Hypersensitivity

Hypersensitivity or allergy refers to a condition in which immune response results in excessive or exaggerated reactions leading to tissue damage, disease or even death. The term “allergy” (allos, altered; ergon, action) originally coined by von Pirquet ‘(1905) was describd as the altered reactivity of an animal to repeated contacts with a foreign antigen. Hypersensitivity occurs in certain individuals who have previous contact with an antigen and exposed to the second dose of the same antigen. Antigens involved are usually innocuous, noninjurious or bland substances, e.g. serum protein. Inducing antigens are called allergens, which may be complete antigen or hapten. Initial contact with antigen (sensitising dose) sensitises the immune system by priming or sensitising appropriate B or T cells. Subsequent contact (shocking dose) with the same antigen leads to a variety of abnormal reactions. Classification Hypersensitivity reactions are of two main types, immediate and delayed, based on time required by sensitised host to respond to the shocking dose of the antigen. Immediate form is mediated by humoral antibody and manifests in a few minutes to few hours while delayed form appears more slowly, usually after 24 hours which reaches a peak after 48-72 hours and is mediated by T cells .

Classification of hypersensitivity reactions made by Coombs and Gel (1963) is widely accepted which includes 4 major types, types Ito IV. First 3 types (I, II, III) are antibody mediated (immediate type.) and type IV is cell mediated (delayed type). Two additional types, type V and VI are recently proposed; type V is antibody mediated and type VI is both antibody and cell mediated.

Distinguishing features of immediate and delayed types of hypersensitivity

Immediate type Delayed type

Timing: Reaction appears immediately within minutes and recedes rapidly usually in one hour.

Appears slowly in 24-72 hours and lasts longer for days.

Immune response : Antibody mediated reaction Passive transfer possible by serum

Cell mediated reaction Transfer possible only with lymphoid cells or’ their extracts.

Desensitization Easy but short lived Difficult but long-lasting

Cellular response Limited polymorphonuclear infiltration

Predominantly mononuclear cell infiltration

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Type I (Anaphylactic ) reaction Anaphylaxis is a type of IgE mediated hypersensitivity reaction, which develops quickly after introduction of a large shocking dose of antigen following one or more small sensitizing doses. Anaphylaxis is the typical

example of type I hypersensitivity reaction. The term anaphylaxis (ana, against; phylaxis, protection) was. first described by Richet (1902) to describe a fatal reaction in dog that followed a second sub lethal injection of toxic extract of sea anemones. Systemic anaphylaxis is a rare event in man and occasionally observed in hypersensitive individuals by insect stings (bees), injection of foreign serum (ATS, horse serum) or penicillin. Factors influencing anaphylaxis 1. Sensitization : Sensitization may occur by any route such as injection, ingestion, inhalation or contact. Parenteral administration of antigen is most effective. Pollens, animal danders, house dust and certain foods are common offenders. Minute dose (0.1 ug) of antigen can sensitize susceptible animals. Once sensitized the individual remains so for long

period. 2. Waiting period: An interval of 2-3 weeks between sensitizing and shocking dose is required, during which cytotrophic antibody IgE produced against the antigen attaches to mast cells and basophils. It is the level of IgE produced in the host in response to a particular antigen that will determine whether an anaphylactic reaction will occur on subsequent exposure to the same antigen. 3. Shocking or eliciting dose: Shocking dose is most effective when administered intravenously; less effective

intraperitoneally or subcutaneously and least effective intradermally. When a massive shocking dose (0.1 to 10 mg) of soluble form of the same antigen, is injected rapidly intravenously, the antigen combines with cell bound antibody on mast cell rapidly. The Ag-Ab complex stimulates mast cell for prompt release of mediators that causes clinical manifestations of anaphylaxis. Mechanism of anaphylaxis IgE is the major antibody (previously known as reaginic antibody) responsible for anaphylaxis. It does not pass through placenta.

