophages, Langerhans cells and mast cells frequent the MALT, eosinophils and neutrophils are there if they have been recruited
• Commensal bacteria present

Question 9

immune privilege

Answer 9

cells are able to tolerate the induction of an antigen without eliciting an inflammatory immune response

Question 10

where in the body is immune privilege exhibited

Answer 10

brain/CNS, testes, placenta, foetus and eyes

Question 11

is immune privilege an active or passive process

Answer 11

active - antigens from immune privileged regions interact with T cells to induce tolerance rather than a destructive response

Question 12

what physical factors contribute to immune privilege

Answer 12

lack of lymphatic drainage, limiting the immune system’s ability to enter the site

blood-tissue barrier

lack of blood vessels

Question 13

what other factors contribute to immune privilege

Answer 13

• immunological ignorance:* low expression of MHC class I and no expression of MHC class II
• inhibition:* development of an intraocular immunosuppressive microenvironment by production of immunosuppressive cytokines and surface molecules that inhibit complement activation ​
• ACAID*

Question 14

ACAID

Answer 14

peripheral tolerance to ocular antigens: induces a systemic tolerance to a foreign antigen in the ocular microenvironment by actively downregulating the immune response

- generation of primed CD4+ T and B cells that produce non-complement fixing antibodies and inhibit delayed hypersensitivity B cells that secrete conplement fixing antibodies

Question 15

what is the purpose of ACAID

Answer 15

Protects the eye and visual axis from the collateral damage of an immune response to infection by suppressing a response to infection that could potentially be damaging in the future

Question 16

Sympathetic Ophthalmia

Answer 16

A rare, bilateral granulomatous uveitis due to trauma (most common) or surgery to one eye. Can lead to blindness.

Question 17

describe the pathogenesis of Sympathetic Ophthalmia

Answer 17

It is thought to be an autoimmune inflammatory response towards ocular antigens. The immune system, which is not normally exposed to ocular proteins, is introduced to the contents of the eye following traumatic injury. Once exposed, these antigens are sensed as being foreign and are attacked by the immune system, with an elevated response.

The primary mediators are thought to be T cells. There is an initial infiltrative wave of CD4+ helper T cells, followed later by a wave of CD8+ cytotoxic T cells.

The injured eye is known as the ‘excited eye,’ and the fellow eye is known as the ‘sympathizing eye.’ Clinically, both eyes appear the same and the exciting eye can only be identified through the history.