Hypersensitivity Reactions PDF

Hypersensitivity Reactions Dr. Raymond F Adebiyi Type I, II, III and IV Hypersensitivity Learning Objectives: 1. Type I Hypersensitivity a. Discuss the mechanism underlying type I hypersensitivity. Identify the key effector cells driving the reaction. Name the key inflammatory mediators and describe...

Hypersensitivity Reactions Dr. Raymond F Adebiyi Type I, II, III and IV Hypersensitivity Learning Objectives: 1. Type I Hypersensitivity a. Discuss the mechanism underlying type I hypersensitivity. Identify the key effector cells driving the reaction. Name the key inflammatory mediators and describe how they contribute to the reaction. Describe how the effect of the reaction depends on the site of exposure to the allergen Explain systemic anaphylaxis. How do immunologic and non-immunologic anaphylactic reactions differ? Describe the immediate and late-phase reactions that occur in type I hypersensitivity. Relate how allergic rhinitis, urticaria and gastrointestinal allergies are mediated by type I hypersensitivity reactions. 2. Explain the mechanism underlying Type II hypersensitivity. Describe the 3 outcomes that are a result of the binding of self-reactive antibodies to antigen. List examples of type II hypersensitivity-mediated diseases. 3. Explain the mechanism underlying type III hypersensitivity. a. Describe the Arthus reaction and serum sickness. How do the two differ? How are they alike? Under what situations does the Arthus reaction occur; when do we most often see it occur? b. Explain the link between the site of immune complex deposition and resulting disease. c. List examples of type III hypersensitivity-mediated diseases. 4. Describe the immune response underlying type IV hypersensitivity. a. Outline the steps that occur in order to generate a delayed type hypersensitivity (DTH) reaction. Why is it "delayed"? b. Name the key cytokines contributing to a DTH reaction and describe their functions. c. Explain type IV hypersensitivity responses using poison ivy or poison oak and the tuberculin skin test as examples. d. Name some other allergens that commonly cause type IV hypersensitivity reactions. b. c. d. e. 1 Types I, II, III and IV Hypersensitivity In order for the immune system to protect the body against foreign invaders, it must be able to distinguish and mount responses against non-self-antigens. Humans encounter many molecules that are foreign but do not necessarily threaten heath (e.g., plants, microbes, and animals that we eat and in the environment) all the time. Most of these molecules stimulate neither inflammation nor adaptive immunity. Some kinds of innocuous molecules stimulate adaptive immune responses that, on subsequent exposure, produce inflammation that can range from a skin irritation to a threat to life. Over-reactions of the immune system to a harmless antigen are called hypersensitivity reactions or allergic reactions. There are many different kinds of substances that are common causes of hypersensitivity. They can be inhaled, ingested, injected. Sometimes simple physical contact with the allergen can drive the reaction. It is important to remember that with hypersensitivity reactions, the patient must first have been sensitized with an antigen and developed a humoral (Types I, II, and III) or cell-mediated (Type IV) immune response against the antigen. Type I Hypersensitivity – Allergy Type I hypersensitivity reactions are a result of degranulation of mast cells and release of inflammatory mediators. Degranulation is triggered by antigen binding to antigen-specific IgE which is bound to FcRI on the surface of the mast cell. These reactions only occur after a person has become sensitized to the antigen; the person must have synthesized IgE antibodies when they first encountered the antigen. IgE-mediated activation of mast cells, basophils, and activated eosinophils is important for the elimination of large parasites which are highly prevalent in tropical regions or regions with poor sanitation. Some figures suggest that onethird of the world’s population is infected by one or more species of parasite. In developed countries, these parasitic infections tend to be rare, though they are becoming increasingly common. Instead, IgE antibody responses tend to be driven by non-threatening substances in the environment. Indeed, in the past couple of decades the incidence of allergy has doubled in the United States. 