A substance that will cause a clumping of particles such as bacteria or erythrocytes. Of major importance are the specific or immune agglutinins, which are antibodies that will agglutinate bacteria containing the corresponding antigens on their surfaces. Agglutinin activity is frequently displayed by purified antibody preparations that also precipitate or give other serological reactions. Agglutinins are readily determined, and their presence is of diagnostic value to indicate present or past host contact with the microbial agent sufficient to result in antibody formation.