Types of allergic reactions
Anaphylactic (type I)¹
When an allergen enters the body for the first time, it causes the appearance of specific antibodies (immunoglobulins type E - IgE), which are fixed on the surface of mast cells.
This condition is called sensitization. Upon repeated contact of an already sensitized organism with the irritant, IgE-dependent activation of mast cells occurs. This causes the release of histamine, heparin and other inflammatory mediators responsible for allergic reactions. Type I allergic reactions are most often caused by components of plant pollen (grass, flowers, trees), household, epidermal and food allergens, some medications, as well as foreign blood serum⁶.
Type I includes anaphylactic shock, allergic rhinitis, atopic bronchial asthma, atopic dermatitis, allergic urticaria and Quincke's edema, anaphylactic reactions, for example to food, insect bites and a number of others⁶.
Cytotoxic (type II)
With this type of reaction, antibodies act against substances that have previously attached to the cells of the body’s own tissues. As a result, the process of their death begins. It is for this reason that type II allergic reactions are called cytotoxic¹.
Antigens in this type of reaction are most often bacterial, viral and other infectious allergens, autoallergens and drugs⁶. Type II can be a drug allergy; the same reaction sometimes occurs during blood transfusions: the body considers the cells that have entered it to be foreign and defends itself against them¹.
Immune complex (type III)
It is characterized by the formation of complexes of the allergen with antibodies, which activate the release of mediators that damage surrounding tissues. This reaction can develop in response to food, drugs, inhalation, viral and bacterial allergens and antitoxic serums¹,⁷.
Diseases caused by type III reaction: serum sickness, drug allergies and autoallergic diseases (rheumatoid arthritis, systemic lupus erythematosus)¹,⁷.
Delayed type hypersensitivity (type IV)
In response to an allergen, sensitized T-lymphocytes are formed. On their surface there are receptors that can interact with the corresponding antigens. When the allergen is re-entered, they combine with sensitized lymphocytes, which leads to the release of mediators - lymphokines, which cause allergy symptoms.
The causes of type IV reactions can be components of various microorganisms, parasites, fungi, helminths, viruses, as well as virus-containing and tumor cells, the body’s own but altered proteins or foreign proteins, haptens⁸.
The simplest example of this kind of reaction is the Mantoux test, which probably everyone has encountered. Doctors conduct tests for the presence of antibodies, just armed with knowledge of delayed-type hypersensitivity.
Features of the pathogenesis of an allergic reaction of the cytotoxic type.
Allergy is a state of increased sensitivity of the body to various substances in response to repeated contact with them, which is based on immunological mechanisms that lead to tissue damage.
Allergic reactions type II (cytotoxic).
Allergen: altered components of cellular and basement membranes (autoallergens).
^ Immunological stage: formation of immunoglobulins IgG1-3, IgM upon initial contact with the allergen. Upon repeated contact with the allergen, the formation of an allergen + antibody complex on the surface of target cells.
^ Pathochemical stage: activation of complement components, release of lysosomal enzymes (cathepsins, DNase, RNase, elastase) and superoxide radicals (O-, OH., H2O2) during phagocytosis.
^ Pathophysiological stage: lysis of target cells, destruction of basement membranes
(complement-dependent cytolysis, complement-independent cytolysis)
The cytotoxic type of damage underlies the development of autoimmune hemolytic anemia, leukopenia, thrombocytopenia, thyroiditis, myocarditis, nephritis, hepatitis, etc.
Features of the pathogenesis of an allergic reaction of the immunocomplex type.
Allergy is a state of increased sensitivity of the body to various substances in response to repeated contact with them, which is based on immunological mechanisms that lead to tissue damage.
Allergic reactions of type III (immune complex).
Allergen: soluble proteins, medications, medicinal serums.
Immunological stage: formation of immunoglobulins IgG1, IgG3 IgM upon initial contact with the allergen. Upon repeated contact with the allergen, the formation of soluble allergen + antibody complexes, circulation of the complexes and their fixation on the walls of microvessels.
^ Pathochemical stage: activation of complement components, activation of phagocytosis and release of lysosomal enzymes and superoxide radicals by phagocytes, activation of mast cells, their degranulation and release of histamine, heparin, serotonin, chemotactic factors; formation of prostaglandins, leukotrienes, platelet aggregation factor, activation of kallikrein-kinin, coagulation, anticoagulant and fibrinolytic systems.
Pathophysiological stage: damage to the walls of blood vessels, increased vascular permeability, development of inflammation.
The immunocomplex type of damage underlies the development of serum sickness, allergic vasculitis, rheumatism, diffuse glomerulonephritis, systemic lupus erythematosus, rheumatoid polyarthritis, Arthus phenomenon.
Features of the pathogenesis of a delayed type allergic reaction.
