What are the sequences of events that occur in type 1 Immediate Hypersensitivity?

1. In response to certain antigens (allergens), a Th2 and Tfh response is activated. Allergens include pollen, food, insect venom, animal dander, penicillin, causing hay fever, food allergies, bronchial asthma and anaphylaxis. 2. Th2 and Tfh secrete IL-4 and IL-13 that induce switching to IgE production by B cells. IgE bind to mast cells via Fc receptor. (Individual becomes sensitized). 3. On second exposure to allergens, Fc-bound IgE binds to antigens and becomes cross-linked. 4a. Crosslinking of surface IgE causes mast cell degranulation with release of vasoactive amines (histamine) causing immediate hypersensitivity: increase in vascular permeability smooth muscle contraction 4b. Other products released from mast cells include cytokines like IL-4 and TNF- that attract eosinophils and neutrophils causing more severe inflammatory response and tissue injury, called the late-phase reaction.