1. The skin-sensitizing and hemagglutinating antibody titers of most ragweed hay fever patients rise after an injection of emulsified ragweed extract reaching a peak at 3 to 6 weeks and showing a gradual decline by the end of 12 weeks.

  2. The least sensitive group of patients who received the highest dosage (10,000 PNU) showed no significant changes in skin-sensitizing antibody titer, but did respond with increased hemagglutinating and blocking antibody titers.

  3. All groups of patients showed a rise in blocking antibody after treatment reaching a peak at approximately 6 weeks. The group that received 10,000 PNU remained essentially unchanged from the 6th to 12th weeks whereas those that received 1000, 2500, or 5000 PNU showed a decrease in titer.

  4. Patients who received two injections of emulsified ragweed extract showed further increases of all antibody titers after the second injection.

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