2 May-Aug 2013 Types I, II, III and IV Hypersensitivity 3 There are 4 kinds of hypersensitivity reactions grouped according to the effector mechanisms producing the reaction: Type I hypersensitivity reactions are a function of antigen recognition by IgE antibodies binding to FcRI receptors. IgE has an exceptionally high affinity to FcRI and, unlike other isotypes, will bind to FcRI in the absence of antigen. FcRI is expressed by mast cells, basophils and eosinophils. Once antigen binds to IgE (which is bound to FcRI), the cell becomes operational immediately – no proliferation is required – and the cell releases mediators within seconds. IgE antibodies of varying specificities can bind to FcRI receptors on an individual mast cell so multiple mast cells have multiple “specificities” and can share “specificity”. THE CELLS INVOLVED IN TYPE I HYPERSENSITIVITY: MAST CELLS, EOSINOPHILS, AND BASOPHILS Mast cells are important players in innate immunity. They are resident in mucosal and epithelial tissues and are capable of alerting the immune system to local trauma and infection (express Toll-like receptors and Fc receptors for IgA and IgG) by secreting cytokines that recruit neutrophils and effector T cells. They also contribute to the repair of damage caused by infection or trauma. IgE-mediated allergic reactions are a result of the release of inflammatory mediators by tissue mast cells. The combined effect of these mediators is to attempt to attract circulating leukocytes (eosinophils, basophils, neutrophils, and effector Th2 T cells) to eliminate the antigen. Sneezing, coughing, wheezing, vomiting or diarrhea can all be symptoms of the allergic reaction. These symptoms are a result of the release of these mediators. A few more notes on mast cell mediators: 4 Types I, II, III and IV Hypersensitivity Histamine is an amine derivative of the amino acid histidine. The physiological effects of this molecule are mediated through 3 kinds of histamine receptors (H1, H2 and H3) on different kinds of cells. In acute allergic reactions, histamine binds to H1 receptor on endothelial cells. **Pharm tie-in: Diphenhydramine (Benadryl), a common over-the-counter drug used to treat allergic reactions is an H1 receptor antagonist** Mast cells also secrete prostaglandins which promote dilation and increased permeability of vessels and act as chemoattractants for neutrophils. Eosinophils are graunulocytes that mostly reside in tissues, particularly in connective tissue. Activation triggers the release of granules. The granules contain highly toxic molecules (like collagenase, peroxidase, and major basic protein) intended to kill microorganisms or parasites directly but can also damage the host. The activated eosinophil then synthesizes and secretes prostaglandins, leukotrienes and cytokines that amplify the inflammatory response. Basophils are thought to be a key cell in inducing a TH2 response. These cells are recruited to a site of infection where they become activated through Tolllike receptors and other innate receptors. They then traffic to the secondary lymphoid organs where they secrete IL-4 and IL-13 which, in turn, skew antigenstimulated CD4 T cells towards a TH2 response. Basophils also express CD40L and, in combination with IL-4 and IL-13, can provide B cells with the signals that promote isotype switching to IgE and IgG4. Mast cells, eosinophils, and basophils are thought to work in concert: Mast cell degranulation triggers an inflammatory response  Eosinophils and basophils are recruited  Eosinophils release major basic protein  Major basic protein triggers mast cell and basophil degranulation  Cytokine production (IL-3, IL-5 and GM-CSF) affect the growth, differentiation and activation of eosinophils and basophils 5 Types I, II, III and IV Hypersensitivity Dust Mite Poo: A common allergen A very common allergen in North America is a cysteine protease derived from Dermatophagoides pteronyssimus, the house dust mite. The mite deposits feces which become airborne particles (sweeping, dusting, or vacuuming stirs them into the air) that are inhaled. Improved heating and cooling systems are thought to promote an environment where the mite can grow and its feces can get desiccated; air currents created by air conditioners or forced-air heaters move the particles into the air where they get breathed in. ATOPY It is estimated that 40% of individuals in the US have a predisposition towards development of type I hypersensitivity towards