Allergy (overview) A1.0

Allergies manifest themselves in numerous organs, with skin and mucous membranes being particularly frequently affected (interfaces at which the individual organism's confrontation with the environment takes place. A general classification can be made as follows according to:

• Organs (allergy of the skin, lungs, nose, eye)
• Pathomechanisms (classification according to the classification scheme of Cooms and Gell)
• Allergen sources (food allergy, animal dander allergy; insect venom allergy)
• Molecular allergens (e.g. Bet V 1, Ara h1, etc.)
• Course and prognosis (acute or chronic allergy)
• Genetic aspects (e.g. hereditary allergy, sporadic allergy)
• Age (childhood allergy, allergy in old age)

Classification according to pathomechanisms (Coombs and Gell 1963): According to the (classical) classification of Coombs and Gell, 4 or 6 immunological reaction types of hypersensitivity can be distinguished (for didactic reasons), although this classification is difficult in the clinic due to overlapping reaction sequences.

Type I reaction (immediate-type hypersensitivity): Here, after antigen contact (allergen contact e.g. of a drug), cross-linking of juxtaposed, membrane-bound IgE antibodies on mast cells and basophilic granulocytes leads to cell activation and release of preformed mediators ( histamine, heparin, tryptase, ECP = eosinophilic cationic protein) and newly generated mediators such as leukotrienes (LTC4, LTD4, LTB4), prostaglandins, thromboxanes and platelet aggregating factor ( PAF). Typical diseases of type I allergy:

• Allergic asthma

• Allergic conjunctivitis

• Rhinitis allergica

• Urticaria

• Angioedema

• Anaphylactic shock

• Jones-Mote reaction (delayed type I reaction)

• Drug allergy (type I reactions are rather rare UAWs)

• Food allergy.

Type II reaction (humoral cytotoxic reaction): After binding of antibodies (IgG, IgM) to cellular antigens, activation of the complement cascade and action of cytotoxic cells (killer cells, platelets, eosinophilic and neutrophilic granulocytes, monocytes/macrophages) results in lysis of the target cell. While the sensitization phase with the formation of corresponding antibodies lasts about 5-10 days, the effector phase until the appearance of clinical symptoms lasts about 2-3 days. Typical diseases underlying a type II allergy:

• allergic hemolytic anemia

• Thrombocytopenia (thrombocytopenic purpura)

• Agranulocytosis

• Transfusion incidents

• Goodpasture's syndrome

Type III reaction

• Immune complex reaction: Triggers are aggregates of allergen (often drug) hapten + carrier protein and the antibodies or complexes induced thereby, which are composed exclusively of allergen-specific immunoglobulins. These may precipitate in the postcapillary venules and cause leukocytoclastic vasculitis. The sensitization phase lasts about 10 days, and the effector phase lasts 2-5 days until the onset of clinical symptoms.

• Serum sickness: In this case, an immune complex reaction proceeds rapidly due to an excessive activation of the complement system. For example, after two parenteral administrations of foreign protein components, massive and acute complement activation and leukocytoclasia occur due to the formation and deposition of circulating immune complexes.

• Immune complex disease in autoimmunity: E.g., chronic polyarthritis (rheumatoid arthritis); lupus erythematosus, systemic; vasculitis, leukocytoclastic; polyarteritis; fibrosing alveolitis.

• Immune complex disease due to sensitization to environmental antigens: e.g., exogenous allergic alveolitis (avian lung, farmer's lung, byssinosis, cotton fever).

Type IV reaction (cell-mediated reaction, delayed-type hypersensitivity): Effector cells are allergen-specific inflammatory and cytotoxic T cells.

• Type IV reaction: Type IV allergy is the most common form of allergy after type I allergy. Local inflammatory reaction starting within 24 hr due to infiltration of the epidermis with basophilic granulocytes. The clinical prototype of the type IV reaction is allergic contact dermatitis. Within 48-72 hr onset reaction to allergen contact in which the allergen (contact allergen) penetrates the epidermis. Contact with specific T cells leads to alteration of the epidermis by release of lymphokines. Depending on the lymphocyte population significantly involved, an immunological subdivision of the reactions into classes IVa-IVd has been proposed (Pfützner W 2018).

