Immunology
HYPERSENSITITVITY
An intact immune system is essential for defending an individual against microorganisms and thus for survival; however, inappropriate or excessive activation of the immune system can be harmful to the host. These detrimental responses (usually the heightened responses) are referred to as hypersensitivity reactions. Classification of hypersensitivity reactions (Modified from Gell and Coombs) Antibody mediated hypersensitivity: Type I Anaphylactic: immediate, IgE mediated Type II Cytotoxic Type III Immune complex; Arthus: Serum sickness Type V Stimulatory hypersensitivity Cell mediated hypersensitivity: Type IV Delayed type hypersensitivity TYPE I HYPERSENSITIVITY Type I hypersensitivity is also known as immediate hypersensitivity or anaphylactic hypersensitivity because of the speed with which the immunologic reactions take place which may culminate into anaphylaxis within minutes following antigen challenge. The most common type of immediate hypersensitivity reaction is seasonal rhinitis, or hay fever. Other types of immediate hypersensitivity reactions are bronchial asthma, allergic dermatitis and some food allergies. Insect venoms and injected drugs are associated with systemic reactions involving the cardiovascular system (anaphylactic shock). For immediate hypersensitivity reactions to occur, an individual must first come in contact with an antigen and produce IgE antibody in response to that antigen. A genetic factor may play role in determining whether a given individual will produce IgE or antibody of any other class in response to an antigen. Once IgE antibody is formed, it gets fixed to mast cells in the tissues and basophils in the blood. These cells possess high affinity receptors specific for the Fc portion of the IgE antibodies. Thus, Fab portion of IgE remains exposed. The interaction of IgE with the mast cells surface is a relatively stable one and found to persist upto 12 weeks. Upon re-exposure to the sensitizing antigen, the IgE molecules on the mast cell surface become cross linked, thus activating the mast cells. The mast cell activation is accompanied by the release of intracellular granules that contain potent mediators of inflammation. It is these mediators which are responsible for symptoms in type I hypersensitivity. In addition to mast cells, the blood basophils also bind IgE and participate in immediate hypersensitivity reactions.
Mediators of Immediate Hypersensitivity These can be divided into two groups: The substances that are preformed and packaged into granules and those that are membrane derived lipids. The first group includes heparin, histamine, proteases (tryptase, chymase), eosinophil chemotactic factor and neutrophil chemotactic factor. The cumulative biological effects of these include constriction of smooth muscles in bronchioles, vascular dilatation with increased permeability and increase in nasal secretions. The second class of mediators released by mast cells as a result of allergic exposure are the newly formed mediators, which are produced by the metabolism of phospholipids within the mast cells. These induce vasodilatation, increase in vascular permeability and bronchoconstriction. The leukotrienes C4, E4 and D4 (previously known as slow reacting substances-SRS) also mediate bronchoconstriction, vasopermeability and mucus secretions. Detection Antigen is injected intra-dermally and a wheal and flare response determined within 15-30 minutes. The reaction is caused by mediators, especially histamine released from skin mast cells and is correlated with sensitivity to antigen. A skin test of this sort should be conducted if it is necessary to administer an antigen systemically to a person, because it will reveal if the person is susceptible to anaphylaxis. PCA and P-K Reactions To perform the passive cutaneous anaphylactic (PCA) reaction, serum from an anaphylactically sensitized animal is injected into the skin of a normal animal. Several hours later, when antigen is injected systemically, an oedematous and erythematous reaction develops at skin site. The antigen injection can be delayed for several days and still immediate skin reaction can be elicited. When the PCA reaction is conducted in man, it is known as Prausnitz-Kustner (P-K) reaction. P-K test for immediate hypersensitivity is performed in a normal subject who has been passively sensitized by immunoglobulin from the allergic individual. Schultz-Dale Reaction Schultz and Dale found that the uterus or a segment of ileum removed from a sensitized guinea pig, if heavily perfused, can be washed free of circulating Ig. When suspended between a fixed and movable pole in an isotonic bath solution, muscles in these tissues will contract when the sensitizing antigen is added to the bath. These contractions can be recorded on kymograph. The Schultz-Dale reaction is thus an in-vitro anaphylactic reaction that proves the need of cell bound antibody to produce anaphylaxis. Type I hypersensitivity can also be demonstrated by measurement of increased serum IgE antibodies against a particular antigen.