common environmental allergens. This state of exaggerated tendency to mount IgE responses is referred to as atopy. Atopic individuals have higher numbers of eosinophils and total levels of IgE in their circulation compared to non-atopic individuals. They also have a higher susceptibility to allergic diseases (discussed below). Atopy runs in families and there appear to be a number of genes that are linked to it. WHAT CONDITIONS FAVOR IGE RESPONSES THAT CAN BECOME TYPE I HYPERSENSITIVITY REACTIONS? Type I hypersensitivity require the presence of allergen-specific IgE. The production of IgE antibodies is favored when the immune system is challenged by small quantities of antigen and when basophils produce IL-4 that skews antigenspecific CD4 T cells towards the TH2 pathway. The TH2 T cells will secrete IL-4 and other cytokines that will promote activated, antigen-specific B cells to undergo class switching to IgE. The antigens, or allergens, that drive type I hypersensitivity responses are invariably proteins or chemicals that chemically modify human proteins (like certain drugs). Most allergens are small, soluble proteins derived from plants and animals that are either inhaled or ingested. Environmental antigens, like most of those that drive type I responses, drive the development of regulatory T cells (Tregs) in non-allergic individuals. These Tregs would inhibit Th2 cell responses against the allergen. In those who have a predisposition towards the development of allergies, these antigens drive Th2 responses. INFLAMMATORY RESPONSE FOLLOWING IGE-MEDIATED MAST CELL ACTIVATION OCCURS AS AN IMMEDIATE RESPONSE FOLLOWED BY A LATE-PHASE RESPONSE When an allergen to which a person is sensitive is injected into the skin, a characteristic wheal and flare reaction occurs at the injection site. This immediate reaction is a consequence of the IgE-mediated mast cell degranulation. Histamine and other mediators trigger vascular permeability of local vessels causing edema. The swelling produces the wheal and the redness triggered by the increased blood flow into the site produces the flare. This reaction can last up to 30 minutes and the severity of the reaction can vary. Six to eight hours later, the chemokines, cytokines and leukotrienes synthesized by the mast cell trigger a late-phase reaction; this reaction is characterized by more widespread swelling at the site of injection. 6 Types I, II, III and IV Hypersensitivity If the allergen is inhaled, immediate and late phase reactions also occur. The allergic reaction causes the airways to become inflamed, constricted and blocked with mucus. The reactions can be monitored by measuring a patient’s breathing capacity. THE EFFECT OF THE IGE-MEDIATED ALLERGIC REACTION VARIES WITH THE ALLERGEN AND THE SITE OF MAST CELL ACTIVATION Only the mast cells at the site of exposure to allergen will undergo degranulation. As the mediators that are released are short-lived, both the immediate and late-phase responses are confined to the site of allergen exposure. GI tract: increased fluid secretion, increased peristalsis ➔ Expulsion of GI tract contents (diarrhea, vomiting) Airways: Decreased diameter; increased mucus secretion ➔ Expulsion of airway contents (coughing, phlegm) Blood vessels: Increased blood flow, increased permeability➔Edema, Inflammation, Increased lymph flow and carriage of antigen to lymph nodes ALLERGENS IN THE BLOOD CAUSES SYSTEMIC ANAPHYLAXIS Systemic anaphylaxis is a dangerous hypersensitivity reaction that can occur when an allergen enters the blood stream. Disseminated mast cell activation causes and increase in vascular permeability with widespread constriction of smooth muscle. Anaphylactic shock occurs when fluid leaves the blood causing a drastic drop in blood pressure. The loss of fluid into the connective tissues causes swelling and the major organs sustain major damage and impaired function. Airway constriction can cause death by asphyxiation. **Pharm tie-in: Epinephrine can counter anaphylaxis by stimulating the reformation of tight junctions between endothelial cells, reducing the amount of vascular leakage** How does the allergen get into the bloodstream? Sting by wasps, bees or other venomous insects Drug injections (Example: penicillin) Orally-administered food or drugs that are rapidly absorbed from the gut (example: peanuts) Anaphylactoid reactions (an old term) resemble anaphylaxis. These reactions, however, are IgE-independent; mast