29 Hypersensitivity-II and III

Dr. M. N. Gupta

  1. Objectives
  • To know about experiments which led to the realization that anaphylaxis and allergic reactions become systemic to create a atopic individual. How allergy becomes systemic to create a atopic individual.
  • How food allergens can use gut as a gateway in development of systemic allergy.
  • How the clinical picture correlates with IgE production and histamine release.
  • To appreciate that we still do not have a complete understanding of how environmental factors can combine with genetics to produce atopy. To understand the molecular basis of various treatments of allergy.
  1. Concept Map
  1. Description

The immune system offers protection or prophylaxis. For individuals who are allergic or hypersensitive to a particular antigen, the immune response is “excessive”. Such individuals are said to be in a state of hypersensitivity.

 

As the response causes tissue damage, rather than prophylaxis, allergic responses are also called anaphylaxis and type I hypersensitivity is called the anaphylactic hypersensitivity.

 

In different animals, different organs serve as experimental systems to demonstrate anaphylactic shock. In guinea pig, it is lung. For rat it is intestine; heart and pulmonary blood vessels in rabbits , hepatic veins in dogs; and lungs and larynx in humans.

 

An injection of 1 mg of egg albumin in a guinea pig followed by another after 2-3 weeks is a good demonstration of systemic or generalized anaphylaxis. The guinea pig starts wheezing and dies of asphyxia within few minutes

 

Pathological examination shows:

  • Intense constriction of bronchioles and bronchi
  • Contraction of smooth muscles
  • Dilation of capillaries

In certain humans, similar consequences may occur following a wasp or bee sting or administration of adrenalin counters smooth muscle contraction and capillary dilation and prevent fatal consequences.

 

Sir Henry Dale discovered that administration of histamine mimics results of systemic anaphylaxis. He also discovered that uterus from a sensitized guinea pig releases histamine and contracts on being exposed to the antigen. This demonstration is called Schultz-Dale technique.

 

It was also found that a serum from such an animal can sensitize the uterus of another normal guinea pig.

 

This is how the phenomenon of allergy was discovered and role of histamine, mast cells etc  was ultimately uncovered. Medical people still prefer the term anaphylaxis and the sensitized animal called atopic. It is only much later that people discovered that “milder” form of anaphylaxis occurs in response to wide variety of substances which were termed allergens.

 

Atopy

 

Let us try to delve deeper into why only some individuals are allergic to specific substances or are „atopic‟ (sensitized). First, let us trace the origin of the various terms.

 

Atopy was first described in 1923 by Coca and Cooke and the term is used to describe clinical symptoms of type I hypersensitivity which includes asthma, eczema, hay fever and urticarial.

 

By that time, it was clear that anaphylaxis in animals discovered by Portier and Richet in 1902 had the same origin as hay fever or asthma in humans.

 

However, Coca and Portier believed that the allergic diseases in humans were different from anaphylaxis in animals. Human allergy was known to have some hereditary aspect. There were no corresponding data for anaphylaxis! Hence, Coca and Portier called these diseases “atopic diseases”. Medical books use the term „atopic‟ to denote that a particular disease has hypersensitivity as its origin.

 

Now we know that atopy, anaphylaxis, allergy, type I hypersensitivity reactions are same

.

Different terms are sometimes preferred depending upon the context.

 

Prausnitz and Kustner in 1921 found that allergy can be transferred passively by serum. Kustner who was allergic to fish injected his serum into the skin of Prausnitz who promptly developed strong skin symptoms. About 45 years later, Ishizaka‟s group showed the involvement of IgE in allergy.

 

In a multi-disciplinary subject like immunology, sometime it is confusing to find different terms in different books/papers for what appears to be same or similar phenomenon. In such cases, brief historical perspectives such as what we discussed are invaluable. Immunology is an integral part of clinical medical practice. Let us further look at atopy or sensitized state from that angle.

 

Prausnitz-Kustner (P-K) test

 

The passive mode of sensitization occurs as the serum of the sensitized animal contains IgE to the specific allergen. As this was discovered by Prausnitz and Kustner the test is named after them.

