People generally follow an egg-free diet for one of two reasons: either they wish to maintain a vegan (pure vegetarian) diet, or they are allergic to eggs. Most people who are allergic to eggs are allergic to egg protein and usually react to the proteins in the egg white. However, because it is impossible to completely avoid cross-contamination between yolk and white, people who are allergic to eggs need to avoid eggs completely. The most allergenic proteins in egg white are ovalbumin, ovomucoid, ovotransferrin, and lysozyme. Lysozyme is an unlabeled additive used in cheese preparation. Egg yolk contains three proteins (apovitellenins I & VI and phosvitin) that are also allergenic.