An Overview of Allergic Reactions and Allergic Diseases

1. Concept

• Allergic reaction, also known as hypersensitivity, is a condition of increased immune response of the body to fight against a certain agent from outside the body, leading to tissue damage and clinical symptoms.

• Consequences of allergic reactions are the production of biologically active substances, most of which are non-toxic, causing pathological symptoms.

Note:

• Allergic reactions are an immune response, meaning that the body is trying to protect itself from foreign agents. However, in this case, the immune response is excessive, harming the body itself.

• Not everyone who comes into contact with an allergen will have allergies. Allergic predisposition is genetically inherited, and environmental factors also play an important role in disease development.

2. Origin of the term

• “Allergy” reflects the clinical manifestations of the disease, which are the symptoms that patients experience.

• “Hypersensitivity” reflects the pathogenesis of the allergic reaction, which is the disrupted immune mechanism that leads to an exaggerated response.

3. Genetics

• Parents without allergies: Giving birth to a child with a 15% allergic predisposition.

• Father or mother with allergies: Giving birth to a child with a 25-30% allergic predisposition.

• Father and mother with allergies: Giving birth to a child with a 50-60% allergic predisposition.

Note:

• Allergic predisposition is a risk factor, not a determining factor.

• Environmental factors such as pollution, processed food, exposure to chemicals… also contribute to increasing the risk of allergies.

4. Genetic mechanisms

• Genetic mechanisms of allergic reactions:

• (1) High levels of total IgE in serum are related to dominant genes.

• (2) HLA type with corresponding antigens.

• (3) Hypersensitivity predisposition to many types of antigens is associated with HLA-B8 and HLA-Dw3.

• HLA-B8 is very common in people with autoimmune diseases.

• People with HLA-B8 often have reduced function of single-cell or multi-cell T cells.

Note:

• HLA is the major histocompatibility complex, playing an important role in immunity.

• Some HLA genes are associated with the risk of allergies, but not all people with these genes develop allergies.

5. Classification

#### 5.1. Classification by time

• Fast allergy (within 24 hours):

• Immediate (seconds to minutes): Reactions occur immediately after contact with the allergen.

• Rapid allergic reaction (30 minutes to hours): Reactions occur later, but still within 24 hours of contact with the allergen.

• Late allergy (after 24 hours, maximum 48-72 hours): Reactions occur later, usually from 24 hours to 48-72 hours after contact with the allergen.

#### 5.2. Classification by pathogenesis

• Type I – fast allergy:

• Characteristics: Immediate allergy, mainly caused by IgE.

• Type II – cytotoxic allergy:

• Characteristics: Allergies caused by IgG or IgM antibodies, leading to destruction of target cells.

• Type III – immune complex disease:

• Characteristics: Allergies caused by antigen-antibody complexes deposited in the tissues.

• Type IV – late allergy:

• Characteristics: Allergies caused by T cells, reactions occur later, usually after 48-72 hours.

#### 5.3. Classification by different pathogenesis mechanisms:

• Participating components: Cells and antibodies participating in the reaction.

• Tissue damage mechanism: How the allergic reaction causes damage to the body.

6. Types of allergic reactions

#### 6.1. Type I allergic reaction (immediate allergy)

• Name: Immediate allergy (seconds to minutes).

• Clinical manifestations:

• Anaphylaxis: Systemic reaction, life-threatening.

• Atopic disease: Local reaction, commonly found in the skin, respiratory tract, and digestive tract.

• Participating components:

• Antibodies (Ab): IgE bound to the surface of Mast cells and Basophils.

• Antigens (Ag): Allergen enters the body for the second time.

• Conditions for activation of Mast cells and Basophils: Cross-linking of Fce receptors.

• Types of cross-linking Fce receptors:

• Antigen.

• Anti-IgE antibodies.

• Anti-Idiotype.

• Anti-Fce antibodies.

• Self-cross-linking of IgE.

• Mechanism of activation of Mast cells and Basophils:

• Adenylcyclase – cAMP – opening of Ca LNC channels – Ca influx into the cytoplasm – Ca binds to calmodulin – activation of enzymes including Protein kinase and promoting cytoskeletal proteins to contract microfibers gradually releasing granules.

• Creation of Fusagenic lipids increases the fusion of the cell membrane with the granule membrane.

• Production of Arachidonic acid, from which intermediate chemicals are synthesized.

• Type I allergic reactions, in addition to Mast cells, Basophils, also involve: Eosinophils, Neutralphils when they are activated.

Note:

• Anaphylaxis is a serious reaction that can be life-threatening.

• Atopic disease is a chronic disease, usually with milder manifestations than anaphylaxis.

#### 6.2. Type II allergic reaction (cytotoxic allergy)

• Clinical manifestations:

• Blood transfusion mismatch ABO system.

• Rh blood group incompatibility between mother and child.

• Hemolysis after using some medications (antibiotics and derivatives).

• Participating components:

• Ab: Mainly IgM, IgG.

• Ag: Antigens on the cell surface, autoantigens, foreign antigens, haptens when they enter the body adhere to the cell surface.

