Sir Patrick.  I could have told you that.  Ive tried these modern inoculations a bit myself.  Ive killed people with them; and Ive cured people with them; but I gave them up because I never could tell which I was going to do.

Ridgeon [taking a pamphlet from a drawer in the writing-table and handing it to him] Read that the next time you have an hour to spare; and youll find out why.

Sir Patrick [grumbling and fumbling for his spectacles] Oh, bother your pamphlets.  Whats the practice of it? [Looking at the pamphlet] Opsonin?  What the devil is opsonin?

Ridgeon.  Opsonin is what you butter the disease germs with to make your white blood corpuscles eat them. [He sits down again on the couch].

Sir Patrick.  Thats not new.  Ive heard this notion that the white corpuscles—­what is it that whats his name?—­Metchnikoff—­calls them?

Ridgeon.  Phagocytes.

Sir Patrick.  Aye, phagocytes:  yes, yes, yes.  Well, I heard this theory that the phagocytes eat up the disease germs years ago:  long before you came into fashion.  Besides, they dont always eat them.

Ridgeon.  They do when you butter them with opsonin.

Sir Patrick.  Gammon.

Ridgeon.  No:  it’s not gammon.  What it comes to in practice is this.  The phagocytes wont eat the microbes unless the microbes are nicely buttered for them.  Well, the patient manufactures the butter for himself all right; but my discovery is that the manufacture of that butter, which 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.  If we had inoculated Jane Marsh when her butter factory was on the up-grade, we should have cured her arm.  But we got in on the downgrade and lost her arm for her.  I call the up-grade the positive phase and the down-grade the negative phase.  Everything depends on your inoculating at the right moment.  Inoculate when the patient is in the negative phase and you kill:  inoculate when the patient is in the positive phase and you cure.

Sir Patrick.  And pray how are you to know whether the patient is in the positive or the negative phase?

Ridgeon.  Send a drop of the patient’s blood to the laboratory at St. Anne’s; and in fifteen minutes I’ll give you his opsonin index in figures.  If the figure is one, inoculate and cure:  if it’s under point eight, inoculate and kill.  Thats my discovery:  the most important that has been made since Harvey discovered the circulation of the blood.  My tuberculosis patients dont die now.

Sir Patrick.  And mine do when my inoculation catches them in the negative phase, as you call it.  Eh?

Ridgeon.  Precisely.  To inject a vaccine into a patient without first testing his opsonin is as near murder as a respectable practitioner can get.  If I wanted to kill s man I should kill him that way.