Types of Hypersensitivity Reactions Explained with Examples for NEET PG

The study of the immune system is very important in medical science. It helps us understand how the body protects itself from harmful substances. However, sometimes the immune system reacts in an inappropriate or exaggerated way. These reactions can harm the body instead of protecting it. Such responses are known as hypersensitivity reactions.

Understanding Types of Hypersensitivity Reactions is essential for students preparing for NEET PG medical postgraduate exam. It is a frequently asked topic in pathology. It also helps in understanding many clinical conditions, such as allergies, autoimmune diseases, and transplant rejection.

Here, you will learn about the different Types Of Hypersensitivity Reactions. The aim is to help you build a strong foundation and improve your preparation.

What is Hypersensitivuty Reactions?

Hypersensitivity reactions are exaggerated or inappropriate immune responses against an antigen or allergen. These reactions can cause tissue damage and disease.

Normally, the immune system protects the body. It identifies harmful substances like bacteria, viruses, and toxins. It then removes them safely. But in hypersensitivity, the immune system overreacts. This leads to inflammation and injury.

Hypersensitivity reactions can occur due to:

• Environmental allergens like pollen and dust

• Food substances

• Drugs such as antibiotics

• Autoimmune responses against self-antigens

These reactions are classified into different types based on their mechanism and immune components involved. This classification helps in better understanding and diagnosis.

Immunity - Hypersensitivity Reactions

The classification of Types Of Hypersensitivity Reactions is based on the Gell and Coombs system. It divides hypersensitivity into four main types. A fifth type is sometimes added for better understanding.

Type I – Anaphylactic (Immediate) Hypersensitivity

This type is mediated by IgE antibodies. It occurs quickly after exposure to an allergen.

Mechanism: During the first exposure, the allergen enters the body. Antigen-presenting cells process it and present it to T cells. These T cells become TH2 cells. TH2 cells release interleukins such as IL-4, IL-5, and IL-13.

• IL-4 helps in antibody production

• IL-5 activates eosinophils

• IL-13 increases mucus production

These signals lead to the formation of IgE antibodies. IgE attaches to mast cells. This stage is called sensitization.

During the second exposure, the allergen binds to IgE on mast cells. This causes mast cell degranulation. Histamine and other mediators are released.

Effects:

• Vasodilation

• Bronchoconstriction

• Increased mucus secretion

Phases:

• Early phase: occurs within 2 hours

• Late phase: occurs after 2 to 24 hours due to eosinophils

Examples:

• Allergic rhinitis

• Asthma

• Allergic conjunctivitis

• Drug reactions

Type II – Cytotoxic Hypersensitivity

This type involves IgG or IgM antibodies directed against fixed antigens on cells or tissues.

Mechanism: Antibodies bind to antigens present on the surface of cells. This leads to:

• Opsonization and phagocytosis

• Complement activation

• Antibody-dependent cellular cytotoxicity

In this process, immune cells destroy the targeted cells.

Examples:

• Blood transfusion reactions

• Hemolytic anemia

• Rh incompatibility

• Pemphigus

Some conditions, like myasthenia gravis and Graves disease, involve antibodies against receptors. These are sometimes classified under a separate type.

Type III – Immune Complex Hypersensitivity

This type involves antigen-antibody complexes that circulate in the blood.

Mechanism: It occurs in three steps:

1. Formation of immune complexes

2. Deposition in tissues

3. Activation of complement system

These complexes get deposited in organs like skin, joints, and kidneys. This leads to inflammation and damage.

Key Feature: Both antigen and antibody are free in circulation before forming complexes.

Duration: Symptoms usually appear after several days.

Examples:

• Systemic lupus erythematosus

• Serum sickness

• Post-streptococcal glomerulonephritis

• Arthus reaction

Type IV – Delayed Type Hypersensitivity

This type is cell-mediated. It does not involve antibodies.

Mechanism: Antigen-presenting cells present the antigen to T cells. These T cells become TH1 cells. TH1 cells release cytokines such as interferon-gamma.

This activates macrophages and leads to inflammation. CD8 T cells also play a role by causing cell death.

Key Feature: The reaction is delayed. It usually appears after 24 to 72 hours.

Examples:

• Tuberculosis skin test

• Contact dermatitis

• Leprosy tests

• Granuloma formation

Type V – Stimulatory Hypersensitivity

This type is sometimes considered separately.

Mechanism: Antibodies bind to receptors on cells. Instead of destroying them, they stimulate the receptors.

Examples:

• Graves disease

• Myasthenia gravis

In Graves disease, antibodies stimulate the thyroid gland. This leads to increased hormone production.

Summary of Types Of Hypersensitivity Reactions

To revise quickly, here is a simple summary of Types Of Hypersensitivity Reactions:

• Type I: Immediate, IgE mediated, allergic reactions

• Type II: Cytotoxic, antibody-mediated cell destruction

• Type III: Immune complex deposition

• Type IV: Delayed, cell-mediated response

• Type V: Receptor stimulation by antibodies

Understanding these Types Of Hypersensitivity Reactions helps in identifying disease patterns and treatment approaches.

Controversial Hypersensitivity

Some diseases do not fit into a single category. They involve more than one type of hypersensitivity. These are called controversial or mixed hypersensitivity reactions.

Examples include:

Rheumatoid Arthritis

It shows features of both Type III and Type IV hypersensitivity. Immune complexes are involved, and T cells also play a role.

Hypersensitivity Pneumonitis

This condition involves immune complex deposition and cell-mediated immunity. Hence, it includes both Type III and Type IV mechanisms.

Systemic Lupus Erythematosus

This disease mainly involves Type III hypersensitivity. However, some features also resemble Type II reactions.

Transplant Rejection

Different types of hypersensitivity are involved depending on the stage:

• Hyperacute rejection: Type II

• Acute rejection: Type II and Type IV

• Chronic rejection: Type IV

These conditions show that the immune system is complex. A single classification may not always explain the full mechanism.

The topic of Types Of Hypersensitivity Reactions is important for both NEET PG exam and clinical understanding. It explains how the immune system can sometimes cause harm instead of protection.

A clear understanding of Types Of Hypersensitivity Reactions also helps in solving case-based questions. It improves accuracy in exams and strengthens conceptual knowledge.

Regular revision and practice will make this topic easier. Focus on mechanisms, examples, and key differences. With consistent effort, you can build a strong command over this important area of pathology.