Hypersensitivity reactions. There are four types of hypersensitivity reactions, usually labeled in Latin numbers. In type 1, the antigen, which is usually an external allergen such as pollen or certain food molecules, binds to IgE antibodies attached to the surface of mast cells or basophils. This antigen-antibody binding induces a cascade of intracellular reactions resulting in the release of large amounts of histamine from the storage vesicles. The release histamine induces various degrees of allergic reactions.
In hypersensitivity type 2, the antigen is not an external allergen floating in the blood, but rather certain molecules attached to the external surface of the patient cells. Binding of IgG or IgM antibodies stimulate a cascade of immune reactions resulting in the activation of the complement system, natural killers and macrophages, which induce further damage to affected tissues. In hypersensitivity type 3, the small antigen-antibody complexes float through the blood and accumulate in various tissues, such as joints, glomeruli and blood vessels. This accumulation triggers the complement system to attack the hot tissues. Unlike other types, the hypersensitivity reaction type 4 is not antibody mediated, but mainly cell mediated immunity.
In this type, the T helper cells, especially CD4 cells, recognize the intracellular antigens such as fungi or mycobacterium tuberculosis. The stimulated CD4 cells release cytokines which activate various immune system components such as phagocytes. The activated phagocytes attack and engulf the affected body cells.
This table summarizes the main differences between the four types of hypersensitivity reactions. Now we have a comprehensive overview of the hypersensitivity reactions. At the end of this video, Dr. Jihad Hamad and I wish you a very happy reading.
The narration is read by Daniela Vizicciu.