cell degranulation is caused by other stimuli. Thus, they are often referred to as non-immunologic anaphylactic reactions. Epinephrine is also used to treat these reactions. CLINICAL SYNDROMES CAUSED BY TYPE 1 HYPERSENSITIVITY REACTIONS Airway: Allergic rhinitis (hay fever) is characterized by violent bursts of sneezing and a runny nose. Here, inhaled allergens diffuse across the mucosal membrane of nasal passages triggering activation and degranulation of mucosal mast cells. This causes local edema that can obstruct nasal airways, nasal discharge of mucus that is rich in eosinophils, and irritation of the nose due to the release of histamine. In allergic conjunctivitis the same antigen that can trigger rhinitis can affect the conjunctiva of the eyes causing redness, itchiness and inflammation. 7 Types I, II, III and IV Hypersensitivity While uncomfortable, both allergic rhinitis and conjunctivitis are generally short lived and cause no long-lasting tissue damage. Allergic asthma is triggered by inhaled allergens triggering activation of submucosal mast cells in the lower airways. This causes an increase of the fluid and mucus secreted into the respiratory tract as well as bronchial constriction. Patients experience difficulties in breathing such as shortness of breath and wheezing. Chronic inflammation subsequently develops and can become chronic asthma. In chronic asthma the airways can be totally occluded by plugs of mucus and the airways can be hyper-responsive to environmental factors. Skin: Urticaria (hives) occurs when allergens trigger mast cell degranulation in the skin. Histamine release causes raised, itchy swellings which are essentially the same as the immediate wheal-and-flare reaction. Insect bites are a good example of urticaria. When mast cells in deeper subcutaneous tissue become activated by the allergen, a more diffuse swelling called angioedema can occur. In these reactions, the allergen can come in direct contact with the skin or can be carried to the skin through the bloodstream. Certain individuals who are predisposed towards allergic responses can develop prolonged allergic responses in the skin called eczema (atopic dermatitis). Here, a chronic inflammatory response causes a chronic and itching skin rash characterized by skin eruptions and fluid discharge. GI: The foodstuffs humans eat contain proteins that are a potential source for peptides that can drive an IgE response. Proteins in food get degraded by proteases and the resulting derivatives are potential peptides for presentation to CD4 T cells that will become TH2 T cells that can drive an IgE response. People sensitized to a particular protein will be allergic to any food that contains that protein. Foods that commonly cause allergies include grains, nuts, fruits, legumes, fish, shellfish, eggs, and milk. When allergen passes across the epithelial wall of the gut, it will bind to IgE on mucosal mast cells associated with the GI tract. This triggers mast cell degranulation. Blood vessels become permeable, fluid leaves the blood and passes across the gut epithelium into the lumen resulting in diarrhea. Additionally, constriction of smooth muscle of the stomach wall and smooth muscles of intestines cause cramps and vomiting. 8 Types I, II, III and IV Hypersensitivity PREVENTION AND TREATMENT A simple approach to manage the effects of allergic reactions is to avoid contact with the allergen. Often this involves modification of behaviors – avoiding food that contains the allergen, keep pets outdoors, or get out of town when the pollen count is high. Pharmacologically blocking the effector pathways can also reduce the effects of allergic reactions and limit the inflammation that can occur as a consequence of the allergic reaction. Antihistamines prevent histamine from binding H1 histamine receptors on vascular endothelium thereby preventing the vascular permeability that drives rhinitis and urticaria Corticosteroids generally suppress leukocyte function; these drugs are typically used to suppress chronic inflammation in asthma, rhinitis and eczema Cromolyn sulfate prevents degranulation of activated mast cells; this drug is an inhaled prophylactic used by asthmatics Epinephrine is used to treat acute anaphylactic reactions Desensitization is an immunological method to prevent the synthesis of allergen-specific IgE. Here, a patient is injected with a very small dose of allergen. Over time, the amount of allergen is increased. The idea is that by changing the route and dose of the allergen, the immune response will shift from dominated by IgE production to one dominated by IgG. TH2 responses also drive the production of IgG4 antibodies that, when bound to antigen, form complexes that do not recruit effector cells. A terrible complication of this technique can be anaphylaxis so the patients are monitored carefully when the shots are administered. Peanut powder has been approved for desensitization to peanut (2020). 