In common parlance,"allergy" is usually classified as this type I allergy. This type occurs when there are abnormally many Th2, which is the destination of information on low-risk antigens.
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① Antigen presentation to Th2
Macrophages and dendritic cells present antigens to Th1 when they detect highly dangerous viruses or bacteria, and present antigens to Th2 when they detect allergens that are less dangerous.
Like Th1, Th2 also makes B cells make antibodies. However, Th2 makes antibodies of the type called "IgE antibody" and Th1 can not make IgE antibodies. This IgE antibody causes troubles.
IgE antibody binds with "mast cell". This is called "sensitization". Sensitized mast cells release "histamine" that were contained in themselves. If this histamine touch the skin, inflammation of the skin occurs and if it adheres to the trachea, the trachea will shrink and asthma will occur. These are allergic reactions.
Th2 works on B cells using substances such as "interleukin 4" and "interleukin 13". However, these substances create a "receptor" that makes dendritic cells and IgE antibodies easier to bind.
④ Receptor of IgE antibody made into dendritic cells
Receptor is a protein that makes specific substances be easier to combine, and it is on the cell surface. The fact that dendritic cells can form a receptor for IgE antibodies means that dendritic cells and IgE antibodies are more likely to bind. Dendritic cells take up more allergens because IgE antibodies are easy to bind to antigens. Therefore, allergic reactions are getting worse more and more.
This type of allergy is also called "immediate allergy" because there is a characteristic that the time from the invasion of allergens to the actual response is short.
Examples of allergens are pollen, food, etc. Examples of symptoms are skin symptoms, mucosal symptoms, respiratory symptoms, gastrointestinal symptoms, etc.
If it becomes chroni, it causes atopic dermatitis and bronchial asthma. Also, if you become severe, anaphylactic shock etc. may occur.