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The immune system is a crucial component of the human body for protection against dangerous diseases. However, people with allergies have hypersensitive immune systems that react to unharmful substances. Allergic disease is a common chronic disease that occurs in one in five people.[1] The common symptoms are swelling, red eyes, runny nose, and anaphylaxis.

What are the causes?

Causes for allergic reactions can be split into two categories: host and environmental. The host factor includes heredity, sex, race and age. This is the most common causation. There are four types of environmental factors: exposure to infectious diseases during early childhood, environmental pollution, allergen levels, and dietary changes. Exposure to allergens (substances causing allergic reactions) that are either inhaled, ingested or entered through the skin initiates the immune systems response by mass producing immunoglobulin E (IgE) which specifically react against certain allergens. [1]


What are the different types of allergies?

Allergies are classified into four types.

Type I: Immediate Hypersensitivity (Anaphylactic Reaction)

Type II: Cytotoxic Reaction (Antibody-dependent)

Type III: Immune Complex Reaction

Type IV: Cell-Mediated (Delayed Hypersensitivity)


Type I: Immediate Hypersensitivity (Anaphylactic Reaction)

The allergic reaction occurs when an antigen cross-links with membrane-bound IgE antibody of a mast cell. Histamine (a compound released by cells in response to the reaction causing contraction of muscles and capillary dilation), serotonin (a compound that constricts blood vessels) and bradykinin (causes similar reaction as histamines) released during anaphylactic reactions potentially cause tissue damage. This is the most common type and is often equated with the term ‘allergy’. Examples of type 1 reactions are hay fever, food allergies, insect venom allergies etc.


Type II: Cytotoxic Reaction (Antibody-dependent)

In this type of reaction, the antibody reacts directly with the antigen bounded to the cell membrane and induces cell lysis. This reaction is controlled by Immunoglobulin G (IgG) and Immunoglobulin M (IgM).


Type III: Immune Complex Reaction

IgG and IgM bind to antigens forming antigen-antibody complexes. These activate PMN, a type of white blood cell, which releases tissue damaging enzymes. This can cause other autoimmune diseases and chronic infectious diseases as well.


Type IV: Cell-Mediated (Delayed Hypersensitivity)

Cell-Mediated reactions are initiated by T-lymphocytes and mediated by effector T-cells and macrophages. The response involves interaction of antigens with the surface of lymphocytes. Sensitized lymphocytes produce cytokines, which biologically activate substances that affect the functions of other cells. This typically takes 48-72 hours to fully develop after contact with antigens. Tuberculosis and fungal infections exhibit delayed hypersensitivity. [2]


What happens to my body when I have an allergic reaction?

  1. Allergens like pollen or dust come into contact with the skin or mucous membrane and peptides are released

  2. The peptides attach themselves to IgE antibodies that were previously bounded to a mast cell (a migrant cell of connective tissues that contain histamines and other compounds that help the immune system) that is located in places where the body frequently comes in contact with allergens

  3. Histamines stored in mast cells are released the moment allergens bind to IgE antibodies

  4. The release of the inflammatory substance causes swelling of the skin, production of secretions, itching or narrowing of the airways[1]


Allergies cannot be cured but it can be treated and controlled by either taking prescribed medication or avoiding contact



[1] WebMD. (2018) What Are Allergies? The Basic Info You Need to Know. [online] Available at [Accessed July 24, 2019]

[2] John Hopkins Medicine. Allergies and the Immune System. [online] Available at [Accessed July 24, 2019]

[3] Dentalcare. Types of hypersensitivity reactions. [online] Available at [Accessed July 24, 2019]

[4] ECARF (July 2016) What happens in my body when I have an allergy? [online] Available at [Accessed July 24, 2019]

Written by Pavitra (Rita) Hsieh

Bangkok Patana School

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