Immunology- Hypersensitivity

Disorders which are caused by an inappropriately vigorous innate and/or adaptive response to antigens that pose little or no threat are called as hypersensitivities. The term allergy first appeared in the medical literature in 1906, when the pediatrician Clemens von Pirquet noted that the response to some antigens resulted in damage to the host, rather than in a protective response

Hypersensitivities are classically divided into four categories which are given as type I–IV hypersensitivity reaction which differ by the immune molecules and cells that cause them, and the way they induce damage.

Type I hypersensitivity reactions

• Also called as Allergy and atopy
• They are mediated by IgE antibodies that bind to mast cells or basophils and induce vasoactive mediator release.
• These reactions include the most common responses to respiratory allergens, such as pollen and dust mites, and to food allergens, such as peanuts and shellfish.
• Typical manifestations such as hay fever, asthma, hives, food allergies and eczema.

Type II hypersensitivity reactions

• Also called as antibody-mediated hypersensitivity
• It results from the binding of IgG or IgM to the surface of host cells, which are then destroyed by complement- or cell-mediated mechanisms. Ab directed against cell surface antigens mediates cell destruction via antibody-dependent cell-mediated cytotoxicity (ADCC) And mainly its target red blood cells (RBCs).
• For example, this is the fate for transfused red blood cells in transfusions between people differing in ABO blood types.
• Typical manifestation includes erythroblastosis fetalis and immune hemolytic anemia.

Type III hypersensitivity reactions

• Also called as immune complex-mediated hypersensitivity
• In these type of reaction, antigen-antibody complexes (such as those generated by the injection of foreign serum proteins) deposited on host cells or tissues which activates complement or the release of mediators from granulocytes, often causing inflammatory responses.
• Typical manifestations include serum serum sickness, rheumatoid arthritis and system lupus erythematosus.

Type IV hypersensitivity reactions

• Also called as Delayed-type hypersensitivity.            
• Mechanism of action is Sensitized T cells release cytokines that activate cytotoxic t cells or macrophages which mediate direct cellular damages.
• Common examples are the skin reactions caused by poison oak or poison ivy.
• Typical manifestations include contact dermatitis, tubercular lesions and graft rejection.