Delayed Type Hypersensitivity Response

• DTH is a cell-mediated immune response, not antibody-mediated, typically occurring 24-72 hours post-exposure.
• It involves T lymphocytes, particularly CD4+ helper T cells, and macrophages as primary effector cells.
• Common triggers include exposure to certain metals, chemicals, and antigens from intracellular pathogens.
• Symptoms often involve localized skin reactions such as redness, swelling, and hardening of the tissue.
• Understanding DTH is vital for diagnosing conditions like contact dermatitis and tuberculosis.

What is Delayed Type Hypersensitivity Response?

What is delayed type hypersensitivity? It is a specific type of immune reaction that takes several hours or even days to develop after exposure to an antigen. This response is primarily mediated by T lymphocytes and macrophages, rather than antibodies, distinguishing it from immediate hypersensitivity reactions. It is also known as Type IV hypersensitivity, representing a critical component of the adaptive immune system’s defense against intracellular pathogens and certain foreign substances. The delayed nature arises because it takes time for T cells to migrate to the site of antigen exposure, become activated, and orchestrate the inflammatory response.

This cell-mediated immunity is essential for host defense against various intracellular bacteria, viruses, fungi, and parasites. However, it can also be responsible for adverse reactions, such as those seen in contact dermatitis or transplant rejection. The characteristic feature is the slow onset and prolonged duration of the inflammatory response, which can lead to tissue damage if not properly regulated.

Mechanism and Causes of Type 4 Hypersensitivity

The delayed hypersensitivity mechanism involves a complex interplay of immune cells. Upon initial exposure to an antigen, antigen-presenting cells (APCs), such as macrophages or dendritic cells, process the antigen and present it to naive T helper (CD4+) cells. These T cells become sensitized and differentiate into effector T helper 1 (Th1) cells. During a subsequent exposure to the same antigen, these sensitized Th1 cells are rapidly activated at the site. They release various cytokines, including interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and chemokines. These cytokines recruit and activate macrophages, which then become the primary effector cells, causing localized inflammation and tissue damage. This cellular cascade typically peaks 24 to 72 hours after antigen re-exposure.

The causes of type 4 hypersensitivity are diverse, often involving small molecules that act as haptens, which bind to self-proteins to become immunogenic, or antigens from persistent infections. Common examples include:

• Contact allergens: Substances like nickel (found in jewelry), urushiol from poison ivy/oak, certain chemicals in cosmetics, latex, and specific dyes.
• Intracellular pathogens: Antigens from bacteria such as Mycobacterium tuberculosis (responsible for the tuberculin skin test reaction), various fungi, and some viruses.
• Certain drugs: A range of medications can induce DTH reactions in susceptible individuals.
• Transplant antigens: In cases of organ rejection, T cells recognize foreign major histocompatibility complex (MHC) molecules on donor cells.

According to the American Academy of Dermatology, contact dermatitis, a common manifestation of DTH, affects a significant portion of the population, with estimates suggesting up to 15-20% of individuals experiencing it at some point in their lives.

Common Symptoms of Delayed Reactions

The symptoms of delayed hypersensitivity reaction are typically localized and manifest at the site of antigen exposure. These reactions usually appear 24 to 72 hours after contact and can persist for several days. The most characteristic symptoms include:

• Erythema: Redness of the skin, indicating increased blood flow to the inflamed area.
• Induration: Hardening or thickening of the skin, caused by the infiltration of immune cells and fluid accumulation.
• Edema: Swelling due to fluid leakage from blood vessels into the surrounding tissues.
• Pruritus: Intense itching, which can be a significant source of discomfort.
• Vesicles or Bullae: In more severe reactions, small fluid-filled blisters (vesicles) or larger blisters (bullae) may form, particularly in cases of severe contact dermatitis.
• Warmth: The affected area may feel warm to the touch due to inflammation.

For instance, in a positive tuberculin skin test (Mantoux test), an individual previously exposed to Mycobacterium tuberculosis will develop a firm, red, raised bump (induration) at the injection site within 48-72 hours. This reaction is a classic example of a DTH response, indicating prior sensitization to mycobacterial antigens. The severity of symptoms can vary widely depending on the individual’s immune response and the nature and concentration of the antigen.