The purposes of this study are to assess the efficacy, safety and tolerability of a single dose of REGN1908-1909 in allergic adult participants, to collect information about how much REGN1908-1909 is in blood over time and to collect information about how the body reacts to REGN1908-1909.

Inclusion Criteria:

1. Healthy men and women between the ages of 18 and 55
2. Positive allergen skin prick test
3. Willing and able to comply with clinic visits and study-related procedures
4. Provide signed informed consent

Exclusion Criteria:

1. Persistent, chronic, or active recurring infection requiring treatment with antibiotics, antivirals, or antifungals within 4 weeks prior to the screening visit
2. Any clinically significant (determined at the investigator's discretion) abnormalities observed during the screening physical examination
3. Onset of a new exercise routine or major change to a previous exercise routine within 4 weeks prior to the screening visit. Patients must be willing to maintain a similar level of exercise for the duration of the study and to refrain from unusually strenuous exercise for the duration of the trial
4. Hospitalization for any reason within 60 days prior to the screening visit
5. Participation in any clinical research study evaluating another investigational drug or therapy within 30 days or at least 5 half-lives (whichever is longer) of the investigational drug prior to the screening visit
6. Any medical or psychiatric condition that in the opinion of the investigator or Regeneron, would place the patient at risk, interfere with participation in the study or interfere with the interpretation of study results

The information listed above is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial and not all inclusion/ exclusion criteria are listed.