In type IV hypersensitivity reactions (cell-mediated, delayed type), it is not AT that take part, but T cells that interact with the corresponding Ag (sensitized T cells), which attract macrophages to the site of allergic inflammation. Sensitized T cells, after binding Ag, have either a direct cytotoxic effect on target cells, or their cytotoxic effect is mediated by lymphokines.
Examples of type IV reactions are allergic contact dermatitis, tuberculin testing for tuberculosis and leprosy, and transplant rejection.
Causes
• Components of microorganisms, single- and multicellular parasites, fungi, helminths, viruses, as well as virus-containing cells.
• Own, but modified and foreign proteins.
• Haptens: for example, drugs (penicillin, novocaine), organic small molecular compounds (dinitrochlorophenol).
Pathogenesis
Sensitization stage
• Antigen-dependent differentiation of T lymphocytes occurs. These sensitized T cells circulate in the internal environment of the body, performing a surveillance function. Some lymphocytes remain in the body for many years, keeping the memory of Ag.
• Repeated contact of immunocompetent cells with Ag (allergen) causes blast formation, proliferation and maturation of a large number of different T-lymphocytes, but mainly killer T-cells. It is they, together with phagocytes, that detect and destroy foreign Ag, as well as its carrier.
Pathobiochemical stage
• Sensitized killer T cells destroy the foreign antigenic structure, directly acting on it
• Killer T-cells and mononuclear cells form and secrete allergy mediators in the area of the allergic reaction, regulating the functions of lymphocytes and phagocytes, as well as suppressing the activity and destroying target cells.
A number of significant changes occur at the site of type IV allergic reactions.
– Damage, destruction and elimination of target cells
— Alteration, destruction and elimination of unchanged cells and non-cellular tissue elements.
— Development of an inflammatory reaction. In the focus of allergic inflammation, predominantly mononuclear cells accumulate: lymphocytes and monocytes, as well as macrophages. Often these and other cells (fanulocytes, mast cells) accumulate around small veins and venules, forming perivascular cuffs.
— Formation of granulomas consisting of lymphocytes, mononuclear phagocytes, epithelioid and giant cells formed from them, fibroblasts and fibrous structures. Granulomas are typical of type IV allergic reactions. This type of inflammation is designated as granulomatous (in particular with tuberculin, brucellin and similar reactions).
— Disorders of microhemo- or lymphatic circulation with the development of capillary-rotrophic insufficiency, degeneration and tissue necrosis.
AllergyN. Y. Onoiko, 2013
General concept
Some 30-35 years ago, allergic diseases seemed irrelevant and low-risk. Now, allergization of the planet's population (especially in industrialized countries) has reached such alarming proportions that it is becoming one of the main problems of modern medicine. All over the world there is an increase not only in the frequency, but also in the complexity of allergic diseases. Almost every day, a doctor is faced with allergic diseases, cases of drug and food intolerance, unusual reactions to chemicals in the household or professional environment, including clothing made from synthetic fabrics, jewelry, cosmetics and much more. Along with the progress of medicine (the discovery of antibiotics, sulfonamides, a number of antiseptics), the number of diseases such as sepsis and purulent complications has significantly decreased, and many infectious diseases have ceased to be fatal. Other diseases came to the fore, including allergic ones, that is, those that depend mainly not on microbes, but on changes in the body’s reactivity.
The basis of allergic diseases is precisely the altered reactivity of the body, i.e. there is hypersensitivity to a certain substance called an allergen. Hypersensitivity means that a person’s immune system, which protects the body from infections, diseases and foreign bodies, does not respond adequately to an allergen. How does this inadequacy manifest itself? And in the damaging nature of those clinical manifestations that we call an allergic reaction. After all, we talk about the presence of an allergic reaction when a person’s immune system, when faced with an allergen, reacts particularly violently. Inflammation, swelling, bronchospasms, itching, runny nose, lacrimation, and sometimes shock appear. It would seem that the immune system fights the allergen, protecting the body. In this case, the above reactions appear. This is - on the one hand, on the other hand - these reactions themselves can pose a pronounced danger to the normal functioning of the body. Thus, an allergic reaction is both protection and damage at the same time. Therefore, it is quite possible to join the opinion of many scientists that, from a general biological point of view, allergic reactions should be considered as reactions that have both destructive and protective features in equal proportions. Thus, an allergy is a kind of immune reaction of the body, accompanied by damage to the body’s own tissues. However, you should not be upset in advance, because the occurrence of damage does not mean the presence of the disease. An allergic disease develops only when the damage to the body is too severe, causing changes in it that fall under the strict criteria of the concept of disease.
What do we call allergens? Allergens are any substances that cause an allergic reaction. For example, in case of allergies when inhaling polluted air, in particular with hay fever, the allergens most often are pollen of plants, trees, mold spores, etc. Allergic diseases include bronchial asthma, urticaria, Quincke's edema, hay fever, eczema . Allergic mechanisms also play a huge role in the development of such serious diseases as rheumatism, glomerulonephritis, systemic lupus erythematosus, hemorrhagic purpura, etc.
