Synonyms and key differences
Type IV allergic reactions are known in medical literature by several equivalent terms, each reflecting a specific aspect of the pathological process. Most commonly, the term delayed-type hypersensitivity (DTH) is used. In addition, this type of reaction is referred to as cell-mediated, infectious-allergic, and tuberculin-type.
The fundamental, key difference between type IV reactions and all other types of allergy lies in the complete absence of any role for antibodies (Abs) in the mechanism of their development. The main and only effectors here are specific T cells. It is they that take on the task of recognizing the foreign agent and triggering the subsequent cascade of tissue damage.
Initiation and cellular implementation mechanism
The development of delayed-type hypersensitivity represents a strict sequence of cellular interactions. The process begins when a foreign antigen (Ag) enters the body and is captured by specialized structures.
- Sensitization. The first stage of the implementation mechanism is the interaction of T cells with a foreign antigen. This process does not occur directly—it requires the obligatory participation of antigen-presenting cells. They prepare and "present" the antigen to T lymphocytes. As a result of this contact, sensitized T cells are formed, ready for active action.
- Phagocyte recruitment. In the second stage, macrophages are actively recruited to the focus of developing inflammation.
- Destruction. The collaborative work of various T lymphocytes (predominantly T killers) and phagocytes aims to detect the foreign antigen. Upon successful detection, these cells subject not only the antigen itself to destruction, but also its carrier, which can be either a whole cell or a non-cellular structure.
Mechanisms of damaging action
The pathological impact on tissues in type IV allergic reactions is realized through two main pathways, in which sensitized T lymphocytes play the main role.
- Direct cytotoxic effect. Sensitized T lymphocytes (primarily T killers) come into direct contact with target cells bearing a foreign antigen and exert a direct destructive effect on them.
- Mediated effect. In addition to direct destruction, T cells secrete specific biologically active substances—lymphokines. Through these mediators, an indirect damaging effect on surrounding tissues is realized, enhancing the inflammatory response and attracting new batches of effector cells to the lesion.
Pathophysiological changes at the tissue level
The development of DTH is accompanied by pronounced pathophysiological changes in tissues. The main processes include alteration, a specific inflammatory response, and severe microcirculatory disorders.
Alteration in type IV reactions has a pronounced destructive character. Large-scale destruction and subsequent elimination (removal) of not only damaged but completely unchanged cells, as well as non-cellular elements of the body's own tissues, take place.
In parallel, serious microcirculatory disorders develop. Hemocirculation (blood circulation) and lymph circulation disturbances occur in the lesion focus. These vascular failures inevitably lead to the development of capillariotrophic insufficiency—a condition in which tissues stop receiving adequate nutrition. The logical outcome of such insufficiency is profound dystrophy and subsequent necrosis (death) of the affected tissue.
Granulomatous inflammation
The most typical and characteristic morphological manifestation of type IV allergic reactions is granulomatous inflammation. In response to the presence of antigen in tissues, active formation of specific nodules—granulomas—begins.
The composition of such an allergic granuloma is highly specific and reflects the cellular nature of the immune response. Initially, lymphocytes and mononuclear phagocytes accumulate in the focus. Subsequently, these phagocytes undergo a complex morphological transformation: they turn into epithelioid cells and fuse to form giant cells. In addition, fibroblasts, which participate in tissue organization processes, are part of the forming granuloma.
Classic clinical examples of reactions proceeding with the formation of such granulomas and the development of DTH are diagnostic tests—the tuberculin and brucellin tests.
Briefly: answers to topical questions
What specific lymphokines do T cells secrete in type IV allergic reactions?
In type IV allergic reactions, sources specify the lymphokines/cytokines of T_H1 cells involved in DTH:
- Interleukin-2 (IL-2)—the main growth factor for T lymphocytes, ensuring the proliferative expansion of T cell clones.
- Interferon-γ (IFN-γ)—a key mediator of DTH; a potent macrophage activator that enhances phagocytosis and the destruction of microorganisms.
- TNF-β (lymphotoxin-α)—listed among the cytokines of T_H1 lymphocytes.
What clinical diseases develop via the mechanism of a type IV allergic reaction (aside from diagnostic tests)?
According to sources, clinical pathologies and manifestations associated with delayed-type hypersensitivity include:
- Insulin-dependent diabetes mellitus (type 1)—an example of organ-specific autoimmune pathology due to DTH caused by autoreactive T lymphocytes; accompanied by infiltration of the islets of Langerhans by lymphocytes and macrophages and destruction of β-cells.
- Lichen planus—in some cases, the development mechanism is associated with DTH to unknown antigens.
- Contact dermatitis—the main manifestation of delayed-type hypersensitivity in reactions to antibacterial chemotherapeutic agents.
Type IV allergy is CMA: Cell-mediated, Granulomatous, Antibody-free.
Class and exam questions
Are antibodies involved in type IV reactions?
No, this is the key feature of delayed-type hypersensitivity. The reaction is entirely cell-mediated—by specific T-lymphocytes and macrophages.
Which cells exert a direct cytotoxic effect on targets?
Direct damaging (cytotoxic) action on target cells is exerted by sensitized T lymphocytes, predominantly T killers.
What is included in the cellular composition of the allergic granuloma?
The basis of the granuloma is formed by lymphocytes and mononuclear phagocytes, which transform into epithelioid and giant cells. Fibroblasts are also present in the composition.
What do microcirculatory disorders lead to in the focus of DTH?
Disorders of hemo- and lymph circulation cause capillariotrophic insufficiency. This condition naturally leads to dystrophy and necrosis of the affected tissue.
What else to study on this topic
- The role of lymphokines in the mediated damage of target tissues
- Mechanisms of transformation of mononuclear phagocytes into epithelioid and giant cells
- Pathogenesis of capillariotrophic insufficiency development in allergy
- Features of the destruction of unchanged cells in allergic alteration
- Clinical significance and mechanisms of tuberculin and brucellin tests
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