Hypersensitivity

Question 1

What is the difference between the sensitization stage and the effector stage?

Answer 1

Sensitization stage is the primary immune response (clinically silent)
Effector stage is a secondary immune response

Question 2

Hypersensitivity is…

Answer 2

An excessive or abnormal secondary immune response to a sensitizing agent

Question 3

Explain the initial sensitization phase of type I hypersensitivity. How long does it take?

Answer 3

Specific allergen is present on APC eliciting a TH2 response which makes IL-4, IL-6. B cells are stimulated to make specific IgE’s for the allergen. [4 weeks]

Question 4

Explain the effector phase of type I hypersensitivity. Time frame of reaction.

Answer 4

The next time you see the allergen you get allergen cross-linking to IgE and within ~15 minutes mediators are released from mast cells or basophils.

Question 5

What are some of the immediate mediators released?

Answer 5

Histamine
Tryptase [Mast cells]
Leukotrienes
PGD2 [Mast cells] 
IL-4

Question 6

What happens 4-6 hours after a type I hypersensitivity reaction?

Answer 6

Damage to epithelium and cellular recruitment through the release of MBP/ECP/EDN, leukotrienes from Eosinophils

Question 7

Explain the difference between an allergen and an irritant.

Answer 7

Allergens produce an IgE mediated disease, they require sensitization for a response and it affects only those that are sensitized to the allergen.
An irritant does not produce an IgE mediated response, it is dose dependent and will affect everyone at high enough doses.

Question 8

What is the receptor on mast cells and basophils that allows them to bind IgE with high affinity?

Answer 8

FceRI

Question 9

Has a short half life in the serum, is only produced by mast cells and basophils and has at least 3 receptors. It is toxic to parasites, increases vascular permeability and causes smooth muscle contraction.

Answer 9

Histamine

Question 10

Remains identifiable in the serum for up to 4 hours later, only made my mast cells, single best marker of mast cell activation, leads to remodeling of connective tissue

Answer 10

Tryptase

Question 11

What are the two forms of tryptase and which is released from mast cell activation??

Answer 11

alpha – constitutively released

beta – only released with mast cell activation

Question 12

When and what cytokines are released in type I hypersensitivity?

Answer 12

Cytokines must be made by activation so their release is delayed (4-6 hours) after degranulation.

IL-4, IL-13: associated with TH2 cells and lead to Ig class switching in B cells to produce IgE
IL-3, IL-5 and GM-CSF: promote the survival and activation of eosinophils
TNF: activates endothelium and leads to adhesion molecule expression

Question 13

When and what chemokines are released in type I hypersensitivity?

Answer 13

Must be made with activation so release is delayed (4-6 hours)

MIP-1alpha (CCL3): chemotactic for monocytes/macrophages/neutrophils/T cells and eosinophils
RANTES (CCL5) and Eotaxin (CCL11): chemotactic for T cells and eosinophils

Question 14

What lipid mediators are released in type I hypersensitivity?

Answer 14

LTC4, LTD4, LTE4: lead to eosinophil migration, smooth muscle contraction, vascular permeability and mucus hyper secretion

Platelet activating factor (PAF): attracts eosinophils and other leukocytes; activates platelets

Question 15

What do you clinically consider with an elevated eosinophil count?

Answer 15

Neoplasia
Asthma
Allergy
Connective tissue disease
Parasitic disease

Question 16

What is the unique receptor on eosinophils? What does it bind?

Answer 16

CCR3
Binds Eotaxin (CCL11) -- found at sites of inflammation

Question 17

How do steroids fight acute inflammation?

Answer 17

Induce apoptosis of eosinophils
Inhibit the production of IL-5
Mediated by steroid binding to GR-alpha and inhibiting AP-1 and NFkB

Question 18

How does IL-5 effect eosinophils?

Answer 18

Prolongs survival

Question 19

What are some of the main eosinophil products?

Answer 19

Lysophospholipase: degrades lysophospholipids
MBP: mast cell activation, helminthotoxic
ECP: same as MBP plus neurotoxic
EDN: neurotoxin
PAF: bronchoconstriction and activates platelets
LTC4: bronchoconstriction, mucus hypersecretion, edema

Question 20

What are some clinical examples of type I hypersensitivity?

Answer 20

Allergic rhinitis
Asthma
Anaphylaxis
Urticaria

Question 21

Explain Type II hypersensitivity.

Answer 21

Involves antibody (IgG or IgM) binding to cell or matrix bound antigen. Then phagocytic or NK cells bind to the antibody via the FC receptor. The target tissue is then destroyed. Complement could be involved (increased binding by C receptors on phagocytic cells).

Question 22

What are some clinical examples of type II hypersensitivity?

Answer 22

Goodpasture syndrome: Ab to Type IV collagen in basement membranes of glomeruli and lung alveoli
Bullous pemphigoid: Ab to epidermal basement membrane proteins
Pernicious anemia: Ab to intrinsic factor and gastric parietal cells
Acute rheumatic fever: antibodies against strep antigens cross-react with heart

Question 23

Explain how some drugs (penicillin, quinidine or methyldopa) can cause Type II hypersensitivity disease.

Answer 23

Some drugs maybe become antigenic when bound to erythrocytes or platelets –> IgG may be made against the combined drug-cell antigen this leads to destruction of the erythrocytes (hemolytic anemia) or platelets (thrombocytopenia) which may develop with the spleen as the major site of destruction.

IgG does not react with just the free drug, only the cell bound version.

Question 24

Explain Type III hypersensitivity.

Answer 24

Occurs due to the production of IgG against a soluble antigen –> form an antigen-antibody immune complex that then activates complement – the complexes cause damage by both activation FCgammaR expressing cells as well as activating complement at sites of deposition

Question 25

Where do you commonly see damage with type III hypersensitivity?

Answer 25

Kidneys
Vessels
Joins
Skin 
--> preference for sites with increased vascular permeability

Question 26

When sensitized individuals are exposed to a specific antigen it creates a reaction in the skin. Involved activation of mast cells and other leukocytes primarily by FcgammaRIII and not complement

Answer 26

Arthus reaction

Question 27

Explain how serum sickness occurs.

Answer 27

Type III hypersensitivity
Systemic reaction from injection of large quantities of foreign protein. Occurs 7-10 days after exposure (time needed to switch from IgM to IgG). Flu-like symptoms – urticarial rash, arthritis, glomerulonephritis

Question 28

Alveolitis due to type III reaction against hay dust or mold spores

Answer 28

Farmer’s lung

Question 29

Explain type IV hypersensitivity.

Answer 29

Does NOT involve antibody.
Antigen enters the skin (usually target) and then binds to self proteins. Bound antigen is then taken up, processed by APCs and presented to T cells.
Takes 24-72 hours to develop.
Involved CD4+ Th1 cells or CD8+ T cells

Question 30

What mediators do T cells secrete during Type IV hypersensitivity?

Answer 30

IFNgamma: induces expression of adhesion molecules, activates macrophages
TNFalpha and Lymphotoxin: local tissue destruction, induces expression of adhesion molecules
IL-3 and GM-CSF: stimulate monocyte production from BM
Chemokines: recruite macrophages

Question 31

What are some examples of Type IV hypersensitivity response?

Answer 31

Tuberculin response (skin test) 
Contact hypersensitivity reactions (Rhus dermatitis)