1. Over-reactions of the immune system
Dr Kathy Triantafilou
University of Sussex
School of Life Sciences

2. Reactions of the immune system
The immune system possesses recognition events that distinguish molecular components of infectious agents from those of the human body
Besides infectious agents, humans come into contact with numerous other molecules that are equally foreign but do not threaten health
These molecules are derived from plants and animals that are present in the environment where we live

3. Over-reactions
In some circumstances, molecules stimulate the adaptive immune response and the development of immunological memory
on subsequent exposures to the antigen the immune memory produces inflammation and tissue damage
The person feels ill, as though fighting an infection, when no infection exists
These over-reactions of the immune system to harmless environmental antigens are called hypersensitivity or allergic reactions

4. Gell and Coombs classification
P.G.H. Gell and R.R.A. Coombs proposed a classification system for hypersensitivity reactions:
Type I
Type II
Type III
Type IV

6. Type I hypersensitivity
Antigens (allergens) induce a humoral immune response
commonly cause by inhaled antigens (i.e. plant pollen)
This immune response results in the generation of antibody-secreting plasma cells and memory cells
The plasma cells secrete IgE
this class of antibody binds with high affinity to Fc receptors (mast cells, basophils, etc)
these IgE-coated cells are said to be sensitised

8. Degranulation
Exposure to the same allergen later cross-links the membrane bound IgE on sensitised mast cells and basophils
This causes degranulation of these cells
The pharmacologically active mediators released from the granules act on surrounding tissue causing:
vasolidation and smooth muscle contraction
either systemic or localised (depending on the extent of mediator release)

9. Components of Type I Allergens IgE antibodies mast cells and basophils
IgE binding Fc receptors
IgE-mediated degranulation
receptor crosslinking
Mediators
histamine
Leukotrienes, postaglandins and cytokines

12. Allergens IgE responses are mounted against parasites
Some persons, however have an abnormally called atopy:
hereditary pre-disposition to the development of hypersensitivity reactions
IgE regulatory defects suffered by atopic individuals allow non-parasitic antigens to stimulate inappropriate IgE production
Allergen refers specifically to non-parasitic antigens capable of stimulating type I hypersensitivity reactions

13. Allergens
Common allergens include rye grass pollen, ragweed pollen, codfish, birch pollen, timothy grass pollen, and bee venom
What makes these agents allergens?
Allergens possess diverse properties
most are small proteins (15,000-40,000)
no common chemical properties
allergenicity is a consequence of a series of interactions involving:
dose, sensitising route, genetic condition of the individual

14. IgE
The existence of a human serum factor that reacted with allergens was first demonstrated by K. Prausnitz and H. Kustner in 1921
The response that occurs when an allergen is injected into an individual is called a P-K reaction
In the mid 1960s K. and T. Ishizaka isolated the new isotype of antibody, IgE

15. IgE
Serum levels in normal individuals are in the range of mg/ml
IgE was found to be composed of two heavy chains and two light chains with a combined molecular weight of 190,000
It has an additional constant region than IgG
This additional domain changes the conformation of the molecule and enables it to bind to receptors on mast cells and basophils
Half-life in the serum of 2-3 days, once bound to receptors is stable for a number of weeks

16. Mast cells and basophils
Blood basophils and tissue mast cells can bind IgE
Mast cells are found throughout the connective tissue, near blood and lymphatic vessels
skin and mucous surfaces of the respiratory and gastrointestinal track (10,000 mast cells per mm of skin)
mast cell populations in different sites differ in the types and amounts of allergic mediators they contain

17. IgE-binding Fc receptors
The activity of IgE depends on its ability to bind to a receptor specific for the Fc region of the heavy chain
Two classes of Fc receptors:
High affinity receptor (FceRI)
mast cells and basophils (40,000-90,000 receptors on a cell)
binds with 1000 fold higher affinity
Low affinity receptor (FceRII)

18. High affinity receptor (FceRI)
The high affinity receptor contains four polypeptide chains:
an a, a b chain and two identical g chains
Displays immunoglobulin-fold structure, and thus belongs to the immunoglobulin superfamily
The a chain binds the IgE molecules
The b chain spans the membrane four times and is thought to link the a to the g homodimer
The g chains contain ITAMS similar to CD3

20. Low affinity receptor (FceRII)
The low affinity receptor (CD23) is specific for the CH3/CH3 domain of IgE
It has a lower affinity for IgE
Allergen crosslinkage of IgE bound to FceRII has been shown to activate B cells, alveolar macrophages and eosinophils
When this receptor is blocked, IgE secretion by B cells is diminished
A soluble form of the receptor exists that has been shown to enhance IgE production by B cells
Sensitised individuals have higher levels of CD23