9 Types I, II, III and IV Hypersensitivity Type II Hypersensitivity In Type II hypersensitivity, damage is mediated by antibodies (of the IgG or IgM isotypes) that are specific for self antigens on cell surfaces or in the extracellular matrix. Antigenic determinants can also be exogenous antigens (like drug metabolites) that bind to the cell surface or matrix. The binding of antibodies can lead to one of three outcomes: 1. Opsonization, phagocytosis and eventual destruction of the cell. Examples include: a. Autoimmune hemolytic anemia, agraulocytosis and thrombocytopenia – where individuals produce antibodies against their own blood cells which then are destroyed b. Erythroblastosis fetalis – where Anti-Rh antigen antibodies developed by Rh-neagtive mothers pregnant with Rh-positive babies (antibodies are generated during the mother’s first pregnancy with an Rhpositive baby) cross the placenta, enter the fetal circulation, activate the classical complement pathway and destroy fetal RBCs. c. Certain drug reactions – where the drug attaches to surface molecules of red blood cells creating an antigenic epitope against which antibodies are produced d. Transfusion reactions – antibodies against ABO blood group antigens bind to transfused RBCs and fix complement, leading to the destruction and clearance of the transfused RBCs 2. Activation of complement or ADCC resulting in inflammation and tissue injury. An example is Goodpasture’s syndrome, a glomerulonepritis due to anti-basement membrane antibodies that bind to the glomerular basement membrane and the alveolar basement membrane. Antibody is deposited in a characteristic linear fashion, different from the 'lumpy bumpy' deposition that is seen with immune complexes. These antibodies activate complement (via the classical pathway) and trigger ADCC thereby driving an inflammatory process that triggers polymorphonuclear leukocyte recruitment and activation. The recruited PMLs release inflammatory mediators and other molecules like reactive oxygen species, lysosomal enzymes, and proteases that damage tissues, collagen, elastin and cartilage. 3. Cellular dysfunction by activating or inhibiting the function of a receptor. Two good examples are myasthenia gravis and Graves disease. In myasthenia gravis, antibodies specific for acetylcholine receptors block neuromuscular transmission thereby causing muscle weakness. In Graves’ disease, antibodies against thyroidstimulating hormone receptor stimulate thyroid epithelial cells causing hyperthyroidism. Penicillin: An example of how drugs can drive Type II hypersensitivity reactions Type II Hypersensitivity reactions can occur as a side effect after the administration of certain drugs. A common example is penicillin. Penicillin is an important antibiotic that targets bacterial cell wall synthesis. The drug can covalently bind to the surface of human cells and create new epitopes that appear foreign to the patient’s immune system. The drug most commonly binds to red blood cells (but can also bind to platelets). Complement component C3b can coat the penicillin-modified RBCs, facilitating phagocytosis of the modified RBC and presentation of penicillinmodified protein to CD4 T cells; these CD4 T cells become activated and differentiate into Th2 T cells which can provide help to antigen-specific B cells. Hence, the patient can then develop IgG and IgM antibodies specific for the drug-cell surface conjugate. The binding of these antibodies to the drug-conjugated RBCs activates the classical complement pathway, leading to RBC lysis, or trigger phagocytosis by macrophages in the spleen. The result is the development of hemolytic anemia (or thrombocytopenia when the drug binds to platelets). 