Type IV Hypersensitivity in T Cells: Mechanism, Reactions, Reasons and Diseases

The type IV hypersensitivity reaction is mediated primarily by T cells and macrophages.

In the Gell and Coombs (1963) classification of hyper­sensitivity, the term type IV or delayed-type hyper­sensitivity (DTH) was used to describe all those hyper­sensitivity reactions which took more than 12 hours to develop. Later it became evident that several different types of immune reactions can produce delayed hypersensitivity.

Type I, II, and III hypersensitivity reactions (which are mediated by antibodies) can be transferred from one animal to another animal through serum (which contains antibodies), but can’t be transferred by sensitized T cells. On the other hand, type IV hypersensitivity reaction can be transferred from one animal to another through sensitized T cells only, but not through serum.

Delayed-type hypersensitivity is a localized inflam­matory reaction induced by some subpopulations of TH1cells against certain antigens. The characteristic feature of DTH reaction is influx of macrophages into the site of local inflammation. Robert Koch injected culture filtrates of Mycobacterium tuberculosis into the skin of tuberculosis patients. The injection caused local swelling at the site of injection and the reaction was called ‘tuberculin reaction’.

Later many antigens were found to induce similar responses and the term delayed-type hyper­sensitivity was coined. Since the reactions took longer time to develop it was called delayed-type hyper­sensitivity reaction. It was also thought that the reaction caused extensive tissue necrosis, and hence it was called hypersensitivity reaction.

However, later it was realized that the host tissue damage is usually minimal and the DTH reaction is an important defense mechanism against intracellular microbes (such as Mycobacterium tuberculosis) and some contact antigens (such as poison ivy).

Generally CD4+ TH1subset of lymphocytes (also referred to as TDTH lymphocytes) induce the DTH response. However, in few cases CD8+ T cells also induce DTH responses.

Mechanism of Type IV Hypersensitivity:

Upon entry of a microbe, which is dealt by DTH immune responses, the APCs of the host engulf and present the microbial antigens to the TDTH cells. The TDTH subpopulation of helper T cells secretes many cytokines. The cytokines in turn activate the nearby lymphocytes and macrophages (Table 18.1).

Table 18.1: Effects of cytokines in DTH reaction




Monocyte chemotactic factor and monocyte activating factor Migration-inhibition factor


IFNγ and TNF

Amplify the proliferation of cytokine producing T cells

Attracts monocytes to the site of inflammation and activate monocytes


Inhibits the migration of monocytes away from the cells harboring the microbe, so that the attracted monocytes are retained in the site of inflammation Activate macrophages

Some of the cytokines and their functions are given below:

i. Macrophage migration-inhibiting factor (MIF): Inhibits the migration of macrophages away from the inflammatory site.

ii. IFNγ, GM-CSFs, and TNFα enhance the microbicidal and cytolytic activities of macrophages.

iii. IFNγ has many other functions. The IFNγ increases the expression of MHC class-II molecules on the macrophage surface and consequently, the antigen presentation capacity of macrophages increases. The increased antigen presentation in turn activates TDTH cells, which secrets many cytokines that enhances the functions of macrophages. Thus an interaction between TDTH cells and macrophages amplifies the immune repossesses at the local inflammatory site.

iv. Leukocyte-inhibiting factor inhibits the random migration of leukocytes.

v. IL-8 is a chemotactic factor for neutrophils and T cells.

vi. IL-2 stimulates the growth of activated T cells. IL-2 also activates cytotoxic lymphocytes and macrophages.

The activated macrophages in turn secrete a number of cytokines and biologically active substances that cause inflammation and destruction of microbes.

i. Cytokines: IL-1, IL-6, IL-8, IL-12

ii. Reactive oxygen metabolites, such as superoxide anion, hydroxyl radical, and hydrogen peroxide.

iii. Proteases and lysosomal enzymes.

The DTH responses usually eliminate the intracellular pathogens. But in some individuals the pathogen is not eliminated in spite of the DTH responses. Consequently, more macrophages accumulate around the site of microbial presence. The macrophages adhere to each other and can assume an epitheliod shape (and such cells are called epithelioid cells) or many macrophages fuse with each other to form multinucleated cells called ‘giant cells’ . The macrophages secrete many cytokines and lytic enzymes that can cause extensive tissue necrosis.

A number of intracellular microbes and contact antigens can induce DTH responses. Mycobacterium tuberculosis (a pathogen that resides inside macrophages of host) induces a DTH reaction in lungs and results in the formation of a granuloma-type lesion called tubercle. The DTH response walls off the tuberculosis bacteria and prevents the spread of the bacteria. However, the cytokines and lytic enzymes secreted by the macrophages and other cells in the granuloma cause extensive damage to lung tissue.

Skin Test for Delayed-Type Hypersensitivity Reaction:

Delayed hypersensitivity skin test is a simple test, which helps in the diagnosis of certain infectious diseases. The test detects cutaneous (skin) hypersensitivity to antigens. With respect to skin tests for infectious diseases, one should remember that, a positive test does not necessarily indicate active infection with the agent being tested for; a positive skin test only indicates that the person has been infected with that infectious agent, though he may or may not suffer from the disease (caused by that particular agent) at the time of testing.

Skin tests are done to find whether the individual is already exposed (i.e. sensitized) to a particular antigen. The antigen is injected intra-dermally into the skin. Appearance of swelling (called induration) in 48 to 72 hour suggests that the individual is already exposed to the antigens, which was injected into the skin.

Skin tests are done to detect previous exposure to Mycobacterium leprae (the skin test is called lepromin test), Mycobacterium tuberculosis (the skin test is called Mantoux test), and many fungi (such as Coccidioides immitis).