 

The serum from an allergic individual is injected into the skin of a normal individual. In the next 1-2 days, IgE diffuses towards nearby mast cells. The injection site responds with a articarial (hives) reaction if the allergen is injected there again. This is called passive cutaneous anaphylaxis.

 

Cutaneous anaphylaxis versus systemic anaphylaxis

 

Intradermal injection of an allergen results in local anaphylaxis. The response is in the form of “wheel and flare”. Wheal is edema (swelling due to serum into tissue as there is increase in permeability). Flare is erythema which is redness due to blood vessel dilation.

In many cases the wheal and flare reaction which occurs within minutes can be followed by late phase reactions which can start within hours and can last for 24 hours. The edema is more pronounced.

 

Intravenous injection, on the other hand results in distribution of allergen throughout the body and may cause systemic anaphylaxis.

 

Respiratory allergy:

 

Allergens like pollens and mite antigens are colloidal suspensions in the air and enter the body through inhalation. Apart from the nasal tract and respiratory tract mucous membranes, eye also is the contact point. Prominent respiratory allergies are hay fever and asthma. These are essentially local allergic reactions. In hay fever, the local release of vasoactive factors leads to irritation, increase in vascular permeability leads to runny nose, tears and inflammation of tissues.

 

In the upper respiratory tract the similar local reaction leads to bronchoconstriction and difficulty in breathing (dyspnea). Usually, inhalation is possible but exhalation is difficult. This over inflation of lungs causes damage in air sacs (emphysema).

 

Food Allergy:

 

Here the local allergic reactions in the intestinal walls causes smooth muscle constriction and built up of fluids. Intense discomfort follows diarrhea. Essentially (to relate it to molecular picture!) allergen binds to specific IgE on mast cells in the gastrointestinal tract to produce local reactions.

 

If the allergen enters the body when gut permeability changes due to release of mediators, the allergen-IgE complex can reach elsewhere such as joints, skin or lungs. That is why asthma is not caused by inhalation of allergens only but often due to food allergies. Eating an allergen can cause urticarial or may precipitate an asthma attack.

 

Gut of a sensitized individual acts as a gate to allergens. This is confirmed by the fact that oral intake of sodium chromoglycate after ingestion of an allergic food prevents asthma. Sodium chromoglycate is known to prevent degranulation of mast cells.

The importance of IgE level in blood

 

The serum concentration in humans of IgE is about 100 IU/mL (1 IU= 2 ng), which is 105 less than serum IgG concentration of 10 mg/mL. IgE is only 0.001 % of the total Ig in serum. Higher levels of IgE indicates atopy.

 

Genetic and environmental factors control clinical symptoms. So, a normal IgE level does nottt exclude atopy. However, if worm infection is ruled out and IgE level is high, atopic allergy is strongly indicated.

 

While positive skin correlates well with IgE level, neither correlates perfectly with clinical symptoms of atopy. That is, neither high IgE level nor positive skin test necessarily guarantee that clinical symptoms of systemic anaphylaxis will be present.

 

Regulation of IgE production

 

Tada‟s lab in early 1970s used rats immunized with DNO-ascaris along with Bordetella pertussis as adjuvant. The rise in IgE level was enhanced wit hrats from which thymus was removed/ destroyed.

 

Passive transfer of thymocytes/spleen cells from another population of primed animals suppressed the IgE production.

 

It was found out that Ts cells reduce IgE production but do not affect IgE and IgM levels.

 

Curious enough neonatal thymectomy led to loss of IgE production.

 

It is now known that in both humans and other animals Th2 cells which produce IL-4 are required for IgE production. Th1 cells producing IFN-g inhibit IgE production.

 

Th2 cells are stimulated by APCs to release IL-4 which induces isotype switch to produce IgE.

 

Responding to allergen, Th2 can inhibit Th1 response by:

 

(a) producing IL-10 which interferes with allergen presentation to Th1

(b) Inhibiting IFN-g

 

The above insights have led us to understand the role genetics may play in type I hypersensitivity. That is why some individuals respond differently from others. Studies with humans as well as other animals have been carried out.

