A brief overview of hypersensitivity reactions and their types

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What is a hypersensitivity reaction

Hypersensitivity reaction can be defined as the immune system’s response like allergy and autoimmunity against any foreign body called antigen or allergen. Hypersensitivity reactions are troubling, damaging, and sometimes prove to be fatal. Such a response of the immune system is known as hypersensitivity reactions. 

Hypersensitivity reactions can also be defined as the harmful immunological response of the body against any foreign particle in the host body. It is also known as allergic reactions or immunological responses or most commonly known as allergy.

Why do hypersensitivity reactions take place?

Hypersensitivity reactions are the natural body immunological response or defensive mechanism that protects the body from foreign particles ( exogenous or endogenous ) like (dust, pollen, chemicals, or drug ). As a result, the immune system starts a chain of reactions associated with chemical-induced reactions resulting in rashes and allergies. Hypersensitivity reactions’ objective is to alert the body’s immune cells and start working to eliminate the foreign particle from the body.

Insensitive individuals’ immune systems become over-activated; thus, they get allergies, rashes, and inflammation so easily compared to other individuals, so such people become easy victims of allergies, hay fever, and other symptoms hypersensitivity reactions. 

Types of hypersensitivity reactions

First type of hypersensitivity reaction

Type 1 hypersensitivity reactions are also known as anaphylaxis or immediate hypersensitivity, as these reactions take place rapidly. These are the allergic reactions that are systemic or local, and the antibody involved is IgE. Type 1 hypersensitivity reactions occur when an antigen interacts with the membrane-bound antibody IgE of mast cells or basophils. Potential tissue damage takes place through histamine release. 

Pathophysiology of type 1 hypersensitivity reaction

Type 1 hypersensitivity reactions start with the sensitization due to contact with an antigen. This sensitization leads to the formation of antibody IgG that coats mast cells and basophil cells. These mast cells and basophils cells release histamine and other chemical mediators like prostaglandin leukotriene etc. This histamine causes the following physiological changes in the body. 

  • Vasoconstriction
  • Peripheral vasodilation
  • Bronchospasm
  • Pruritis
  • Rhinitis
  • Abdominal cramping
  • Hypoxia
hypertension reaction
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Hypersensitivity symptoms

Clinical findings revealed that type 1 hypersensitivity reactions are immediate and initiate within seconds to minutes upon exposure with the antigen.  While the late phase may occur after the immediate phase for the duration of hours (24 to 72 hrs). The common symptoms are 

  • Pruritus
  • Rash
  • Rhinitis
  • Edema 
  • Abdominal cramping
  • Allergic asthma
  • Anaphylaxis

Second type of hypersensitivity reaction

Type 2 hypersensitivity reactions are also known as cytotoxic reactions. These cytotoxic reactions are initiated by the antibody’s cytotoxic activities attached with antigen on the outer surface of the cell. These antibodies activate the complement-dependent lysis result in tissue damage. Antibodies involved in cytotoxic reactions are IgM and IgG. Antibodies in cytotoxic reactions directly respond to the antigen attached to the cell membranes that may be self or nonself. Examples of type 2 hypersensitivity reactions are blood transfusion reactions and good posture syndrome.

Pathophysiology of type 2 hypersensitivity reactions

The pathophysiology involves the attachment of antibody IgM and IgG with antigens that activate the complement pathways and Fc, mediated immune cells that lead to phagocytosis or cell lysis of antigen. It is called a cytotoxic reaction because it damages or eradicates the cells.

Its symptoms are almost the same as the type 1 hypersensitivity reactions mentioned above.

Third type of hypersensitivity reaction

Type 3 hypersensitivity reactions are also called immune complex reactions.  It involves the formation of antigen-antibody complexes. These antigen-antibody complexes participate in different tissues like skin, joints, vessels, and glomeruli and start the complement cascade pathway. This complement activation leads to the activation of inflammatory cells like monocytes and neutrophils—these cells else oxygen free radicals and lysosomal enzyme leads to tissue damage. The most common disease that includes type 3 hypersensitivity reactions is serum sickness and rheumatoid arthritis.

Hypersensitivity symptoms

One of the typical examples of type 3 hypersensitivity reactions is serum sickness. However, in general, the symptoms of type 3 hypersensitivity reactions appear after 1-2 weeks following initial exposure. Hypersensitivity symptoms include

  • Fever
  • Rash
  • Myalgia
  • Headache
  • Blurred vision
  • Diarrhea
  • Nausea
  • Vomiting
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Fourth type of hypersensitivity reactions

Type 4 hypersensitivity reactions are also known as delayed hypersensitivity or cell-mediated hypersensitivity reactions. Hypersensitivity reactions in type 4 are induced by T lymphocytes, followed by the phagocytotic activity of macrophages. These lymphocytes some chemicals called cytokines that cause cell damage. Many long-lasting and chronic disease shows type 4 hypersensitivity reactions like tuberculosis.

Pathophysiology of type 4 hypersensitivity reactions

It is a T cell-mediated reaction that starts with sensitization followed by penetration of antigen by the skin. This antigen is reuptake by Langerhans cells. These Langerhans cells migrate to lymph nodes and play their role in the formation of sensitized T lymphocytes. Repeated exposure with antigen results in macrophages’ activation and secretion of lymphokines and cytokines, leads to an inflammatory reaction in tissues.

Hypersensitivity  symptoms:

The most common example of type 4 hypersensitivity reactions is allergens contact dermatitis that affects the skin. Symptoms associated with it are mentioned as follows.


  • Rash
  • Redness
  • Papular acne
  • Serious oozing 
  • Puss formation 

What is crossreactivity?

The team cross-reactivity in hypersensitivity reactions refers to a few particles with antigen and initiation of hypersensitivity reactions. By simple means,  particles like antigen may initiate the hypersensitivity reactions upon linkage with antibody due to structure resemblance. Such particles are called cross reactant allergens. Examples of such cross reactant allergens  are 

  • Pollen I,e primary allergen (apricot, peaches, hazelnut, kiwi, carrot, apricot are  cross reactant allegiance)
  • Bird danger ( egg yolk)
  • Cat danger ( pork)
  • Latex ( avocado, banana , kiwi)
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Hypersensitivity reactions are the most prominent reasons for different types of allergies and allergic asthma. Basically, hypersensitivity is not itself a pathological condition. It works to protect the body from antigens and allergen but sometimes becomes fatal in hypersensitive patients or those individuals when the immune system is overactive. Sensitivity with some particular antigen is due to many reasons like it may be genetic or environmental but need immediate consideration to relieve the body from unwanted chemical reactions and inflammations.