Is Type 1 hypersensitivity immediate or delayed?

• HYPERSENSITIVITY REACTIONS
• TYPE-I HYPERSENSITIVITY REACTION
  • Types of Antigens
    • Antigens examples:
    • Type of Antibodies
    • The Fc-receptor are of two types:
    •  Formation of IgE
    • Normal IgE level:
  • Mast cells:
    • The cells with Fcε-receptor are:
  • Stages of Type 1 Hypersensitivity Reactions are:
    • Degranulation is an active process where:
    • Degranulation leads to the release of:
    • Chemical Mediators of type 1 Hypersensitivity Reaction are:
    • Primary or Preformed Mediators:
    • Secondary Mediators [Arachidonic Acid Metabolites]:
    • Mast Cell Associated Cytokines:
    • Histamine
    • Histamine clinically leads to:-
    • Histamine target areas show:-
    • Secondary Mediators
    • Mediators of the type 1 hypersensitivity reactions are:
    • Clinically Anaphylactic Reaction may be:
    • ATOPY
    • Atopy may be due to:
    • Nature Check Mechanism
  • Diagnosis Type 1 hypersensitivity reaction
  • Treatment
    • Hypersensitivity reactions and their summary:
    • Add Comment

HYPERSENSITIVITY REACTIONS

Definition

Hypersensivity reactions are defined as any of the following:

• Hypersensitivity reactions are excessive immune responses leading to damage in the host.
• These are inappropriate immune responses resulting in pathological changes in the host.
• Inappropriate responses to innocuous foreign substances are called allergy or hypersensitivity reactions.

Classification

1. Historically these are divided on a time basis:

1. 1. Immediate reaction: Reaction develops in less than 24 hours.
   2. Delayed reaction: When reaction develops within 24-48 hours.

1. Coomb’s and Gell’s classification: This was given in 1963.

• Ab-dependant reactions [B-L dependant]

1. 1. Type I        =          Anaphylactic and immediate reaction.
   2. Type II      =          Cytotoxic reaction.
   3. Type III     = Immune-complex disease.

• Ab-independent reaction [T- L dependant]

1. 1. Type IV     =          Delayed or cell-mediated immunity.
2. Kirk Patrick classification: This was given in 1987:

• 1. Class-I =          Reagin dependant injury
  2. Class-II =          Cytotoxic reaction
  3. Class-III =          Immune complex disease
  4. Class-IV =          Anti-receptor Ab-reaction
  5. Class-V =          Delayed and cell-mediated

The Coombs and Gell classification is more popular and is commonly used among these three classifications.

TYPE-I HYPERSENSITIVITY REACTION

1. Definition of type 1 hypersensitivity reaction
   This is also called immediate hypersensitivity when an IgE response is directed against the antigens like pollens and leads to the release of pharmacological mediators, such as histamine IgE-sensitized mast cells, and produces an acute inflammatory reaction with S/S like asthma or rhinitis.
2. This type 1 reaction can range from the life-threatening anaphylactic reaction to milder forms associated with food allergies.
3. Atopic allergy, including hay fever, asthma, and food allergy.

Type 1 Hypersensitivity reaction and the role of mast cell

Mast cell and basophil cell

1. This is also called an anaphylactic reaction [anaphylaxis – harmful] or allergic reaction. It has two presentations:
2. Immediate reaction: This reaction starts within 5-30 minutes and subsides in 60 minutes.
3. Late phase reaction: This starts in 2-8 hours and lasts 2-3 days.
4. The term allergy was used by Von Pirquet in 1906, meaning the changed reactivity of the host when encountering the antigen on a second or subsequent occasion.

Types of Antigens

1. Characteristics of antigen or allergins:
   1. Any substance which gives rise to the wheel and flare and symptoms of allergic diseases are from various sources.
   2. When these substances are purified, they have almost all been found to be proteins.
   3. Their molecular weight varies from 10,000 to 40,000 daltons.
   4. These proteins are all freely soluble in the aqueous solution but have many different biological functions.

The most common examples of antigens [allergens] are:

Antigens examples:

1. Heterologous proteins: e.g., antisera, hormones, enzymes, venom, and pollen.
2. Polysaccharides: e.g., Iron-dextran, gives severe reaction, and some other diagnostic agents like iodide and bromosulphophthalin, etc.
3. Therapeutic agents: e.g., drugs like antibiotics [Penicillin gives severe reaction] and vitamins.
4. Food: e.g., milk, wheat, fish, nuts, and chocolate.
5. Insects venoms like bees and wasps.

Type of Antibodies

1. IgE is the main Ab and is called reagin. It is cytotoxic Ab.
2. Once produced, it has love and affinity and attaches to high-affinity receptors [IgεR] present on Mast cells and basophils.

IgE receptor [IgεR] consists of 2-α [molecular weight of 45,000] and 2-β [molecular weight of 30,000] chains.

Fig 88: IgE Receptor structure [Fc receptor]

Type 1 Hypersensitivity reaction and Fc receptor for IgE on Mast/Basophils

The Fc-receptor are of two types:

1.  High affinity is called IgεR1 [IgεRI].
2.  Low affinity is called IgεR2 [IgεRII]

 Formation of IgE

1. Antigen stimulates B-L with the help of antigen-presenting cells and Th2. These activated B-lymphocytes are transformed into plasma cells and produce IgE. One hundred molecules of IgE are needed to initiate the reaction.

   Fig 89: Formation of IgE molecule by activation of B-lymphocyte

   Mast cells and Basophils release histamine and heparin

2. IgE is a local response occurring at the entry site of the allergins.
3. Locally produced IgE will first sensitize the local mast cells, and the rest of the IgE enters the circulation and binds to the Fc-receptors on basophils and tissue fixed mast cells throughout the body.
4. The half-life of serum IgE is only 2.5 days, while mast cells may remain sensitized for up to 12 weeks.

IgE structure and Fc receptor

1. IgE level is raised in allergic diseases and markedly raised in parasitic infestation.
2. There is no transfer across the placenta. In contrast, IgG can cross the placental barrier.

Type 1 hypersensitivity reaction mechanism

Normal IgE level:

• Serum of normal individual =