 

Role of genetics in IgE response

 

Mutations in mice that inactivate IL-4 gene lead to abolition of IgE production even after a nematode infection.

 

Patients given IFN-a show reduction in serum IgE level. IFN-a acts similar to IFN-g but has fewer adverse effects.

 

Th2 cells also produce IL-5 which causes B-cells to produce IgA as a secretory antibody. IgA is known to stimulate eosinophile development and activation.

 

Th2 cells produced IL-4 and IL-5 is the basis of eosinophilia which is quite frequently associated with allergy. Eosinophile count is a clinical aid in diagnosis infections.

 

Animal differ in the amount of IgE they produce in response to identical exposure to allergen There are strains of mice such as SJL which do not produce high level of IgE even under optimum protocols Low responder SJL mouse makes only IgG in response to a single large dose of antigen and makes no IgG or IgE when repeated low dosage of antigen are given.

 

A high responder C3H strain mouse produces IgE with both kinds of exposure. The repeated low dosage injection mimic natural exposure.

 

As early as 1920s, there was data that allergy is hereditary. Both genetic background and high level of IgE in serum are risk factors for atopy.

 

While nonidentical twins show concordance of atopy only slightly greater than general average, even identical twins show much less than 100% concordance. This shows that heredity alone does not determine atopic state. Among known environmental factors are:

  •  Nutritional status of the individual
  •  Presence of underlying chronic infections
  •  Acute viral illness
  •  Quantity of the exposure

If allergic persons are identified by skin tests with different allergens, those testing positive show higher frequency of HLA-B8 and HLA-Dw3 but not of HLA-A1. Higher responsiveness also correlated well with higher levels of total IgE in individuals with HLA-B8. HLA-B8 also shows strong correlation with incidences of autoimmunity.

 

It is suggested that association of HLA-B8 to hyperactivity is via depressed Ts cell activity as depressed Ts is involved in IgE responses as well as autoimmunity.

 

Morphology of Mast Cells

 

Mast cells between different specie and from different sites in the same specie differ in their morphology and functional behavior.

 

Connective tissue mast cells (CTMC) stain with toluidine blue after formalin fixation of tissues. Mucosal mast cells do not show up well with this stain.

 

CTMCs are found around blood vessels throughout the body, contain large amount of histamine and heparin. Their degranulation is inhibited by sodium chromoglycate. Mucosal mast cells (MMCs) are present in gut and lung. These depend upon IL-3 and IL-4 for proliferation and their number increases during parasitic infection of the gut. They also increase in ulcerative colitis and Crohn‟s disease.

 

CTMCs number increases in the synovium of rheumatoid arthritis patients.

 

It is believed that both CTMCs and MMCs arise from a common precursor cell and local environment dictate their differentiation.

 

Tryptase and chymase are two neutral proteases which are found in mast cells in different ratios depending upon the tissue. Both proteases have been cloned and sequenced.

 

These proteases are of clinical interest. Tryptase is associated with bronchial hyperesponse and chymase controls bronchial mucus secretion in asthma. Both proteases can degrade vasoactive intestinal peptide (VIP) which mediates bronchial relaxation.

 

Mast cells are seen to infiltrate the local sites in patients. Patients with hay fever have MMCs collecting in nasal epithelium during pollen season. In asthma patients, higher number of mast cells is seen in bronchoalveolar lavage fluid.

 

Mucosal surface is the first site of contact between allergen and these superficial mast cells. This leads to release of mediators. That increases the permeability of mucosa to allergen. Allergen now contacts submucosal mast cells which now release mediators. This is why clinical symptoms amplify.

 

The structure and function of Fc receptors

 

IgE binds to mast cells and basophils via high affinity FceR1 receptor. This receptor is a member of Ig supergene family and is called classical receptor.

 

Leucocyte and lymphocytes have another receptor for Fc of IgE. This is a low affinity receptor and its structure is homologous to animal lectins.

 

FceR1 is a tetramer with ab2g composition. It has one a-chain (45 kDa) which is a glycoprotein and is involved in binding of IgE. The b-chain (33 kDa) is linked to g-chains (9 kDa) via disulphide bridges. The binding constant of IgE with this receptor is 1010 M-1. It is only the crosslinking of several IgE crosslinked on the surface which triggers degranulation of the cells.

