Hypersensitivity
Muhammad Iqbal
Lecturer
KMU

Introduction
• Hypersensitivity is overreaction to an allergen.
• Or it is an antigenic response beyond that is
considered normal.
• It is also known as allergy.
• Hypersensitivity occurs when a sensitized
person re-exposes to the allergen.
• Allergy -- 4 types

Type 1(Anaphylactic) Reaction
• quickly developing (2—30 min) after contact of Ag
(allergen) with Ab after previous exposition.
• IgE produced in response to an antigen like insect
venom or plant pollen grains, bind to the surface of
mast cells and basophils
• mediated through histamine, leukotriens,
prostaglandins released by mast cells & basophils.
• increase of vascular permeability, vasodilatation,
bronchoconstriction, increased mucoproduction

• Local reaction - skin or mucosa
• Bee sting, food allergy, hay fever (pollinosis),
asthma, urticaria (hives)
• Systemic reaction ( Anaphylactic shock)
• Parenteral administration of Ag (e.g. penicillin)
• Within minutes-- itching, redding of skin,
breathing problems, abdominal pain, vomiting,
diarrhea.
• During several min -- death may occur due to
collapse of circulation (drop in blood pressure)

Type II (Cytotoxic) reaction
• Antigens –
• IgG and IgM are involved
• Incompatible blood transfusion -- destruction
of RBCs
• Rh-incompatibility - erythroblastosis fetalis
(anti-Rh Ab)

Type III (Immune Complex ) Reaction
• Formation of Ag-Ab complexes (immune complexes)
• Ag-Ab complexes are deposited in organs
• IgG is usually involved
• Acute inflammation of tissues and organs
• e.g- repeated exposure to animal serum (antitetanic)
• Vessel wall -- thrombosis - ischemic necrosis
• Manifestation …….. 2—8 hours
• For example
• Glomerulonephritis
• systemic lupus erythematosus (SLE)

Type IV (cell mediated) Reaction
• Cellular immunity --- T-cells
• e.g. tuberculin reaction - Mantoux test,
• Similarly IDDM…
• person previously exposed to TBC develops after
intradermal injection of Ag skin induration
• manifestation in 1-3 days .

Transplantation rejection
• Transplantation:
• - autologous (own tissue)
• - homologous - Other human tissue
• - heterologous - animal tissue (e.g, pig skin)
• Contact dermatitis is another example