To be more comprehensive, I'm going to expand upon immunoglobulins and mast cells.
An immunoglobulin (Ig), also called an antibody, is a Y-shaped molecule that binds to anything foreign in our bodies. In this way, the Ig marks what it binds to for other cells to look at and destroy. IgE is the least common of the 5 different kinds of immunoglobulins, unless you're allergy-prone, like me!
In this pic, you'll see how each type of Ig is structured differently, but made from an identically-shaped repeating structure. Keep in mind that a molecule's shape is extremely important in chemistry, and that the principle that each object binds to another object in a very specific way, holds true. So, if these Igs have different shapes, you can assume that they will bind to different structures in very specific ways. Likewise, if their shape is similar (as with the three in the top row), they may be able to bind to similar or the same type structure on another cell.
For an additional description of each type of Ig, where they are found, and what they do, see: http://www.webmd.com/a-to-z-guides/immunoglobulins