• Mechanism of target cell damage:

• Activation of the complement system through the classical pathway.

• Opsonization.

• ADCC: Antibody-dependent cell-mediated cytotoxicity by IgG.

Note:

• Type II allergic reactions are often associated with the destruction of target cells.

• Rh blood group incompatibility between mother and child can cause neonatal jaundice, affecting the health of newborns.

#### 6.3. Type III allergic reaction (immune complex disease)

• Clinical manifestations: Depend on the location where the immune complex is deposited.

• Serum sickness, Systemic lupus erythematosus (systemic manifestations): Immune complexes deposit in many locations in the body, causing systemic manifestations.

• Allergic lung disease, Arthus phenomenon (allergic vasculitis) dermal capillaries, autoimmune glomerulonephritis, rheumatoid arthritis (local manifestations): Immune complexes deposit in certain locations, causing local manifestations.

• Participating components:

• Ab: IgG.

• Ag: Soluble antigen.

• Because IgG, when combined with Ag, creates a complex large enough to move and deposit.

• Pathogenesis:

• Immune complexes are usually dissolved by Neutralphils and Monocytes, the body eliminates them.

• But when there are many, large in size, high affinity for tissue –> move to tissue, fix to the cell surface of the tissue –> activate the complement system and release C3a C5a –> attach to Mast and Basophils causing the release of granules like Type I, causing chemotaxis of Neutralphils, increased vascular permeability –> inflammatory reaction at the deposition site and can cause damage to the cells at the deposition site.

Note:

• Type III allergic reactions are often associated with the deposition of immune complexes in tissues.

• Systemic lupus erythematosus is an autoimmune disease that can damage many organs in the body.

#### 6.4. Type IV allergic reaction (late allergy)

• Name: Late allergy (48-72 hours), T cell (Tc) mediated.

• Clinical manifestations:

• Mantoux reaction: After 72 hours, a red halo > 10 mm appears at the injection site, meaning the IDR is positive, meaning that tuberculosis has been contracted.

• Allergic contact dermatitis: Contact chemicals are haptens that bind to epithelial proteins to form complete allergens.

• Intracellular infection (KST Leishmania, fungi, Mycobacterium Leprae, Listeria monocytogenes, Brucella arbotus Herpes, measles): Intracellular bacteria cause Type IV allergic reactions.

• Participating components:

• Tc: Sensitized before.

• Ag-carrying cells.

• Damage mechanism:

• Sensitization stage: Ag enters the body, APC capture, process, and present to Th.

• Subsequent exposure: Th1 is activated, secreting cytokines that attract Monocytes, stimulating Tc progenitors, NK.

• Monocytes concentrate at the site of Ag invasion: Destroy Ag, create epithelial cell chains, phagocytose bacteria and become targets for Tc destruction.

• Result of the entire reaction: Chronic granuloma.

Note:

• Type IV allergic reactions are usually associated with T cell activation, causing local inflammation.

• Allergic contact dermatitis is common in people who come into contact with chemicals, cosmetics, and skin irritants.

7. Diagnosis

• Clinical: Collect history, clinical examination.

• Tests: Blood tests (IgE, specific blood tests), skin tests (skin tests).

Note:

• Diagnosis of allergies must be based on both clinical and laboratory findings.

• Allergic tests can help identify the type of allergen causing the disease, but they are not always accurate.

8. Treatment

• Avoid contact with allergens: This is the most important thing to treat allergies.

• Medication: Antihistamines, anti-inflammatory drugs, corticosteroids, anti-IgE drugs (Omalizumab)…

• Immunotherapy: Injections of small amounts of allergen on a specific schedule to help the body become accustomed to the allergen.

Note:

• Treatment of allergies should be performed by a specialist.

• Do not self-medicate, especially corticosteroids, as they can cause many dangerous side effects.

9. Complications

• Anaphylaxis: Life-threatening.

• Asthma: Chronic disease, severely affecting health.

• Atopic dermatitis: Causes itching, discomfort, affects aesthetics.

• Allergic rhinitis: Causes nasal congestion, runny nose, affects breathing.

• Allergic conjunctivitis: Causes itching, redness of the eyes, affects vision.

Note:

• Complications of allergies can be very serious, affecting health and quality of life.

• Allergies should be treated promptly to avoid dangerous complications.

10. Prevention

• Avoid contact with allergens: Identify and avoid contact with allergens that cause disease.

• Strengthening resistance: Eating a balanced diet, exercising regularly, getting enough sleep…

• Maintaining a clean living environment: Maintain personal hygiene, clean the house regularly, avoid contact with chemicals, dust…

Note:

• Preventing allergies is better than treating them.

• Proactively learn about allergies and take appropriate preventive measures to protect your health.

11. Conclusion

Allergic reactions are an abnormal immune response caused by a disrupted immune mechanism. Understanding the causes, mechanisms, classifications, diagnosis, and treatment of allergies helps us to proactively prevent and treat the disease effectively, protecting our own health and that of our families.