10 Types I, II, III and IV Hypersensitivity Type III Hypersensitivity Type III hypersensitivity reactions occur when antigen-antibody (IgG) complexes become deposited at a site and produce tissue damage. Small immune complexes (like one IgG antibody molecule bound to 2 soluble antigen particles) are inefficient at fixing complement to facilitate their removal (at least 2 IgG molecules per complex are required to fix complement) and tend to circulate in the blood and be deposited in blood vessel walls. As Clearance of Immune Complexes Immune complexes are cleared primarily by red blood they accumulate they become capable of fixing complement and cells. When the immune complex fixes complement, it activating leukocytes and mast cells via Fc receptors and complement becomes covered with C3b. Red blood cells bind to the receptors. Activation of mast cells by C3a triggers the release of immune complex through CR1 and as they pass through histamine, causing urticaria. More inflammatory cells are recruited the liver and the spleen, tissue macrophages remove into the tissue by C5a. When immune complexes are formed in the and degrade the immune complex, leaving the red blood cell unscathed. circulation, they can get deposited in organs such as the kidneys 11 Types I, II, III and IV Hypersensitivity (glomerulonephritis), joints (arthritis) or in the small vessels of the skin. Platelets can deposit at the site of the immune complex deposition and form clots that can cause the blood vessels to burst, resulting in hemorrhage. ARTHUS REACTION The Arthus reaction is a Type III hypersensitivity reaction that can occur when a soluble antigen is subcutaneously injected into individuals who have been exposed to that antigen in the past and have synthesized IgG against that antigen. Antigen-specific IgG diffuses from the blood into the tissues at the site of the injection. The antibodies bind to the antigen and form immune complexes which activate complement and trigger an inflammatory reaction. Inflammation causes further recruitment of leukocytes and antibodies to the injection site. This inflammation is bolstered by mast cell and other leukocyte activation triggered by immune complexes engaging Fc receptors and complement engaging complement receptors. Arthus reactions are characterized by localized areas of erythema with hard swelling that typically subside within a day. These reactions are often seen at the site of injections used to desensitize IgEmediated allergies. SERUM SICKNESS In the late 19th and early 20th centuries, before there were so many antibiotics, horses were immunized with certain bacteria or bacterial toxins that caused life-threatening bacterial infections in humans (e.g., diphtheria, tetanus, or scarlet fever). The serum from these horses was then injected into infected people to attempt to transfer immunity. While these injections helped control and clear the infection in these patients, a specific Type III hypersensitivity reaction would sometimes occur. Seven to 10 days after the injections, patients would develop fever, chills, rash, vasculitis, arthritis and sometimes glomerulonephritis. These symptoms were mediated by the formation of antibodies against horse proteins and the resulting formation of immune complexes in tissues; the symptoms of serum sickness depend on where the immune complexes are deposited. Serum sickness can occur whenever a patient develops antibodies against an antigen that is infused into the serum in large amounts. It is observed in patients who are treated with monoclonal antibodies as treatment for various diseases (patient mounts antibodies against the infused antibodies), treatment with certain drugs or enzymes; it can occur in patients treated with penicillin and can occur in heart-attack patients treated with streptokinase (a bacterial enzyme used to break up blood clots). The onset of serum sickness coincides with the development of antibodies which form immune complexes with the antigenic proteins. Accordingly, it has a limited duration and ends when the immune complexes are cleared by phagocytes. The systemic effects are related to the fact that the serum is loaded with antigen so the immune complexes fix complement and activate Fc receptor-bearing leukocytes throughout the body. 12 Types I, II, III and IV Hypersensitivity Type IV Hypersensitivity – Delayed Type Hypersensitivity (DTH) Type IV hypersensitivity reactions are mediated by effector T cells specific for the sensitizing antigen. In contrast to antibody-mediated hypersensitivity reactions which are apparent within minutes, they occur 1-3 days after the contact with the antigen (hence they are called delayed type hypersensitivity