  • Type IVa: INF-gamma and/or TNF-alpha secreting CD4+ T helper cells (Th1 type).

  • Type IVb: IL-5 (and IL-4, IL-13) secreting CD4+ T helper cells (Th2 type) are mainly responsible for mobilization and activation of eosinophil granulocytes.

  • Type IVc: Perforin- and granzyme-B-producing CD8+cytotoxic T cells represent the major effector cells of many drug responses. Depending on the extent of T cell activation and the release of perforin- and granzyme-B, reversible keratinocyte damage occurs to varying degrees (dyskeratosis cell necrosis)

  • Type IVd: Characterized by CXCL8- and GM-CSF-producing T cells (CXCL8 stands for CXC motif chemokine 8). They are mainly responsible for the recruitment of neutrophil granulocytes. This mechanism plays a role in acute generalized exanthematous pustulosis (AGEP).

Type V (Granulomatous reaction): 12 hr after injection of tuberculin, local perivascular inflammatory reaction occurs in a sensitized organism due to infiltration of T lymphocytes. The release of lymphokines and cytotoxic factors results in tissue damage. Persistence of pathogens or other exogenous material results in granulomatous inflammation, usually interspersed with giant cells.

Type VI (immunodeviation): V.a. biologics such as TNF-alpha antagonists or versch. Immune checkpoint inhibitors can trigger inflammatory or autoimmunological reactions by stimulating inflammatory T cell populations, inducing autoantibodies or suppressing regulatory T cells(Treg). An example is the induction of autoimmune phenomena in melanoma patients treated with CTLA-4 or PD1 antibodies. Furthermore, autoimmune thyroiditis, myasthenia gravis, forms of insulin resistance, etc. are attributed to this type of allergic reactions.

A classification proposed by Johansson et al. divides hypersensitivity into immmunological and non-immunological(pseudoallergies) based on clinical considerations.

• Allergic hypersensitivity:
• IgE-mediated:
  • Atopic
  • Non-atopic:
    • Insect bites
    • Medications
    • Other.
• Non-IgE-mediated:
  • T-cell mediated (e.g. contact dermatitis, allergic)
  • IgG-, IgM-mediated (e.g. chronic urticaria, allergic alveolitis)
  • Other.
• Non-allergic hypersensitivity (pseudoallergy[intolerance reaction]).

Type I reaction: Within seconds to minutes conjunctivitis, rhinitis, bronchial asthma, shock, anaphylactic, urticaria, angioedema (see also pollinosis), diarrhea, vomiting. Aeroallergens (e.g. pollen) play an important role in the Type I reaction. These allergens can be absorbed not only through the mucous membrane (by inhalation) but also percutaneously through the skin. This mechanism plays a role in the seasonal exacerbation of atopic dermatitis (especially in exposed skin areas). Remarkably, type I sensitizations in children are associated with an increased CRP value (Chawes et al. 2017).

Type II reaction: onset within hours. Haemolytic anaemia (e.g. haemolyticus neonatorum disease, transfusion incident), allergic thrombopenia, allergic granulopenia, Goodpasture's syndrome, myasthenia gravis, autoimmune thyroiditis, pemphigus and pemphigoid.

Type III reaction:

• Arthus reaction: onset within 12 hours of injection. More or less haemorrhagic oedema develops at the injection site and subsides significantly within 48-72 hours, e.g. local vaccination reaction after overvaccination with tetanus antigen.
• Serum disease: onset within a few hours with nephritis, arthritis, urticarial or extensive redness.
• Infectallergic immune complex disease, e.g. leprosy, malaria, bacterial endocarditis, leukocytoclastic vasculitis, hepatitis.

Type IV reaction (late type reaction):

• Contact allergy: Allergic contact dermatitis, Airborn contact dermatitis, dermatitis bullosa pratensis, type IV exanthema: these occur as a result of a systemic absorption of the allergen (macular, mouth-papular
• Tuberculin reaction: macular, papular, hemorrhagic allergic drug exanthema.
• Granulomatous reaction: tuberculosis, sarcoidosis, granuloma anulare, granuloma glutaeale infantum.

Besides causal therapy, allergy prevention also plays a decisive role in treatment.

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