 

FceRII (CD23) which is a 45 kDa membrane bound protein and occurs in two forms.

 

The two forms IIa and IIb differ only in the cytoplasmic part which is the N-terminal. IIa is generally expressed in B-cells and IIb is expressed on T-cells. B-cells, monocytes and eosinophils depend upon IL-4 for induction.

 

IIb+ lymphocytes and monocytes increase in circulation of some atopics especially atopic eczema. In hay fever patients both IgE level and expression of IIb on lymphocytes increases simultaneously during pollen season. Normal alveolar macrophages which express IIb receptor in the presence of IgE+ allergen release enzymes which suggests their involvement in asthma or alveolitis.

 

Mast Cell activators

 

Number of compounds can activate mast cells directly. These include calcium ionophores, mellitin, codeine, morphine. The common feature in their effect is Ca2+ influx into mast cells. Both direct triggering and via FceRI mediated triggering is illustrated in the figure.

 

We are now ready to look at the various factors involved in the development of allergy. The integrated picture had led to the concept of “allergic breakthrough”. Finally, we will look at some possibilities regarding treatment of allergy. By now, it should be clear that allergy is a generic term. All allergies have some common features yet each is distinct in its own way.

 

Allergic Breakthrough

 

The hypothesis of allergic breakthrough is just about recognition that clinical symptoms of allergy arise when overall immunological activity crosses a certain point called “allergic breakthrough”.

 

So, everybody responds to an antigen. This response, once it exceeds this point, we tend to identify it as an allergy.

 

Even genetically predisposed high IgE responder needs a sum total of concomitant factors which can be denoted by „x‟ consisting of decreased Ts cell activity, transient IgA deficiency or upper respiratory tract viral infections.

 

Atopics during upper respiratory tract virus infection display aggravation of asthma.

 

Basophils release histamine if anti-IgE is given, it crosslinks IgE bound via Fc receptors.

 

Herpes simplex virus (HSV-1), live or killed enhanced histamine release. Interferon may mediate this, interferon itself has the same effect as the viral infection.

 

Controlling Allergy

 

(A) Hyposensitization therapy

 

Injection of increasing doses of allergen often benefits. Increase in specific IgG and Ts cells activity and decrease in specific IgE is seen in serum. Specific IgG presumably blocks specific IgE mediated responses.

 

In normal rats and mice, with ovalbumin as antigen, hyposensitization resulted in decrease in IgE response indicating development of tolerance. High IgE responder did not develop tolerance. In their case, antigen specific Ts cells and their IgE mediated responses did not change.

 

We do not have clear understanding of how Be memory cells are maintained. There are possibilities that dendritic cells continue to act as APC or idiotypic network system ensures it.

 

(B) b-adrenoreceptor stimulants such as adrenalin and isoprenaline; salbutamol which causes bronchodilation; a-adrenoreceptor inhibitors such as methoxamine and phenylephrine all have been tried to control allergic reactions.

 

Antihistamines such as pyrilamine, promethazine, diphenhydramine inhibit histamine which is just one mediator. Tryptamine antagonist cyproheptadine blocks receptors for serotonin and histamine and may give relief with some patients.

 

Anti-inflammatory drugs such as salicylates, phenylbutazone and glucocorticosteroids work but are immunosuppressants.

 

IgE is believed to have evolved as a defence mechanism against parasitic worm infection which affect about one-third of world population. Allergy, anaphylaxis or hypersensitivity type I reactions shown by atopic individuals show that these individuals are caught in this crossfire as they are more vulnerable for reasons not yet completely understood but are definitely complex and consist of multiple factors.

 

Summary

  • Atopy, anaphylaxis and allergy turned out to be same phenomenon.
  • The difference between cutaneous and systemic anaphylaxis.
  • Gut can be a gateway to allergens.
  • Importance of IgE level and regulation of its production.
  • Role of genetics and environmental factors in IgE production
  • Role of allergic breakthrough
  • Controlling allergy