22. Receptor crosslinkage
IgE-mediated degranulation begins when an allergen crosslinks IgE that is bound to the Fc receptor on a mast cell or basophil
the binding of IgE to FceRI has no effect on the target cell
It is only after the allergen crosslinks the fixed IgE-receptor complex that degranulation begins
monovalent antigens can not crosslink and thus can not trigger degranulation

24. Intracellular events leading to degranulation
The cytoplasmic domains of the b and g chains of the FceRI are associated with protein tyrosine kinases (PTKs)
Crosslinking of the receptor results in the phosphorylation of tyrosines within the PTKs
Within 15 sec after crosslinking, methylation of various membrane phospholipids is observed, resulting in the formation of Ca2+ channels
An increase in Ca2+ channels reaches a peak within 2 min

26. Ca2+ channels
The Ca2+ increase eventually leads to the formation of arachidonic acid which is converted into two classes of mediators:
postaglandins
leukotrienes
The increase of Ca2+ also promotes the assembly of microtubules and the contraction of microfilaments (necessary for the movement of granules to the cell surface)

27. Mediators
The manifestation of the type I hypersensitivity reactions are related to the biological effects of the mediators released from the granules
The mediators can be classified as:
primary mediators
produced before degranulation (histamine, proteases, eosinophil chemotactic factor, neutrophil chemotactic factor and heparin)
secondary mediators
after degranulation (platelet activating factor, leukotrienes, postaglandins, cytokines

28. Histamine Is formed by decarboxylation of the amino acid histidine
Histidine is a major component of mast cell ganules, accounting for 10% of the granule weight
Once released, it binds to specific receptors on various target cells
Three types of histamine receptors have been identified: H1, H2, and H3
binding to the receptors induces contraction of intestinal and bronchial smooth muscles, increased permeability of venules, and increased mucus secretion

29. Leukotrienes and postaglandins
Secondary mediators which are not formed until the mast cell goes through degranulation, and enzymatic breakdown of membrane phospholipids
Longer time for the biological effects to become apparent
Their effects are more pronounced and longer lived than histamine

30. Leukotrienes and postaglandins
bronchoconstriction
increased vascular permeability
mucus production
1000x more potent as bronchoconstrictors than histamine
prolonged bronchospasm and buildup of mucus (asthmatics)
Postaglandins
bronchoconstriction

31. Cytokines
Cytokines released from mast cells and eosinophils contribute to the clinical manifestation of type I hypersensitivity
Human mast cells secrete IL-4, IL-5, IL-6 and TNF-a
These cytokines alter the local environment leading to the recruitment of inflammatory cells
IL-4 increases IgE production by B-cells
IL-5 is important in the recruitment of eosinophils
TNF-a contribute towards the shock in anaphylaxis

33. Consequences of type I Systemic anaphylaxis Localised anaphylaxis
Allergic Rhinitis
Asthma
Food allergies
Atopic dermatitis

34. Systemic anaphylaxis
A shock-like (often fatal), whose onset occurs within minutes of a type I hypersensitivity reaction
This was the reaction observed by Portier and Richet
Caused by venom from bee, wasp, hornet and ant stings; drugs such as penicillin, insulin and antitoxins, seafood and nuts
Epinephrine is the choice of drug for anaphylaxis (counteracts the effects of mediators by relaxing the smooth muscle, and reducing vascular permeability

35. Localised anaphylaxis
The reaction is limited to a specific target tissue or organ
Often involving epithelial surfaces at the site of allergen entry
The tendency to manifest localised anaphylactic reactions is inherited and is called atopy
atopic allergies afflict about 20% of the population

36. Asthma Common localised anaphylaxis is asthma
There are two types of asthma:
allergic asthma
airborne or blood-borne allergens, such as pollen, dust, fumes, insect products or viral antigens trigger an asthmatic attack
intrinsic asthma
induced by exercise, cold, independently of allergen stimulation

37. Asthma
Like hay fever, asthma is triggered by degranulation of mast cells with release of mediators
Instead of occurring in the nasal mucosa, the reaction develops in the lower respiratory tract
The resulting contraction of the bronchial smooth muscles leads to bronchoconstriction
Airway edema, mucus secretion, and inflammation contribute to the bronchial constriction and to airway obstruction