Recent studies show that certain subsets of T helper cells produce particular profiles of lymphokines which are responsible for the production of IgE by B cells. IgE is formed in response to antigenic stimulus and when large amounts are formed, the stage is set for the reaction. Mast cells are normally present in large number in sub mucosal layers of respiratory and gastrointestinal tract, skin and vascular endothelium. IgE molecules are bound to mast cells of tissues and basophils of blood. A part of the Fc region of IgE (CH3 and CH4) is involved in binding to Fc receptors (FcR) on mast cells and basophils. Mast cells acquire IgE in regional lymph nodes before migrating to tissues. On subsequent exposure to a large dose of the same antigen, antigen combines with Fab fractions of IgE bound to mast cells and basophils. Eosinophils and platelets can also bind IgE. Ag-Ab complex upsets adenylcyclase-cyclic AMP system in cell- membrane leading to degranulations of mast cells and circulating basophils and release of vasoactive amines (contained in granules) that cause anaphylactic reaction

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Mechanism of Type I Hypersensitivity

Chemical mediators Once IgE has bound to the FcR on mast cells and basophils, the signal for the release or production of biologically active molecules is triggered by cross-linking of surface-bound IgE by antigen .The effects of histamine is mediated by histamine receptors, Hi and H2. Chemical mediators are active only for some minutes after release and are then inactivated. 1. Primary mediators: Histamine, serotonin, eosinophil chemotactic factor of anaphylaxis, neutrophil chemotactic factor of anaphylaxis and various proteolytic enzymes. Primary mediators are preformed granules of mast cells and basophils. 2. Secondary mediators: SRS, prostaglandin and platelet activating factor. These secondary mediators are newly formed upon stimulation of mast cells, basophils and other leucocytes. Features of anaphylaxis 1. Reaction occurs immediately within a few seconds to few minutes following administration of shocking dose of antigen. 2. IgE antibody is synthesised probably in man only. It has got affinity for skin cells.

3. Symptoms result from constriction of smooth muscles such as bronchioles, and increased vascular permeability. 4. Duration is short. 5. Slight êllular damage occurs: 6. Artificial induction by serum of sensitised animal is possible. 7. It is not a heritable disease.

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Chemical Mediator of Anaphylaxis

Manifestations of anaphylaxis Manifestation of anaphylaxis depends on species of animal, amount of .shocking dose, site of injection and portal of entry of antigen.

A.Systemic anaphylaxis Systemic anaphylaxis is rare in man. It is a state of shock which usually follows administration of horse serum such as ATS, ADS or AGS. Systemic anaphylaxis includes active anaphylaxis, passive anaphylaxis, cutaneous anaphylaxis and organ anaphylaxis. Lunf is the principle shock organ in human.

B. Localised anaphylaxis 1. Mucosal anaphylaxis : Application of small dose of antigen on mucosal surface such as conjunctiva, nasal mucosa, respirat.9ry tract leads to rhinorrhoea, conjunctivitis and bronchospasm respectively. 2. Cutaneous anaphylaxis : It is manifested by appearance of local wheal and flare response, urticaria or angioneurotic oedema. It is induced both by qitaneous (intradermal) injection of antigen or by ingestion (ingestion is followed by absorption).

