Such an effect may be demonstrated outside the body by making a (actiopsonic suitable mixture of (a) a suspension of the particular bacterium, (b) the serum to be tested, and (c) leucocytes of a normal animal or person.
Thus the serum of the white rat, which is immune to anthrax, kills the bacillus; whereas the serum of the guinea-pig, which is susceptible, has no such effect.
Is to say, if an animal be treated with injections of these bodies, its serum acquires the power of dissolving or of producing some disintegrative effect in them.
In estimating the opsonic power of the serum in cases of disease a control with normal serum is made at the same time and under precisely the same conditions.
It can be shown that in many cases when bacteria are injected the serum of the treated animal has no bacteriolytic effect, and still an immune body is present, which leads to the fixation of complement; in this case bacteriolysis does not occur, because the organism is not susceptible to the action of the complement.
The blood serum has a direct bactericidal action on certain bacteria, as tested outside the body, and this also varies in different animals.
In the case of diphtheria the antitoxic power of the serum may reach Boo units per cubic centimetre, or even more.
Shortly afterwards Widal and also Griinbaum showed that the serum of patients suffering from typhoid fever, even at an early stage of the disease, agglutinated the typhoid bacillus - a fact which laid the foundation of serum diagnosis.
An antitoxic serum has been prepared from horses by the Institut Pasteur in France, but has not met with success.
The second - passive immunity - is produced by the transference of a quantity of the serum of an animal actively immunized to a fresh animal; the term is applied because there is brought into play no active change in the tissues of the second animal.