Mechanism of delayed hyper sensitivity reactions involve activation of T cells by Ags that are bound to the cell.
The activated T cell proliferates to produce clones of TD and Tc lymphocytes. Interaction of Ags with primed T cells induces release of lymphokines from T cells.
Since the lymphokines contain macrophage stimulating factor, macrophage migration inhibiting factor etc., the macrophages are accumulated to phagocytize and destroy foreign cells at the site of infection.
The degradative lytic enzymes released from activated macrophages produce localized inflammatory response, resulting in necrosis and fibrosis in the host tissue along with destruction of foreign agents.
Measurement of Delayed Hypersensitivity
There are different tests to measure delayed hyper sensitivity. They are grouped in to four categories. (1) Skin tests (2) Lymphocyte activation tests (3) T Lymphocyte assays and (4) T cell function tests.
This test is used to establish previous exposure to particular antigen. The antigen to be tested is injected into the skin and studied for positive delayed inflammation reaction at the site of injection.
Lymphocyte activation test:
This test is commonly used to Judge’ cellular immunity in individuals with immune deficiency, autoimmunity, infections, diseases and cancer.
The test depends on culture of purified lymphocytes of patients in media containing mitogens. (Mitogene is a plant or bacteria derived product that have the ability to stimulate lymphocytes division).
Since DNA synthesis is very active during cell division, a radio active precursor of DNA has to be added to the media, before leaving it for incubation. The radioactivity levels are measured after incubation to estimate the rate of division.
High quantity of radioactive substance indicates high rate of cell division in the subject tested.
E Rosette test:
The test helps to estimate the number of stimulated T cells in an individual to identify immune disorders. As human cell bind with sheep erythrocytes, they form flower like rosettes with centrally located sheep erythrocyte. The role of this test is being superseded by the use of the fluorescence activated cell sorter.
Macrophage migration inhibition test:-
This test helps to judge the functioning capacity of lymphocytes through liberated lymphocytes. It is a test which measures the presence of migration-inhibitory factor.
Usually peritoneal macrophages are placed in a capillary tube in the presence or absence of supernatants from activated T-cells. If migration inhibition factor is present, the migration of monocyte/macrophages is reduced.
Type V or Stimulatory Hyper Sensitivity
This hyper sensitivity is some what similar to Type II hyper sensitivity but here the hyper sensitivity reaction stimulates an inappropriate and enhanced function of the target cells. Thyrotoxicosis or graves disease is a very good example for the stimulatory hyper sensitivity.
It is a disease condition due to over activity and secretion of thyroid (Refer Chapter 15 for autoimmune diseases). The reaction causing agent is an IgG antibody, which mimics the thyroid stimulating hormone molecules released from pituitary gland.