38. Asthmatic response The asthmatic response can be divided into:
early response
occurs within minutes of allergen exposure and primarily involves histamine, leukotrienes and postaglandin
bronchoconstriction, vasolidation, and some build-up of mucus
late response
occurs hours later
involves IL-4, IL-5, IL-16, TNF-a, eosinophil chemotactic factor (ECF) and platelet activating factor (PAF)

39. The overall effects is to increase endothelial cell adhesion as well as recruit inflammatory cells into the bronchial tissue
the inflammatory cells are capable of causing significant tissue damage
this lead to the occlusion of the bronchial lumen with mucus, proteins and cellular debris, thickening the basement of the epithelium and hypertrophy of the bronchial smooth muscles

40. Food allergies
Various foods can cause localised anaphylaxis in allergic individuals
allergen crosslinking of IgE on mast cells along the upper and lower gastrointestinal track can induce localised smooth muscle contractions and vasolidation
this leads to symptoms such as vomiting and diarrhea

41. Atopic dermatitis
Atopic dermatitis (allergic eczema) is an inflammatory disease of skin that is frequently associated with a family history of atopy
The disease is observed more frequently in young children
Serum IgE levels are often elevated
The allergic individual develops skin eruptions that are erythematous
The skin lesions have Th2 cells and an increased number of eosinophils

42. Late-Phase reaction
As the reaction begins to subside, mediators released during the course of the reaction often induce a localised inflammatory response, called the late-phase reaction
It develops 4-6 hours after the type I reaction and persists for 1-2 days
Characterised by infiltration of neutrophils, eosinophils, macrophages, lymphocytes and basophils
Mediated by cytokines such as TNF-a, IL-1, IL-3, IL-5

44. Detection of type I Skin testing
Small amounts of potential antigens are introduced at specific skin sites either by intradermal injection or by superficial scratching
a number of tests can be applied to the site on the forearm or back
If the person is allergic, local mast cells degranulate and the release of histamine produces a wheal and flare within 30 min

45. Skin test Advantages inexpensive large number of allergens tested
Disadvantages
sometimes sensitises the allergic individual to new allergens
rarely induces systemic anaphylactic shock
a few manifest a late-phase reaction

47. Detection of type I Another method is to determine serum levels of IgE
Using the radioimmunosorbent test (RIST)
Patient’s serum is reacting with agarose beads or paper disks coated with rabbit anti-IgE

50. Therapy of type I
Identify the offending allergen and avoid contact if possible
removal of house pets, dust-control measures, or avoidance of offending food
elimination of inhalant allergens (such as pollen) is impossible
immunotherapy with repeated injections of increasing doses of allergens (hyposensitization) has been known to reduce the severity of type I

51. Therapy of type I
Antihistamines have been the most useful drugs for symptoms of allergic rhinitis
They bind to the histamine receptor and block the binding of histamine
The H1 receptors are blocked by the classical antihistamines, whereas the H2 receptors are blocked by a newer class of antihistamines
Several drugs block release of allergic mediators by interfering with biochemical steps in mast-cell activation

52. Therapy of type I
Disodium cromoglycate prevents Ca influx in mast cells
theophylline is commonly administered to asthmatics orally or through inhalers (blocks degranulation)
Cortisone and other anti-inflammatory drugs have been shown to reduce type I reactions

54. Type II hypersensitivity (Antibody-mediated cytotoxic)
Involves antibody-mediated destruction of cells
This type is exemplified by blood transfucion reactions
Host antibodies react with foreign antigens on the incompatible transfused blood cells and mediate destruction of those cells
Antibodies mediate cell destruction by activating the complement system or though antibody-dependent cell-mediated cytotoxicity (ADCC) (cytotoxic cells bind to the Fc region of antibodies on target cells)

55. Transfusion reactions
Antibodies to the A, B, and O antigens on red blood cells are usually IgM class
An individual with blood group A has antibodies against B in their blood
If a type A individual is accidentally transfused with blood containing type B cells, the anti-B antibodies will bind to the B blood cells and mediate their destruction by means of complement-mediated lysis

57. Transfusion reactions
Transfusion of blood into a recipient possessing antibodies to one of the blood-group antigens can result in a transfusion reaction
massive intravascular hemolysis (can be immediate or delayed)
Reactions that begin immediately are associated with ABO incompatibilities, which lead to complement-mediated lysis
within hours, free hemoglobin can be detected in the plasma, filtered through the kidneys, some of it gets converted into bilirubin (high levels are toxic)