Atopy

The term atopy (atopy, out of place or strangeness) was first coined by Coca (1923) is a type I hypersensitivity reaction that occurs spontaneously in response to substances encountered in the environment in everyday life. Atopy is typified by hay fever and asthma. About 10% of the population are prone to develop sensitization to various environmental antigens such as pollens of ragweed, grasses or trees; foods and animal danders. Features of atopy 1. Atopy shows marked familial distribution and it is suspected that the sensitization is inherited. 2. Reactions occur at the site of entry of the exciting Ag. e.g. respiratory organs in bronchial asthma and hay fever. 3. Routes of entry Inhalations of pollens etc. affect lungs (bronchial asthma), ingestion leads to cutaneous eruptions or gastrointestinal disorders (fish, mushroom, milk, egg, nuts, drugs) and contact leads to local allergy (conjunctivitis).. 4. Induction of atopy is difficult by artificial means because atopens are poorly antigenic. 5. Atopy is IgE mediated hypersensitivity reaction. The atopen combines with the cell-bound IgE molecules which are already fixed on the surface of mast cells and the basophils in tissues and Ag-Ab complex stimulates the release of mediators that set of the symptoms of atopy. Examples of anaphylactic reaction 1. Food allergy on shell fish, prawn, egg, mushroom. 2. Dust allergy on pollens of ragweeds, grasses or trees, animal danders and house dust. 3. Drug allergy on penicillin, sulphonamides. Prausnitz-Kustner (PK) reaction The special affinity of IgE for skin cells forms the basis of passive cutaneous anaphylaxis in animals as demonstrated by Tests for anaphylaxis These tests are performed before administration of foreign protein or antigen to an individual 1. Skin test : An amount of 0.1 ml of diluted serum or antigen (1:10) is injected intradermally on forearm, characteristic wheal and flare appears in a few minutes in positive reaction. It is short-lasting. 2. Conjunctival test: One drop (1:10) diluted antigen when placed in conjunctival sac of one eye, redness of the eye with itching and lacrimation develops in hypersensitive individuals in 5-20 minutes. Normal saline is placed

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in control eye. Desensitization in anaphylaxis 1. Acute desensiti.sation : Small amounts of antigen are administered at 15 minutes interval for an hour or two. Small scale Ag-Ab complex formed during the process, releases chemical mediators from mast cells which are insufficient to produce a major reaction. This technique is adopted while administering drug (penicillin) or foreign serum (ATS) to a hypersensitive individual. Desensitisation is short-lasting as the hypersensitivity returns after days or weeks. 2. Chronic desensitization : It is a long-term procedure where small amount of antigen is administered at weekly internals to hypersensitive person to the antigen. Antigen stimulates production of IgG-blocking antibodies in serum which prevents the antigen later from reaching IgE antibody on mast cells, thereby blocking a hypersensitivity reaction. Desensitisation is long-lasting. Depot therapy (injection of an antigen with oil adjuvant) also leads to production of IgG-blocking antibodies against the antigen. Anaphylactoid reaction A type of reaction that resembles anaphylactic shock clinically is provoked by intravenous injection of heavy metal salts, trypsin, peptone, starch or polysaccharides. It has got no immunological basis, is a nonspecific mechanism where the offending agent appears to activate the alternative complement pathway with release of anaphylatoxins.

Type II (cytotoxic) reaction

It is a cytotoxic reaction mediated by antibodies directed towards antigens present on the surface of cell or other tissue components resulting in damage of the cell. The antibody is directed against an epitope that can be a microbial product passively adsorbed on to a cell or a drug or a self molecule. Both IgG and 1gM antibodies

are produced in type II reaction. The antibody attaches o the antigen via Fab region and serves as bridge to complement via Fc region of Ab . As a result, type II reaction may be a complement mediated lysis of cells as occurs in autoimmune haemolytic anaemia, ABO transfusion reactions or Rh haemolytic disease. Examples 1. Isoimmune reactions such as ABO transfusion reactions, erythroblastosis foetalis. 2. Autoimmune reactions, e.g. autoimmune haemolytic anaemia, agranulocytosis or thrombocytopenia in which individuals produce autoantibodies for obscure reasons, which destroy red cells, neutrophils or platelets. 3. Drug reactions are seen with penicillin, phenacetin, quinidine, sedermoid (not used now a days) and others.

4. Bacterial reactions are observed in certain salmonella and mycobacterial infections.

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Demonstration of type II reaction 1. Direct antiglobulin test (Coombs test) is usually positive 2. Agglutination test with tanned red cells, CFT, precipitation and immunofluorescence are some important diagnostic tests.

