Opsonins reveal their maximum action in a medium of neutral reaction. No opsonization takes place in a serum which contains an amount of alkali corresponding to more than 1.6 c.c. of a 1/20 N. solution, or acid more than 0.5 c.c. of this concentration per 1 c.c. of serum. Of several normal blood serums titrated (lacmoid used as indicator) the average alkalinity was found to be equivalent to about 0.8 c.c. of 1/20 N. solution.

The opsonic index obtained in the native serums is not the expression of the action of the whole content of opsonins, but only so much as the degree of optimum of the reaction permits to come into action. Estimation of the opsonic power should, therefore, be made in a medium of neutral reaction and in diluted serum.

All serums have their opsonic power increased by diminishing the native alkalinity.

Opsonins whose activity is suspended by an unfavorable reaction become immediately active as soon as the reaction is brought back to the neutral point, unless the acid or alkali employed approaches the strength of 1 N., at which point the alteration becomes permanent.

Treatment of a serum with alcohol robs it of its opsonic power. The opsonic power of serum remains unaltered upon desiccation at 23° C. In the dry state opsonins are preserved for two years.

The temperatures of 100°, 120°, 135° and 150° C. do not destroy opsonins of the dried serum.

Complements of serum are also siccostabile and are preservable in that state for several months. Dry heat of 135° C. reduces but does not destroy the complementary power of the dried serum.

The opsonins and complements of the dried serum regain their original thermolability when they are dissolved in a proper amount of water.

Lastly, it may be emphasized that opsonins exhibit in their sensitiveness to reaction and resistance in the dry state to high temperatures certain properties characteristic of the ferments.

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