What is DTH in Medical?

DTH in medical contexts stands for Delayed-Type Hypersensitivity. It refers to a specific type of immune reaction characterized by a delayed onset of symptoms following exposure to an antigen.

Understanding Delayed-Type Hypersensitivity (DTH)

Delayed-Type Hypersensitivity is an inflammatory response that develops 24 to 72 hours after exposure to an antigen that the immune system recognizes as foreign. Unlike immediate hypersensitivity reactions (like allergies, which primarily involve antibodies), this type of immune response involves mainly T cells rather than antibodies (which are made by B cells). This makes DTH a crucial component of the body's cell-mediated immunity.

Key Characteristics of DTH

DTH reactions are distinct from other hypersensitivity types due to several defining features:

• Delayed Onset: The hallmark of DTH is its late appearance, typically between 24 and 72 hours post-exposure, as opposed to immediate reactions which occur within minutes to a few hours.
• Cell-Mediated Immunity: The primary immune cells involved are T cells, specifically CD4+ helper T cells (Th1 cells) and sometimes CD8+ cytotoxic T lymphocytes. These cells recruit and activate other immune cells like macrophages, which are central to the inflammatory response. B cells and antibodies play a minimal role.
• Antigen Specificity: The reaction is highly specific to the antigen that initially triggered the immune system's sensitization.
• Inflammatory Nature: The response manifests as localized inflammation, often involving redness (erythema), swelling (edema), induration (hardening due to cellular infiltration), and sometimes blistering or necrosis.

Common Examples and Clinical Significance

DTH reactions are crucial in various physiological and pathological processes, serving both protective and sometimes detrimental roles.

1. Diagnostic Tool: Tuberculin Skin Test (PPD Test)

• One of the most widely recognized applications of DTH is the tuberculin skin test (PPD test), used to screen for exposure to Mycobacterium tuberculosis, the bacterium causing tuberculosis.
• A small amount of purified protein derivative (PPD) from the bacteria is injected intradermally. If a person has been exposed to TB in the past, their immune system (specifically memory T cells) will recognize the antigen, leading to a localized DTH reaction (induration and redness) within 48-72 hours at the injection site. This reaction indicates previous exposure or infection.

2. Allergic Contact Dermatitis

• This common skin condition is a classic example of a DTH reaction. It occurs when the skin comes into contact with certain sensitizing allergens (e.g., nickel, urushiol from poison ivy/oak, certain chemicals in cosmetics, dyes, or latex).
• Upon re-exposure, previously sensitized T cells migrate to the contact area, release cytokines, and activate macrophages, leading to inflammation, intense itching, redness, and blisters that typically appear hours to days after contact.

3. Granuloma Formation

• In chronic infections (e.g., tuberculosis, leprosy, fungal infections) or in response to persistent foreign bodies (e.g., sutures, talc), the body's immune system may form granulomas. This process is a specialized form of DTH, where activated macrophages and T cells cluster around the offending agent to wall it off, preventing its spread.

DTH vs. Immediate Hypersensitivity

While both are immune responses, DTH is fundamentally different from immediate hypersensitivity reactions (Type I, II, and III). The primary distinction lies in the effector mechanisms and timing:

Understanding DTH is crucial in medical diagnosis, monitoring of immune responses, and the development of treatments for various immune-mediated diseases and chronic infections.