Hypersensitivity is a condition that causes the body to respond very strongly especially in an undesirable manner to allergic substances or allergens. It can also be referred to as allergy or allergic reaction. Though they can be generally classified as exaggerated in vivo reactions mediated by the immune system to foreign bodies or antigens that entered the body and, immunological responses that are not normally harmful to the host; hypersensitivity reactions may destroy the host cell in the process of destroying the pathogen or allergen. Allergens are substances that provoke hypersensitivity reactions or allergy in an individual. Allergic substances include dust, pathogens, foods (e.g. nuts, egg and milk, sea food and some beans), venoms of some insects such as bees and wasps, drugs (e.g. penicillin and sulphonamides), animal hairs and even pollen grains from some flowers (e.g. poison oak plant).
These allergic substances enter the body via several routes especially through the mouth and nose or nares that are known to be rich in mucous substances. Immunoglobulin E (IgE) is amongst the antibodies that colonize the mucous membranes of these surfaces (aside secretory IgA); and the level of IgE in the serum of people exposed to allergens (e.g. parasites) is usually on the increase compared to normal individuals without exposure to allergic substances. This is because IgE is the primary antibody produced by the immune system against allergens; and IgE is generally known as reagenic antibody because it is the major immunoglobulin involved in anaphylactic reactions – in which the immunoglobulin binds to and brings about the degranulation of mast cells or basophils to produce pharmacologically active substances (e.g. histamine) that cause hypersensitivity reactions in the host.
Individuals react differently to different allergic substances; and allergens generally stimulate adverse immunological response in individuals who come in contact with them. In hypersensitivity reactions, the host’s immune system is provoked (especially by allergens) to act in an exaggerated fashion which is harmful to the body. Depending on the type of invading allergen, some allergic or hypersensitivity reactions could be immediate-type hypersensitivity or delayed-type hypersensitivity. While delayed-type hypersensitivity reactions occur slowly following the introduction of allergens into the body, the immediate-type hypersensitivity occur in a much faster or rapid manner after the exposure of the sensitized individual to a further dose of the allergen. An individual becomes immunologically sensitized after its first contact with an antigen (in this case an allergen); and this illustrates the primary response of the host’s immune system to the antigen – in which antibodies are produced against the invading antigen.
However, when the same host or individual comes in contact with the same allergen or antigen the second time, there is a heightened immunological response known as a secondary response – in which the allergic response or reaction actually occurs. In some cases, the secondary immunological response to the invading allergen or antigen may be excessive; and this phenomenon leads to the production of effector cells of the immune system that stimulate mild subclinical and localized inflammatory reaction in the body. Though such localized inflammatory response may be beneficial in protecting the host organism from the adverse effect of the invading pathogen; the response may become severe and out of control in such a way that it affect the body adversely, leading to the damage of the host’s tissues and even death in some cases. The biological processes leading to the damage of the host’s tissues when immunological responses (inclusive of the B and T cell response) to recognized foreign bodies or antigens that invaded the body generally results to an immunological response known as hypersensitivity reaction or allergy. There are mainly four types of hypersensitivity reactions, and these shall be highlighted in this section.
- Type I hypersensitivity
- Type II hypersensitivity
- Type III hypersensitivity
- Type IV hypersensitivity
Type IV hypersensitivity is generally referred to as delayed-type hypersensitivity because this type of hypersensitivity reaction involves mainly the cell-mediated immunity which may usually take a longer period of time to become apparent. Delayed-type hypersensitivity (DTH) reactions usually occur days or weeks after the body becomes sensitized following the introduction of antigens into the host. Immediate-type hypersensitivity includes Type I, Type II and Type III hypersensitivity because these types of hypersensitivity reactions mainly involves the humoral or antibody-mediated immunity in which immunoglobulins are produced instantaneously upon the body’s encounter with antigens. Unlike the DTH reaction whose symptoms normally take days to occur, the symptoms of the immediate-type hypersensitivity reaction occurs minutes to hours after the exposure of the host to the allergens or antigens. Hypersensitivity reaction is generally a heightened condition of immune responsiveness that causes damage to the host.
Abbas A.K, Lichtman A.H and Pillai S (2010). Cellular and Molecular Immunology. Sixth edition. Saunders Elsevier Inc, USA.
Actor J (2014). Introductory Immunology. First edition. Academic Press, USA.
Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.
Bach F and Sachs D (1987). Transplantation immunology. N. Engl. J. Med. 317(8):402-409.
Barrett J.T (1998). Microbiology and Immunology Concepts. Philadelphia, PA: Lippincott-Raven Publishers. USA.
Jaypal V (2007). Fundamentals of Medical Immunology. First edition. Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India.
John T.J and Samuel R (2000). Herd Immunity and Herd Effect: New Insights and Definitions. European Journal of Epidemiology, 16:601-606.
Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.
Roitt I, Brostoff J and Male D (2001). Immunology. Sixth edition. Harcourt Publishers Limited, Spain.
Zon LI (1995). Developmental biology of hematopoiesis. Blood, 86(8): 2876–91.