Anaphylactoid Reaction It may be defined as any reaction having the characteristics of anaphylaxis but not based on immunologic phenomena. These are acute, life-threatening reactions that follow the intravascular administration of non-antigenic materials. These substances include starch, organic iodine, bromophenol blue and contrast media used in urographic analysis. The reactions with contrast media are unpredictable and rare. Atopy Atopy (atopy, a strange disease) is defined as an IgE dependent allergy often arising from unknown exposure to an antigen or auto-coupling hapten. These allergies are divided into: Inhalant allergy Ingestant allergy Injectant allergy Miscellaneous TYPE II HYPERSENSITIVITY Type II hypersensitivity is often referred to as cytotoxic hypersensitivity because in these reactions IgG or IgM antibody directed against cell surface components causes damage to, or lysis of the affected cell. Complement can participate in these reactions by affecting cell lysis or through opsonisation of the antibody coated cell. The incompatible blood transfusion reaction and autoimmune haemolytic anaemia are important examples. Mechanism The critical event in any of the type II cytotoxic reactions is the binding of IgG or IgM antibody to the cell surface antigens. If the antibody is IgM, IgGl, IgG2 or IgG3, then Clq component of the complement is bound and activated. This initiates complement activation by classical pathway and terminates in lysis of the affected cell. Damage can also be caused by means which are independent of complement. For example, an antibody coated cell can lead to destruction of target cell by the killer cell through antibody dependent cell mediated cytotoxicity. A classic type II cytotoxic hypersensitivity disease is erythroblastosis fetalis, also known as haemolytic disease of the newborn.
TYPE III HYPERSENSITIVITY This is also known as immune complex hypersensitivity since the tissue damage results from the deposition of immune complexes in tissue. Complement participates in the tissue damage that occurs in these reactions through the anaphylactic properties of the split products of complement, C3a and C5a. Some examples of type III hypersensitivity include SLE, rheumatoid arthritis, glomerulonephritis and cutaneous vasculitis. Mechanism In this type of hypersensitivity, following the formation of Ag-Ab complex, complement is bound and activated through classical pathway. This results in the release of anaphylatoxins. These induce basophils to release histamine and increase vascular permeability. Polymorphs bind to immune complex through Fc and C3b receptors and phagocytoze the immune complexes. In the process of phagocytosis, polymorphs release some of its enzymes which cause local tissue damage. The entire process may take place within the blood vessel walls or along the glomerular basement membrane in the kidney. Immune complexes which are continuously being formed in body are removed from the circulation by macrophages located primarily in the liver, spleen and lungs. Factors influencing the immune complex mediated tissue damage include: Size of immune complex Local vascular permeability
Deposition of immune complexes at certain favoured sites in the body. Sites where biological filtration takes place such as renal glomerulus and choroid plexus are particularly susceptible to immune complex mediated damage.
Lesions of type III hypersensitivity Due to locally formed complexes o Arthrus reaction o Reaction to inhaled antigens o Reaction to internal antigens Due to circulating complexes o Serum sickness o Immune complex glomerulonephritis o Complex at other sites. Arthus Reaction
In this an extensive zone of erythema and oedema appears around the bleb created by intradermal injection of antigen. Within a few hours a cyanotic centre develops within an erythematous ring. Later this assumes a deep purplish black cast indicative of cellular necrosis. Over the succeeding day or two this necrotic zone may enlarge to a few centimeters in diameter. The dead tissue dries and over a period of a week or more healing becomes complete. This local reaction is caused by deposition of an intravascular precipitate and thombosis. Diffusion of the antigen into the vascular bed surrounding the injection site creates a zone of sufficient high concentration of antigen and antibody that a precipitate forms. This precipitate becomes as extensive as to physically blockade the small venules which results into tissue destruction. With the appearance of complement and associated changes, further damage occurs which is followed by gradual healing. Variants of the Arthus reaction include the passive Arthus and the reversed passive Arthus (RPA) reactions. The first of these is simply the provocation of Arthus reaction in an animal that has been passively immunized. In RPA reaction, the antiserum is injected into the skin (rather than systemically) and the antigen is then given systemically (rather than intra-dermally). Both injections are made at about the same time so that a sufficient amount of antibody will remain near the injection site to precipitate with the antigen and cause local necrosis. Differences between PCA and RPA PCA Quantity of antibody required Cytotropic antibody required Latent period after transfer Histamine release Antihistamine effective Complement required Little + + + + _ RPA Large +
Serum Sickness Serum sickness develops in approximately 50% of normal human beings who receive a single injection of bovine or horse antitoxin against tetanus, gas gangrene, rabies, diphtheria or other toxins for prophylactic or therapeutic purposes. The symptoms of serum sickness i.e. hives, extensive oedema, joint pain, malaise and fever appear after about 7-10 days of injection, persist for several days after which they gradually subside. It is caused by mutual presence of antigen and antibody in the blood following primary immunisation by about fifth to eighth day of immunisation, antibody starts appearing in the blood and at the same time some antigen is also circulating. This results into formation of immune complexes which are deposited at various locations in the body. These complexes further activate complement and biological activities of complement also get manifested. All these ultimately result into hypo-complementemia (decreased complement levels) oedema, joint pain and eosinophilia seen in patients with serum sickness. As the symptoms of serum sickness do not appear until several days after the injection of the antigen, the disease has also been called as protracted anaphylaxis. Immune complex glomerulonephritis: Many cases of glomerulonephritis are associated with circulating complexes and biopsies stained by fluorescent method confirm this. Well known example is glomerulonephritis following infection with nephritogenic streptococci where complexes of infecting organisms have been implicated. The choroid plexus being a major filtration site is also favoured for immune complex deposition and this could be responsible for frequency of central nervous system disorders in SLE. Apart from SLE, subacute sclerosing panencephalitis (SSPE) and vasculitis skin rashes may also be caused by immune complexes.
