Question

Define the differences between immediate and delayed-type hypersensitivity in terms of immune effectors, target tissues, antigens, and clinical outcome. (Section 24.8)

Short answer

Immediate-type hypersensitivity involves IgE and mast cells causing acute reactions, while delayed-type involves T cells and macrophages leading to chronic inflammation.

Step-by-step solution

Define Immediate-Type Hypersensitivity

Immediate-type hypersensitivity (ITH) involves the production of IgE antibodies by B cells in response to an allergen. These antibodies bind to Fc receptors on mast cells and basophils, which leads to the release of histamine and other inflammatory mediators upon subsequent exposure to the allergen.

Identify Immune Effectors in ITH

The main immune effectors in ITH are IgE antibodies, mast cells, and basophils. These cells and antibodies initiate an immediate allergic reaction.

Define Target Tissues in ITH

Target tissues in ITH typically include the skin, respiratory tract, and gastrointestinal tract as these areas are common sites for allergen entry.

Define Antigens in ITH

Antigens in ITH are generally environmental allergens such as pollen, dust mites, pet dander, and food proteins.

Explain Clinical Outcome of ITH

Clinical outcomes of ITH can include acute responses like hives, asthma attacks, and anaphylaxis, which occur within minutes of exposure to the allergen.

Define Delayed-Type Hypersensitivity

Delayed-type hypersensitivity (DTH) involves a T cell-mediated immune response where T helper cells recognize an antigen presented by antigen-presenting cells, leading to the activation of macrophages and cytotoxic T cells.

Identify Immune Effectors in DTH

The main immune effectors in DTH are T helper cells (Th1 type), macrophages, and cytotoxic T cells, contributing to a delayed immune response.

Define Target Tissues in DTH

Target tissues in DTH are usually areas where antigen-presenting cells interact with antigens, such as the skin (in cases like contact dermatitis) or deeper tissues (like in tuberculosis).

Define Antigens in DTH

Antigens in DTH can include intracellular pathogens (such as Mycobacterium tuberculosis), contact sensitizers (such as poison ivy), and some transplanted tissues.

Explain Clinical Outcome of DTH

Clinical outcomes of DTH include chronic inflammation and tissue damage, often appearing 24–72 hours after exposure to the antigen, manifesting as contact dermatitis or granulomas.

Key concepts

Immediate-Type Hypersensitivity

Immediate-type hypersensitivity (ITH) is a rapid allergic reaction. It occurs within minutes of exposure to an allergen. This type of hypersensitivity involves IgE antibodies, which are produced by B cells.
When an allergen enters the body, IgE antibodies bind to receptors on mast cells and basophils.
Upon subsequent exposure to the same allergen, these cells release histamine and other inflammatory substances. This results in symptoms like hives, asthma, and, in severe cases, anaphylaxis.
Typical allergens that trigger ITH include pollen, dust mites, pet dander, and certain foods.

Delayed-Type Hypersensitivity

Delayed-type hypersensitivity (DTH) is a slower immune response. It takes 24-72 hours to develop after exposure to the antigen.
Unlike ITH, DTH is mediated by T cells rather than antibodies.
Specifically, T helper cells recognize antigens presented by antigen-presenting cells, leading to the activation of macrophages and cytotoxic T cells.
This results in inflammation and tissue damage, as seen in conditions like contact dermatitis and tuberculosis. For example, poison ivy can cause a DTH reaction after skin contact.

Immune Effectors

The term 'immune effectors' refers to the components of the immune system that bring about a response. In ITH, the key immune effectors are:

• IgE antibodies
• Mast cells
• Basophils

These elements work together to initiate an immediate allergic reaction.
In contrast, DTH involves:

• T helper cells (Th1 type)
• Macrophages
• Cytotoxic T cells

These cells contribute to a delayed immune response, causing tissue inflammation and damage.

Clinical Outcomes

Clinical outcomes vary between immediate and delayed hypersensitivity.
For ITH, symptoms appear within minutes and can include:

• Hives
• Asthma attacks
• Anaphylaxis

These manifestations are acute and often require rapid medical intervention.
DTH, on the other hand, causes symptoms that take 1-3 days to appear.
These include:

• Contact dermatitis
• Granulomas
• Chronic inflammation

In both types, the immune system's response is meant to protect the body but can sometimes cause more harm than good.

Allergens

Allergens are substances that can cause an allergic reaction.
Common allergens in ITH include pollen, dust mites, pet dander, and certain foods like nuts and shellfish. These substances are usually harmless but can trigger severe reactions in some people.
In contrast, the antigens in DTH are often more complex. They can include intracellular pathogens like Mycobacterium tuberculosis, as well as contact sensitizers such as poison ivy. Transplanted tissues can also act as antigens, leading to a delayed immune response.

T Cell-Mediated Response

T cells play a crucial role in the immune system, especially in DTH.
In this type of hypersensitivity, T helper cells (Th1) recognize antigens presented by antigen-presenting cells.
This recognition triggers the activation of macrophages and cytotoxic T cells. These activated cells then release inflammatory molecules, leading to tissue damage and chronic inflammation.
This response is essential for combating intracellular pathogens such as viruses and certain bacteria. However, it can also cause problems in cases of contact dermatitis or organ transplantation.