Type III (immune complex) reaction

It is a type of humoral antibody mediated hypersensitivity reaction characterized by deposition of Ag-Ab complexes in tissues (particularly on vascular endothelial surfaces), activation of complement and massive infiltration by polymorphonuclear leucocytes leading to tissue damage. Principle When an antigen

combines with its corresponding antibody, Ag-Ab complex is formed. Normally monocytes and macrophages efficiently bind and remove these immune complexes. These cells of reticuloendothelium system are efficient to remove large complexes. These cells are also able to remove smaller Ag-Ab complexes made in antibody excess but are relatively inefficient in removing smaller complexes formed in antigen excess which occasionally persist and are deposited in tissues causing several immune complex disorders. When there is a defect in the system involving phagocytes and complement in the process of removal of the Ag-Ab complex or when the system is overloaded, and the complexes are deposited in tissues, type III reactions appear .

Two typical type III reactions are Arthus reaction (localized) due to relative antibody excess and serum sickness (generalized) due to relative antigen excess.

A. Arthus reaction Arthus (1903) observed that with repeated subcutaneous injections of antigen (horse serum) into rabbits, high level of precipitating antibody appears in blood. When the same antigen is injected subcutaneously or intradermally in that animal, intense local oedema and hemorrhage develop which reaches a peak in 3-6 hours. This type of reaction is also seen in man and is called Arthus reaction. Antigen-antibody complexes formed at equivalence or slight antibody excess precipitate at the site of antigen injection producing Arthus reaction. The reaction can occur wherever the antigens are injected such as synovial joint space or pericardial sac.

B. Serum sickness Serum sickness is systemic form of immune complex diseases in which only a single

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dose of antigen is sufficient to produce the hypersensitivity reaction. After a single injection of high titre foreign serum (or drug) such as ATS, antigen is slowly cleared from the circulation and antibody production begins. The antibody level in serum reaches high enough titre after 7-12 days. But still some amount of excess antigen remains in circulating blood, which combines with Ab forming small and soluble Ag-Ab complexes. These immune complexes formed in large antigen excess become soluble, which may circulate or may be filtered out in important organs and tissues particularly in the endothelial lining of the capillaries of kidney, muscles, lymph node and joints. The immune complexes damage the tissue in the same way as in Arthus reaction. Typical serum sickness is characterized by fever, urticaria, arthralgia, lymphadenopathy and splenomegaly usually 1-2 weeks after injection of a single dose of foreign serum. Symptoms subside with the elimination of immune complex. Although manifestations of serum sickness appear several days after injection of antigen (serum), it is included under immediate type hypersensitivity reaction because once immune complex is formed, symptoms occur promptly .Some important immune complex diseases include post-streptococcal glomerulonephritis and rheumatoid arthritis1.

Type IV delayed reaction (syn. CMI)

Delayed type of hypersensitivity is a specially provoked, slowly evolving (24-72 hours), mixed cellular reaction involving lymphocytes and macrophages, where tissue damaging events is mediated by T lymphocytes and not by antibody.

It is the principal pattern of immunological response to a variety of intracellular microbial agents such as

tubercie bacilli, brucellae, viruses etc. Delayed hypersensitivity and cell mediated immunity are closely related. Type IV reaction is typified by the tuberculin skin test. Three types of delayed hypersensitivity reactions are well recognized, tuberculin (infection) type and contact dermatitis both occur within 72 hours of antigen challenge, whereas granulomatous reaction develops over a period of weeks.

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Delayed hypersensitivity reactions

DTH type Reaction time

Clinical appearance Histological appearance Antigen

Contact 48-72 hours Eczema

Infiltration of lymphocytes, and later macrophages, oedema of epidermis

Epidermal: e.g. nickel, rubber, poison ivy usually a hapten .

