HYPERSENSITIVE REACTION (HSR) 

Immune response that have deleterious effect resulting in tissue damage are called as hyper sensitivity. The reaction  within humoral branch are called as immediate hypersensitivity which manifest its symptoms within minutes.
These are of 3 types-
1. Type I          2. Type II                 3. Type III

Hypersensitivity reaction with in cell mediated branch show its symptoms to a delay of two days following Ag exposure. *It is delayed type hypersensitivity.


Immediate Hypersensitivity-
In immediate hypersensitivity reaction different isotypes induce different immune effector molecules.
 IgE induces mast cell degranulation
 IgM & IgG induces hypersensitivity reaction by activating complement system.

Type I : initiation time 2 to 30 min
In this type hypersensitivity IgE molecules bound to mast cells & basophils get sensitized by cross linking through allergen molecules which induces degranulation in respective cell. Upon degranulation of both Mast cell and basophils, they secrete various allergic mediators.
Receptors for IgE binding on mast cells and basophils are of two types:-
1. Fc εR  I --- high affinity     2. Fc εR II---- low affinity

Type II HSR
This is also called as antibodies mediated cytotoxic HSR.
In this reaction cytotoxic cells with Fc receptors bind to Fc portion of antibodies on target cell and promote the killing. Binding of antibodies to the surface of the cell can result in:
o Phagocytosis of the cell
o Lysis of the cell
o Damage to molecules on the cell surface (e.g., myasthenia gravis)
o Activation of cell-surface receptors (e.g., thyrotoxicosis)
Initiation time for such reaction is 5 to 8 hrs.

Type III HSR
Whenever the antigen- antibody complexes get deposited very near to site of antigen entry or these complexes flow in blood, they will develop localized and systemic reaction respectively. In such reaction complement split products will causes Mast cell degranulation and consequent increase in local vascular permeability.
Such type of reaction will take 2-8 hrs to get initiated.


Type IV Delayed type of reaction:---  such type of HSR has initiation time – 24 to 72 hrs. These are the important reaction against bacteria and parasites that can live intra cellular. In this HSR cell mediated branch of immune system will sensitized Tdth  which will induce release of various cytokines. The net effect of these cytokines is to cause an accumulation and activation of macrophages which will leads to localized non specific destruction of cell and so intracellular pathogens are also get cleared.