58. Delayed hemolytic reaction
Occurs in individuals who have received repeated transfusions of ABO-compatible blood that is incompatible for other blood groups
The reaction develops within 2-6 days after transfusion
The transfused blood induces clonal selection and production of IgG against a variety of receptors
Blood group antigens that cause this: Rh, Kidd, Kell, and Duffy
Symptoms: fever, low hemoglobin, increased bilirubin, jaundice and anemia

59. Hemolytic disease of the newborn
Develops when maternal IgG antibodies specific for fetal blood-group antigens cross the placenta and destroy fetal red blood cells
Severe hymolitic disease of the newborn, called erythroblastosis fetalis, most commonly develops when an Rh+ expressed an Rh antigen on its red blood cells that the Rh- mother does not express

60. Hemolytic disease of the newborn
During pregnancy, fetal red blood cells are separated from the mother’s circulation by a layer of cells called the trophoblast
During her first pregnancy with an Rh+ fetus, an Rh- mother is usually not exposed to enough antigen to activate her Rh-specific B-cells
At the time of delivery separation of the placenta from the uterine wall allows large amounts of fetal blood to enter the mother’s circulation
The fetal red blood cells activate the Rh-specific B-cells of the mother

61. The secreted IgM antibodies clear the fetal red blood cells from the mother’s circulation, but the memory cells remain
A subsequent pregnancy with a Rh+ fetus can activate the memory cells, which results in secretion of IgG anti-Rh antibodies which cross the placenta and damage the fetal red blood cells
Mild to severe anemia can develop in the fetus, sometimes fatal

62. Prevention
Hemolytic disease of the newborn caused by Rh incompatibility can be almost entirely prevented by administering antibodies against the Rh antigen to the mother within hours after the first delivery
These antibodies are called Rhogam
They bind to fetal red blood cells that have entered the mother’s circulation and facilitate their clearance before B-cell activation

64. Therapy
If hemolytic disease develops, the treatment depends on the severity of the reaction
For a severe reaction, the fetus can be given an intrauterine blood-exchange transfusion
This replaces the fetal Rh+ cells with Rh- cells
This transfusion is given every days until delivery
In less severe cases, a blood-exhange transfusion is not given until after birth

65. Drug-induced hemolytic anemia
Certain antibiotics (penicillin, cephalosprin, and streptmycin) can absorb nonspecifically to proteins on RBCs
In some patients these complexes induce formation of antibodies, which then bind to the cells and induce complement-mediated lysis and thus progressive anemia
When the drug is withdrawn the hemolytic anemia disappears

66. Type III hypersensitivity (immune-complex-mediated)
The reaction of antibody with antigen generates immune complexes
Generally this complexing of antigen with antibody facilitates the clearance of antigen by phagocytic cells
In some cases, large amounts of immune complexes can lead to tissue damaging type III hypersensitivity reactions

67. immune-complex-mediated
Large amounts of immune-complexes are carried and deposited at different sites
The deposition of these complexes initiates a reaction that results in the recruitment of neutrophils to the site
The tissue there gets injured as a consequence of the granular release by the neutrophils
When antibodies or other proteins from non-human species are given therapeutically to patients, type III reactions are the potential side-effect

69. Type IV (TDTH-mediated) Hypersensitivity
Type IV reactions develop when antigen activates sensitised TDTH cells
These cells are generally TH1, although sometimes Tc
Activation of TDTH cells by antigen on appropriate antigen-presenting cells results in the secretion of various cytokines, such as IL-2, interferon gamma, etc)
The overall effect is to draw macrophages into the area and activate them, promoting increased phagocytic activity and increased conc. of lytic enzymes

70. Type IV
As lytic enzymes leak out from the macrophages into the surrounding tissue, localised tissue destruction can ensue
These reactions typically take hours to develop, the time required for the accumulation of macrophages
The hallmarks of type IV are the delay in time required for the reaction to develop and the recruitment of macrophages as opposed to neutrophils

71. Type IV
Many contact dermatitis reactions, including responses to formaldehyde, phenol, nickel, various cosmetics and hair dyes, poison oak and poison ivy are mediated by TDTH cells
Most of these substances are small molecules that can complex with skin proteins
This complex is then internalised by APCs in the skin, processed and presented together with an MHC class II molecule, causing activation of T-cells

72. Poison oak
A pentadecacatechol compound from the leaves of the plant complexes with skin proteins
When T-cells react with this compound displayed by local APCs they differentiate into sensitised TDTH cells
A subsequent exposure to this compound elicits activation of TDTH cells and cytokine production
48-72 hours after the second exposure, macrophages are recruited to the site
Activation of the macrophages and release of their lytic enzymes leads to a IV reaction