This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. An antibody or product of complement activation in blood serum that causes bacteria or other foreign cells to become more susceptible to the action of phagocytes.

n. Any antibody that causes an invading species to be more susceptible to phagocytosis

n. A hypothetical substance, present in blood-serum, upon which the phagocytic action of the leucocytes is dependent.

n. an antibody in blood serum that attaches to invading microorganisms and other antigens to make them more susceptible to the action of phagocytes

"I call opsonin, goes on in the system by ups and downs -- Nature being always rhythmical, you know -- and that what the inoculation does is to stimulate the ups or downs, as the case may be."

antibody