Although agglutination is a mechanism by which your body destroys attacking microorganisms, when this happens on a larger scale it can be dangerous. The human blood groups are a good illustration of the way that antibodies and antigens interact and this is well illustrated at the Nobel prize website (Karl Landsteiner's work made it possible to determine blood groups and thus paved the way for blood transfusions to be carried out safely. For this discovery he was awarded the Nobel Prize in Physiology or Medicine in 1930.)
Note that the example of human blood groups also illustrates that you do not generally have antibodies against antigens that are normally present in your body. This makes a lot of sense and this distinction between self and non-self is crucial. We see the importance of it, and the power of our immune system, when it breaks down in so-called auto-immune diseases (originally known by the splendidly gothic name of “horror autotoxicus”).