Immunoglobulin E is an antibody. It's the antibody that you love to hate because it causes allergies. What's intresting about allergies and what IgE is doing to cause them is that it has no beneficial purpose. When certain harmless substances enter the body - like ragweed pollen or cat dander - B cells create IgE specifically tailored for latching on to these harmless substances (after, the IgE is anchorched to an immune cell called a mast cell, which is found in the lungs, skin and gut). Now the next time the harmless substance comes floating through the body it may latch onto the mast cell-IgE combo and if this happens nothing happens. But if it happens to latch onto two mast cell-IgE combos then the mast cell freaks out sends all sorts of messages, makes your muscles contract, fluids pour in (or out later as the case may be) and this is what causes the sneezing of hay fever and the hives of a bee sting.
So now your thinking "What the fuck?" why have this IgE in the first place and this is what scientists first thought as well. Eventually it was noticed that industrialized, hygenic and medicine filled societies had low levels of IgE in people until allergy season but in poorer nations the levels were always high. This is because IgE is for fighting parasitic animals like blood flukes and tape worms, something which is not prevelant in wealthy nations. To me though its always going to be the antibody I don't need that makes me sneeze.
I learned this from the book "Parasite Rex" by Carl Zimmer.