It is defined as a condition where there is exaggerated immune response.  

Causes includes – 

• Autoimmunity 
• Reactions against microbes 
• Reactions against environmental antigens 

Hypersensitivity reactions are of  4 types 

• Type I – Immediate 
• Type II – Antibody mediated 
• Type III – Immune comple 
• Type IV – T cell mediated 

Components of Immune system - Lymphocytes – B &T cells (CD 4 & CD 8 cells), neutrophils, basophils, eosinophils, mast cells, macrophages and complement system.

Hypersensitivity reaction –Type I 

• Immune response rapidly develops in a previously sensitised person 
• Peak action time is 15-30 minutes 
• Etiology Genetic basis, pollutants and viral infections 
• Mediated by IgE - mediators from mast cells and basophils 
• Eg- food, drug allergy, allergic rhinitis, systemic anaphylaxis and local anaphylaxis 
• Reaction- local, systemic, atopy 

Systemic anaphylaxis includes - administration of antisera e.g. anti-tetanus serum (ATS), administration of drugs e.g. penicillin  and sting by wasp or bee 

Local anaphylaxis includes like hay fever, bronchial asthma due to allergy to inhaled allergens like house dust, food allergy to ingested allergens like fish, cow’s milk, eggs etc, cutaneous anaphylaxis due to contact of antigen with skin, angioedema, an autosomal dominant inherited disorder characterised by laryngeal oedema, oedema of eyelids, lips, tongue and trunk.

Hypersensitivity reaction - Type II 

• There is reaction of humoral antibodies that attack cell surface antigens and cause cell lysis 
• The reaction develops in 15-30 minutes 
• HLA-linked, exposure to foreign / tissue cells is the etiology 
• Antibody mediated –IgG or IgM 
• Effector cell – complement, FcR cells(macrophages, NK cells) 
• Eg Cytotoxic antibodies to blood cells such as transfusion reaction, Cytotoxic antibodies to tissue cells such as Grave’s disease 

Hypersensitivity reaction - Type III

• Results from deposition of antigen-antibody complexes on tissues within 6 hours 
• Etiology is persistence of low grade infection, environmental antigens, autoimmune process 
• Mediated by Immune complex - IgG or IgM 
• Eg. Immune complex glomerulonephritis, Goodpasture’s syndrome, collagen diseases and drug induced vasculitis 

Hypersensitivity reaction -Type IV 

• It is a cell-mediated slow and prolonged response, also called as delayed hypersensitivity reaction. The reaction occurs about 24 hours after exposure to antigen and the effect is prolonged which may last up to 14 days 
• Mediated by T cells 
• Eg contact allergy, granulomas,tuberculin 
• Peak action time is after 24 hours 
• It is cell mediated reaction and CD8+ T cells, cutaneous antigens being the etiology 
• Eg. includes reaction against microbacterial antigen, reaction against virus infected cells, Reaction against tumor cells 

Article by Dr. Siri P. B.