A protein substance commonly found in foods that binds with glycoproteins and glycolipids on the surface of animal cells causing agglutination. Some lectins cause agglutination of erythrocytes in specific blood groups. (medical dictionary definition). Lectins also play a significant role in hormonal reactions.
In lay language, this translates to very tiny molecules found in foods that selectively cause blood and other body tissues to stick together. A lectin that causes the tissues of a person of one blood type to stick together will not necessarily have the same effect on a person of a different blood type.