Detection of Immune Complex Formation Measurement of C3 and C3c Cryoprecipitation of sera at 4°C Immunofluorescence Precipitation and measurement of complex IgG Binding of C3b with conglutinin Estimation of binding of Il25-C1q Detection with radiolabelled anti Ig of serum complexes capable of binding to bovine conglutinin or C3b TYPE IV HYPERSENSITIVITY Type IV hypersensitivity is also known as delayed hypersensitivity since the signs of reactions are observed 24 hours or more after contact with antigen. Unlike types I to III, this hypersensitivity reaction is mediated by T-Iymphocytes and macrophages. Allergic contact dermatitis is a common example. More severe forms of delayed type hypersensitivity reactions have been observed in diseases like leprosy and tuberculosis where an antigen is persistent and cannot easily be eliminated by the macrophages. T -lymphocytes, sensitized by prior contact with antigen, become activated upon re-exposure to allergen and release soluble mediators known as lymphokines.
Characteristics of four types of hypersensitivity Type I Type II Characteristic Main mechanism Other mediators Immediate IgE Mast cells BasophiIs Anaphylactic factors Soluble or particulate Cytotoxic IgG, IgM Complement
Type III Immune complex IgG, IgM Complement EosinophiIs NeutrophiIs Soluble or particulate
Type IV Delayed T cells Lymphokines Macrophages
Antigen
On cell surface
On cell surface
Reaction time Nature of reaction Therapy Example
Seconds to minutes Local flare and wheal Desensitization Anaphylaxis
Variable, usually hours Clumping of RBCs Steroids Thrombocytopenia Haemolytic anaemia
3-8 hours Acute inflammation Steroids Serum sickness
24-48 hours Cell mediated, cell destruction Steroids Tuberculin test
One of these lyrnphokines, macrophage activating factor or gamma interferon causes differentiation of responsive macrophages into activated macrophages which are now capable of enhanced phagocytosis and increased microbicidal activity. Another lymphokine acts to amplify immune response by increasing the number of T cells in the vicinity. In this way lymphocytes and activated macrophages recruited by lymphokines accumulate at local reaction sites. It is often the case that following infection with certain bacteria such as Mycobacterium tuberculosis and Mycobacterium leprae, the host cannot adequately eliminate these agents and they persist within macrophages or in the tissues. This results into an intense area of inflammation around infection. The inflammatory response is characterized by a core of macrophages and epithelioid cells, the latter being the characteristic cells of a granuloma. Giant cells may also be present. Epithelioid and giant cells are derived from macrophages and represent terminal stages of differentiation. An area of actively proliferating lymphocytes is usually seen surrounding the core area of the granuloma. This picture is mainly due to the inability of the host macrophages to eliminate the infecting organism. The end product of the inter action between the T-Iymphocytes and macrophages is tissue death, called necrosis. Differences between immediate and delayed hypersensitivity Immediate Delayed Character hypersensitivity hypersensitivity Appearance Rapid Slow Duration Short Longer Induction by Antigen or hapten Infection Antibody mediated + reaction Cell mediated + reaction Passive transfer with Serum Lymphocytes Desensitization Easy but of short Difficult duration Acute exudation around blood Mononuclear infiltration Lesions show vessels Maximum reaction in 6 hours 24-48 hours
TYPE V -STIMULATORY HYPERSENSITIVITY Many cells have receptors on their surfaces which when come in contact with an appropriate and specific agent get activated and transmit the signal to interior of cell to commence or augment cellular activity. As it happens in thyroid when thyroid stimulating hormone (TSH) of pituitary origin combines with receptors for TSH on surface of thyroid cells. In the sera of patients with thyrotoxicosis an autoantibody directed against an antigen on thyroid surface exists which stimulates the cell and produces same changes which are brought about by TSH. The situation is similar to stimulation of B lymphocytes with Ig receptors which can be stimulated by changes induced through receptor molecules either by binding of specific antigen or antibody to the Ig.