Tuberculin 48-72 hours

Local hardening and swelling ±, fever

Infiltration of lymphocytes, monocytes, and macrophages

Intradermal, used diagnostically: tuberculin, mycobacterial and

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leishmanial antigens

Granulomatous 4 weeks

Hardening e.g. in skin or lung - .

Granuloma consisting of epitheloid cells, giant cells, and macrophages: fibrosis ±, necrosis

Persistent Ag or Ag- Ab complexes in macrophages: or “nonimmunological” e.g. talcum powder.

Contact dermatitis type Delayed hypersensitivity may develop in a localized area of skin after repeated contact with a wide range of sensitizing materials, such as, (a) drugs : topical application of penicillin or other antibiotic in ointment or creams, (b) metals : nickel, chromium, (c) simple chemicals : hair dyes, picryl chloride, formaldehyde, dinitrochlorobenzene. cosmetics, soaps. These substances act as haptens. After percutaneous absorption, small molecules of antigen combine with skin protein and become antigenic. Cell mediated immunity is induced in skin by the antigen. Sensitization is particularly liable to occur when a chemical in an oily base (ointment or cream) is applied on an inflamed area of the skin.

Most of the antigens being fat soluble, their likely portal of entry is the dermal route along the sebaceous glands. The Langerhan’s cells of dermis are the antigen presenting cells for these antigens. They carry the antigen to regional lymph node where T lymphocytes are sensitized. Sensitized T cells proliferate and then mature in lymph node and some return to the site of entry of the offending hapten. On subsequent exposure of the skin to the offending agent, sensitiseci T lympho- cytes release lymphokines which cause superficial inflammation of skin characterised by redness, induration and vesiculation within 12-48 hours. The dermis is infiltrated predominantly by lymphocytes and few macrophages with some oedema. Subsequent avoidance of contact will prevent recurrence. Granulomatous type The most important clinical form of delayed hypersensitivity is the granulomatous hypersensitivity, that usually results from persistence of micro-organisms or other particles within the macrophages, which the cell is unable to destroy. Granulomatous hypersensitivity shows granuloma containing epitheloid cells, giant cells and macrophages e.g. Mitsuda reaction to leprosy antigens. Examples : Diseases manifesting delayed hypersensitivity include mycobacterial, protozoal and fungal diseases, such as, 1. Tuberculosis 2. Leprosy 3. Leishmaniasis 4. Listeriosis 5. Deep fungal infections (e.g. blastomycosis) 6. Helminthic infections (e.g. schistosomiasis). Type V stimulatory type reaction It is a modification of Type II hypersensitivity reaction in which an interaction of antibodies with antigens occur on cell surface that sometimes causes cell proliferation and differentiation instead of inhibition or killing. Such antibodies are non- complement fixing, directed against certain cell surface components and reacts with the key surface components such as hormone receptor of the cell. Antigen-antibody combination enhances the functional activity of affected cell. The classical example of stimulatory type of reaction is grave’s disease in which thyroid hormones are produced in excess amount.

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Thyroid stimulating antibody (LATS) is a specific thyroid autoantibody to thyroid membrane antigen. Lymphocytic infiltration is prominent in thyrotoxic glands.

Antibody-Dependant Cell-Mediated Cytotoxicity (ADCC) ADCC is mediated through natural killer NK cells and the cytotoxic mechanism is independent of complement. Target cells coated with low concentration of antibody are killed by NK cells through an extra cellular nonphagocytic mechanism. Most of them are lacking any surface marker and only a small proportion bear T markers. Antigen, after introduction into body, attaches on the target cell and induces antibody production. Ag-Ab complex forms on target cells. Natural killer cells combine with Ag-Ab complex via Fc fragment of antibody and causes lysis of the target cells (cytotoxicity). Shwartzmann phenomenon It is .not an immune response, although it has superficial resemblance to hypersensitivity reaction. It is probably a specialised type of disseminated intravascular coagulation (DIC) precipitated by endo toxin.

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Source: A Text Book pf Microbiology By P .Chakraborty.

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