Hemagglutinins refers to glycoproteins which bring about agglutination of erythrocytes or hemagglutination. Hemagglutination can be used to identify surface antigens on erythrocytes (with known antibodies) and, hence, the blood type of an individual. Hemagglutinins consist of lectins and antibodies. Lectins (from the Latin legere, "to select") are non-immune glycoproteins that exhibit reversible binding to specific carbohydrate structures, glycans of glycoproteins, glycolipids and polysaccharides. Lectins own at least one non-catalytic domain, and, in some cases, a few carbohydrate binding domains, which enable them to effect agglutination of erythrocytes and other cells or precipitation of glycoconjugates. Lectins are found in a constellation of organisms and display an array of activities. Topics discussed in this book encompass algal lectins, plant type 2 ribosome inactivating proteins, edible legume lectins, jacalin, jacalin-related lectin, wheat lectins, rice lectins, banana lectins and potato lectin, immunomodulatory action of plant lectins, piezoelectric assay using the galactose-binding Bauhinia monandra leaf lectin and its antibody as a potent tool for detection of antigen-antibody recognition, assays using hemagglutinins of different origins to investigate their mechanisms of action, lectins from medicinal herbs and medicinal mushrooms, C-type lectins from a diversity of invertebrates and vertebrates, fish lectins and amphibian lectins.