Why does a type IV allergic reaction cause increased pulmonary capillary permeability?
Allergy and hypersensitivity mean the same thing, that is, what is commonly called an allergy, in which the body's own immune system is overly sensitive to pollen, animal fur, smoke and dust, and reacts in a way that harms the body. This allergic reaction, that is, an allergic response, includes allergic reactions of the skin, respiratory tract, digestive tract and so on. People with an allergic constitution are also prone to being allergic to certain parasites; when they are repeatedly exposed to a certain allergen, an allergy occurs and an allergic or hypersensitivity reaction develops.
Hypersensitivity or allergic reactions have four types of manifestation, namely type I immediate type, type II cytotoxic/cell-lytic type, type III immune complex type, and type IV delayed or cell-mediated type. Type I immediate type is the most common, and the allergic reaction is mainly manifested on the surface, such as allergic asthma of the respiratory tract, allergic gastroenteritis of the digestive tract, and skin rashes such as eczema, as well as dermographism, atopic dermatitis, skin itching and so on.
Type II cytotoxic antibody reactions are strongly related to blood, and specifically include reactions caused by mismatched blood types during blood transfusion, hemolytic anemia of the newborn, autoimmune hemolytic anemia and so on. Type III immune complex type allergic reaction is caused by antigen-antibody complexes and is strongly associated with blood vessels and the kidneys. Specifically, medium-sized molecular immune complexes in the circulation deposit on blood vessel walls, the glomerular basement membrane or tissue spaces, activating complement and causing inflammatory reactions, such as kidney diseases including nephritis, systemic lupus erythematosus, arthritis and myocarditis and so on.
Type IV is mediated by specifically sensitized effector T cells, and is mainly manifested as a localized inflammatory reaction that changes slowly. Generally, only 24 to 48 hours after the appearance of local inflammation is there a relatively strong reaction, so it is also called delayed type allergic reaction. This is because after the antigen meets the body, T cells transform into sensitized lymphocytes, causing the body to become allergic; after the same antigen invades again, T cells differentiate and proliferate, releasing countless lymphokines and forming an inflammatory reaction. Specific examples include tuberculosis, measles, and so on.