The immune system of the body mainly focus on the body’s two subsets of T-lymphocytes, known as T helper cells TH1 and TH2. In the normal immune response, TH1 cells releases the mediators which aids the body to defend against invasion from parasites, bacteria and viruses.
In hypersensitivity individuals, TH2 cells and their mediators encourage the immune system to recognise allergens as an invader, and produce a response against the foreign bodies.Some states that the people living in sterile environment free of exposure to antigen develops reduced immune response to fight against them.
For an allergy to take place,the allergen must be come into contact with the body.. Antigen-presenting CELLS SUCH as the macrophages and dentritic cells which are present in the mucosal surface of the cells sensitize the antigen.. Allergen enters into the immune system in many ways such as through inhalation,through skin contact, and antigen presenting cells sensitize these allergens and provoke an immune response and this allergen is processed and displayed on the surface of the antigen presenting cells. These cells then migrate to the T lymphocytes and presents the antigen to them which in turn activates B cells to secerete IgE antibodies. This IgE antibodies which is specific to the allergen comes into contact with mast cells on the mucosal surface and basophils on the blood.
After the period of sensitisation described above there is a period of latency, and on subsequent re-exposure to the allergen the hypersensitivity response is triggered: allergen cross-links with the IgE on the surfaces of the mast cell or basophil, causing the cell to ‘degranulate’ or release inflammatory mediators. These include largely histamine and other mediators, including cysteinyl leukotrienes, prostaglandins and kinins. They have different actions in terms of symptoms in different organs
TYPES OF hypersensitivity reactions:
TYPE 1 -This reaction is termed as ANAPHYLACTIC REACTION . It is mediated by the IgE antibodies in response to an antigen.
Eg : Administartion of drugs,
sting by wasp or bee,
Hay fever(seasonal hypersensitivity rhinitis) due to pollen sensitization of conjunctiva and nasal passages.
Bronchial asthma due to inhaled allergens such as dust,pollens
Cutaneous(skin) anaphylaxis leading to urticaria.
TYPE 2: (cytotoxix or cytolytic reactions)-
In this reaction humoral antibodies and complement system(a type if immune response) comes into the role.This reaction results in lysis of target cells.Cells take part in this reaction include tissue macrophages,platelets,natural killer cells ,neutrophils and eosinophils and amin antibodies taking place in this reaction include IgG and IgM.This reaction appears after 15-30 minutes after the exposure.
Eg:Transfusion Reactions,Autoimmune haemolytic anaemia,Idiopathic Thrombocytopnic Purpura,Drug induced cytotoxic antibodies,Haemolytic disease of the Newborn(Erythroblastosis foetalis),Graves disease,Myasthenia Gravis,Type 1 diabetes mellitus.
TYPE 3:(Immune complex Mediated)-
Participation of IgG and IgM antibodies,neutrophils,mast cells and complement system plays an important role in this type of reaction.Duration of the reaction is 6 hours after exposure to the antigen.
Eg:Immune complex Glomerulonephritis,Good Pasteur syndrome,Systemic Lupus Erythematosis,Rheumatoid artheritis,Framer’s Lung.
TYPE 4:(Delayed type or Cell mediated Reaction)-
The cells takes part in these reaction include Mast cells,Basophils,Macrophages and CD8+ cells.