Mantoux Skin Test for Tuberculosis:

Purified protein derivative (PPD) obtained from in vitro Mycobacterium tuberculosis culture is used as antigen in the Mantoux skin test. The PPD is injected intra-dermally in the skin. The injected area is checked for erythema (redness of skin) and induration (swelling produced by the inflammatory reaction) after 48 to 72 hours of injection. The reactions usually take 48 to 72 hours to develop (the time required for TDTH activation, cytokine secretion, accumulation of macrophages, and release of lytic enzymes).

The diameter of induration is measured with a ruler. If the diameter of induration is more than 10 mm, the person is said to be skin-test positive. A positive Mantoux skin test simply suggests that the person has been sensitized with Mycobacterium tuberculosis.

A positive Mantoux test does not mean that the person is suffering from tuberculosis at the time of skin test. A positive or negative Mantoux test should be correlated with clinical signs and symptoms of the patient as well as radiological (x-ray) and bacteriological tests.

In an individual who is already sensitized with Mycobacterium tuberculosis, memory T cells against M. tuberculosis antigens are present. Memory T cells remain in the body for longer time, often many years.

Upon injection of PPD, the APCs engulf and present the PPD antigens to the specific memory T cells.

The memory T cells become activated and release numerous lymphokines into the tissue spaces.

The secreted lymphokines recruit many inflammatory cells, retain them (at the site of PPD deposition), and activate them. Among the cytokines secreted by T cells, those acting on the monocytes seem to play important roles (Macrophage chemotactic factors attract the cells to the site of antigen deposition; macrophage migration inhibitory factors prevent the monocytes from moving away from the site so that the monocytes are retained at the site; IFNγ activates the monocytes).

The infiltration at the PPD injected site by numerous immune cells and fluid from the blood vessels cause swelling at the PPD injected site.

The activation of coagulation-kinin system leads to deposition of fibrin at the site. The fibrin deposition imparts a firm consistency (induration) that is characteristic of tissues undergoing DTH reactions.

DTH reaction is a protective response in the following conditions:

i. Tuberculosis and leprosy

ii. Fungal and viral diseases

iii. Tumors

However, DTH is also harmful to the host in some conditions:

i. Contact dermatitis of skin upon exposure to certain substances.

Delayed-Type Hypersensitive Diseases:

1. Contact Dermatitis

1. Contact Dermatitis or Eczematous Contact Allergy:

Inflammation of skin is called dermatitis. Contact hyper­sensitivity is a classical example of type IV hyper­sensitivity reaction. Contact dermatitis is an eczematous skin disease caused by type IV hypersensitivity to environmental antigens. Immune reactions are induced by antigen upon its contact with the skin. There are numerous antigens capable of causing allergic contact dermatitis (e.g., many natural and synthetic chemicals; metals like jewelry and watch; dyes and fabric finish in clothing).

The skin eruption appears as red swelling with blisters. Later the skin disrupts and fluid ooze out from that area. There may be inching in that area.

The most common agents of contact hypersensitivity are haptens. The small hapteruc molecule penetrates the epidermis of the skin and binds with host protein and form an immunogenic hapten-carrier complex. The hapten-carrier complex is recognized by the host T cells.

The T cells responses are directed against the conjugate but not to the hapten or carrier molecule, which is in contrast to the antibody induction against hapten-carrier complex. During the first contact of the hapten with the skin the hapten penetrates the epidermis of the skin and form a hapten-host protein complex.

The Langehans cells in the epidermis capture the hapten-host protein complexes, carry them to the regional lymph nodes and present the complexes to the helper T cell. The helper T cell becomes activated and thus the individual is sensitized to the hapten. The sensitization process takes about 2 weeks to develop.

When the hapten comes in contact with the skin for the second and subsequent times it is picked up by APCs and presented to the sensitized TDTH cells in the skin and regional lymph nodes. The TDTH cells release a number of cytokines.

Cytokines TNF and IL-1 induce expression of adhesion molecules on endothelial cells of blood vessels, which in turn lead to the infiltration of the site by monocytes and lymphocytes from blood. The infiltrating lymphocytes and monocytes secrete a number of cytokines and the area is inflamed. The peak of cellular infiltration is reached at 48 to 72 hours. The majority of lymphocytes are CD4+ with a small number of CDS+ T cells.

The only way of prevention of allergic contact dermatitis is avoidance of the offending antigen. Some contact agents such as dinitrochlorobenzene (DNCB) have the ability to sensitize all individuals and hence DNCB was used to test the CMI response (by skin test) in patients suspected to have immunodeficiency diseases. DNCB should not be used as a skin test antigen in human.

Patch Test for Contact Dermatitis:

Patch test is used to detect the antigens to which the patient is sensitive. A battery of standard contact sensitivity antigens (such as rubber, cosmetics, plant extracts, perfumes, metals) are applied to the back of the patient.

There are two patch test procedures:

i. In open-patch test method, a drop of acetone extract of the allergen is applied on the skin. The acetone dries quickly leaving the allergen on the skin. The site is uncovered and examined after 48 hours.

ii. In the closed-patch test method the allergen in petrolatum is applied to a pad and the pad is tapped to the skin. After 48 hours the pad is removed and the skin site examined.

A positive (i.e. the patient is sensitive to the allergen applied) test is indicated by erythema, papules, or vesicles. If the test site is negative for reaction the site is re-examined at 72 hours and 96 hours (from the time of allergen application on skin) for the reactions. In one sitting about 20 substances can be applied as patch tests.

Submitted by : Professor Sophia, Category : Immunology, Tag : Hypersensitivity