IgE sensitisation
In contrast to IgG that float in the body fluids complexed with their antigen and then bind to Fc-γ receptors, IgE bind to FcεRI on the surface of, for example, mast cells all by itself. Notice in this next video that empty IgE produced in response to the first exposure to the allergen, bind to mast cells by their Fc portion leaving the F(ab')2 region unoccupied. What happens when the allergen enters the body the second time?
The empty F(ab')2 cocks the immunological gun, but it does not fire. At this stage the person is said to be sensitised to the allergen. The immunological "trigger" can only be pulled on second exposure to the same allergen. When this happens, the F(ab')2 of adjacent antibodies bind to repeated epitopes on the antigen causing the FcεRI to become cross-linked. It is FcεRI cross-linking that sends the message for mast cells to degranulate and release their potent chemicals. The gun has been fired and the allergic reaction is underway!