Types of Hypersensitivity

 Hypersensitivity is classified in two ways. They are:

 1. Based on the time taken for the reaction 

2. Based on the different mechanisms of pathogenesis. 

 I. Classification Based on the Time Taken for Reactions 

Based on the time required for a sensitized host to develop clinical reactions upon reexposure to the antigen, hypersensitivity is classified into two categories They are: 

1. Immediate hypersensitivity 

2. Delayed hypersensitivity. 

 1. Immediate Hypersensitivity

 When the immune reactions manifest in a short duration of time, within minutes, the hypersensitivity is called immediate type. Most of the hypersensitivity to drugs like penicillin belong to this category. Hay fever is another example. Immediate hypersensitivity is characterized by the following salient features: 

1. This immune reaction appears and disappears rapidly.

2. Inflammatory response occurs in a few minutes. 

 3. It produces urticaria, wheal and granulocyte accumulation. 

 4. It involves the interaction of antigen and antibody. 

 5. It is handled by B cells by the production of antibodies. Hence immedi te hypersensitivity is antibody - mediated.

 6. It can be passively transferred from one host to another by the transfer of serum. 

 7. Desensitization can be easily done by small dose of allergen.

 8. It is inhibited by antihistamine drugs except for Arthus reaction. 

 

2.Delayed Hypersensitivity 

When the immune reactions manifest slowly after 24 hours to 72 hours, the ypersensitivity is called delayed type. 

 A familiar example for delayed hypersensitivity is Mantoux reaction * ob ined by injection of tuberculin into the skin of an individual in whom previous nfection with the Mycobacterium had induced a cell mediated immunity. Saras Publication The reaction appears after 24 to 48 hours.

 The reaction is characterized by ythema and induration. 

 Delayed hypersensitivity is characterized by the following salient features: 

1. This immune response appears slowly and lasts longer. 

 2. Inflammatory response manifests only after 24 to 48 hours. 

3. It produces erythema *, induction and lymphocyte infiltration. 

 4. It involves the reaction between antigens and T - cells.

 5. As it is handled by T cells, it is a cell mediated immunity.

 6. It cannot be transferred from the sensitized host to another by the transfer serum. But it can be transferred by the transfer of T cells. 

 7. Desensitization cannot be easily done by drugs. 

 8. It is suppressed by corticosteroids.