reactions [DTH]). Additionally, because processing and loading of the antigens into MHC is somewhat inefficient, the amount of antigen necessary to drive a DTH is 100-1000x greater than for antibodymediated hypersensitivity reactions. It is important to note that the DTH response is essentially a cell-mediated immune response. Th1 T cells become activated and secrete cytokines (like INF-) that activate macrophages. The development of Th1 T cells is critical in the successful elimination of intracellular pathogens such as Leishmania, Listeria, Legionella, Mycobacterium tuberculosis that infect macrophages. CD8 T cell activation and the development of CTLs are critical in the successful elimination of intracellular pathogens like viruses, bacteria and parasites that infect non-phagocytic cells. Most DTH responses are driven by cytokines released by Th1 T cells. During the primary exposure to the antigen, the antigen is presented to naïve CD4 T cells which become activated and 13 Types I, II, III and IV Hypersensitivity develop into Th1 T cells. Memory Th1 T cells develop as part of this response. When the patient encounters the antigen again, these memory T cells recognize the antigen, become activated, travel to the site of antigen contact, and secrete cytokines. It is these cytokines (see table above) that drive the DTH response. This is perhaps better explained using an example: Poison ivy or poison oak Pentadecatechol is a small, highly reactive, lipid-like molecule present in the leaves and roots of the poison ivy and poison oak plants. When a person touches the plant, pentadecatechol penetrates the outer skin layers and forms covalent bonds with skin cell-surface proteins and extracellular proteins. When a person is first exposed to pentadecatechol, skin DCs degrade the chemically-modified proteins formed when pentadecatechol binds to the self-proteins. These peptides are presented via MHC class II to CD4 T cells. This drives the development of a Th1 immune response. Additionally, pentadecatechol crosses the plasma membrane of skin cells and chemically modifies intracellular proteins. These chemically-modified proteins are processed and presented via MHC class I to CD8 T cells. Activated CD8 T cells are then armed to kill any cells presenting the chemically-modified proteins at the site of antigen contact. The hypersensitivity response occurs upon subsequent encounters with the plant. Memory Th1 T cells and CD8 T cells recognize the pentadecatechol-modified self-proteins presented by macrophages and secrete inflammatory cytokines which drive the DTH. The DTH response driven by pentadecatechol causes red, raised, weeping skin lesions that are a result of the infiltration of leukocytes and the destruction of skin cells and extracellular matrix holding the skin together. Pentadecatechol is easily transferred from the initial site of contact to other parts of the body. As it takes days for the DTH to develop, the patient may end up with widespread lesions, exacerbating the reaction. Another classic example of a DTH reaction is the tuberculin skin test (TST). This test is used to determine if a person has been exposed to Mycobacterium tuberculosis (Mtb), a pathogen of eminent importance. In the TST, a small amount of antigens extracted from the organism are placed intradermally or intracutaneously. Twenty-four to 72 hours later, patients who have previously been exposed to the pathogen, who have been vaccinated with the BCG-strain of the organism, or who are experiencing a primary infection with the organism develop an inflammatory reaction at the site of antigen administration. 14 Types I, II, III and IV Hypersensitivity The inflammatory reaction occurring in sensitized patients is mediated by a Th1 immune response. Injected antigens are engulfed by tissue macrophages and dendritic cells and presented via MHC class II to Mtb-specific memory T cells that exit the blood and traffic to injection site. These memory CD4 T cells become activated and differentiate into effector Th1 T cells that secrete cytokines that trigger inflammation. Fluid, proteins and other leukocytes are recruited to the infection site. It takes hours for each phase of the reaction to occur, hence the effects are delayed. DTH responses mediate the hypersensitivity reactions that can occur with nickel (and other divalent cations which can modify MHC class II conformation or peptide binding), insect venoms, and latex. 15