op's on in

[G. ops on, sauce, seasoning.] substance occurring in the blood senmi, the action of which is to render microorganisms and po ibly other cells attractive to the phagocytes certain opsonins are present in normal serum and act upon all microorganisms, others are formed in response to special stimuli and are believed to be specific for one bacterial species only, common 0. normal o. immune' o., specific o. normal u., u. present in the blood serum of a non-infected individual; it is thermolabile and acts upon various bacteria, specific 0., u. formed in the serum in response to infection by a specific microorganism or to artificial inoculation with dead cultures of this same microbe it is thermostable and is effective only against this particular bacterial species, thermola'bile c, normal o. thermosta'ble o. test, the serum to be tested is heated to 58° or 60° C. for ten minutes or more, and then its opsonic power is measured by the phagocytic index; as the normal or thermolabile opsonin destroyed by heating, any opsonic power remaining in the serum is a umed to be due to specific opsonin produced as a result of autoinoculatiou or of a previous inoculation with a vaccine.