Delayed Type Hypersensitivity (DTH) is a cell-mediated immune response that unfolds over time, rather than immediately after exposure. It involves specialized immune cells, not antibodies. This delayed characteristic distinguishes it from rapid allergic responses. DTH reactions are common, ranging from mild skin irritations to significant inflammatory conditions.
The Immune System’s Delayed Response
The mechanism behind Delayed Type Hypersensitivity involves a two-phase process initiated by T lymphocytes and macrophages. The first phase, sensitization, occurs upon initial exposure to an antigen. During this exposure, antigen-presenting cells (APCs), such as dendritic cells and macrophages, capture and process the antigen. These APCs then present antigen fragments on their surface, bound to major histocompatibility complex (MHC) molecules, to naive T cells.
This interaction activates specific CD4+ helper T cells, predominantly Th1 cells, which proliferate and differentiate into memory T cells. These memory cells persist in the body, ready to respond to subsequent encounters with the same antigen.
The second phase, elicitation, begins when a sensitized individual is re-exposed to the same antigen. Upon re-exposure, memory T cells are quickly activated by antigen-presenting cells at the site of contact. Activated CD4+ T cells, particularly Th1 cells, release cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α). These cytokines attract and activate other immune cells, including macrophages, to the antigen site. Activated macrophages then contribute to the inflammatory response by releasing pro-inflammatory mediators and hydrolytic enzymes, which cause tissue damage and swelling. The “delayed” nature of the reaction, appearing 48 to 72 hours after re-exposure, is due to the time required for T cells to migrate, activate, and orchestrate the full inflammatory response.
Triggers and Visible Reactions
Delayed Type Hypersensitivity reactions can be triggered by a diverse array of substances. One widely recognized cause is contact with certain plants, particularly those from the Rhus genus, which includes poison ivy, poison oak, and poison sumac. The oily resin called urushiol in these plants induces a classic DTH reaction, manifesting as allergic contact dermatitis.
Metals like nickel, cobalt, and gold are also common culprits, often found in jewelry, belt buckles, or even some medical devices. Various chemicals present in cosmetics, dyes, and rubber products can also act as triggers. Beyond environmental exposures, certain medications can induce delayed drug hypersensitivity reactions, presenting with symptoms ranging from skin rashes to more severe systemic involvement.
The visible signs of a DTH reaction typically emerge between 24 and 72 hours after exposure to the triggering substance. The initial manifestation often involves redness and swelling at the site of contact. This can progress to intense itching, the formation of fluid-filled blisters, and a hardening or thickening of the affected skin. In more severe instances, particularly with widespread exposure or potent allergens, the reaction may encompass fever and involvement of internal organs.
Identifying and Managing DTH
Identifying Delayed Type Hypersensitivity reactions relies on diagnostic tests that observe the delayed immune response. For contact dermatitis, the most common form of DTH, a patch test is used. This test involves applying small amounts of suspected allergens to the skin, typically on the back, under adhesive patches. The patches remain in place for 48 hours, after which a healthcare professional examines the skin for localized reactions, such as redness, swelling, or blistering.
Another widely used diagnostic application of DTH is the tuberculin skin test (PPD test). A small amount of tuberculin protein is injected just under the skin, usually on the forearm. The site is then evaluated 48 to 72 hours later for induration, which is a firm, raised area, indicating a positive reaction and prior exposure to tuberculosis bacteria. Interpretation of these tests involves measuring the size of the reaction and considering other clinical factors.
Managing DTH reactions centers on avoiding the identified trigger. Once the specific allergen is pinpointed through testing or patient history, steps should be taken to minimize or eliminate future exposure. For symptomatic relief, topical corticosteroids are frequently applied to reduce inflammation and itching. Cool compresses can also help soothe the affected area and alleviate discomfort. In cases of widespread or severe reactions, oral corticosteroids may be prescribed to control the immune response. While many DTH reactions resolve with these measures, persistent or severe symptoms, or any signs of systemic involvement like fever, warrant medical attention from a healthcare provider.