This research is being done to study the effects of the drug omalizumab (Xolair) in people with cat allergies. The investigators will use omalizumab to study changes in the cells in the nose, cells in the blood and cells in the skin that cause allergies. The investigators will compare the changes in the nose to changes in the skin and blood cells.

Objective: To test the hypothesis that treatment with omalizumab will decrease the nasal allergen challenge late-phase eosinophil count in nasal brushings at the time when blood basophils have become hypo-responsive to in vitro allergen exposure.

This is a randomized, placebo-controlled, double-blind, parallel group design study that includes 3.5 months of treatment with omalizumab or placebo and a 3 month follow-up. All subjects will be cat allergic.

Twenty four subjects (1:1 randomization) will undergo a cat allergen nasal challenge prior to treatment and another challenge after 2 months of treatment or when their blood basophils become hyporesponsive to cat allergen in vitro. A second group of 10 subjects (1:1 active:placebo), will not undergo nasal challenges. This group will participate in an ancillary study in which the effects of omalizumab on gene expression profiles in peripheral blood cells will be studied.

Inclusion criteria:

1. Male or female, ages 18-50
2. Females must be surgically sterile or postmenopausal or using a specified acceptable form of birth control throughout the duration of the study. Females in certain categories (not sexually active, vasectomized partner) will be admitted at the discretion of the investigator on a case-by-case basis.
3. Females must have a negative urine pregnancy test at Visit A and other visits specified in this protocol.
4. Clinical history of perennial allergic rhinitis for at least two years, with or without seasonal allergic rhinitis, and with or without mild persistent asthma as define by the 2007 NAEPP guidelines.
5. Allergic cat sensitization defined by a positive puncture skin test (mean wheal diameter 3 mm or more greater than diluent control), and a positive CAP-RAST to cat > 0.35 kU/L.
6. Positive intranasal cat allergen challenge defined by the induction of a total of ≥ 5 sneezes at screening(criterion not applicable in the ancillary study).
7. Baseline in vitro histamine release of peripheral blood basophils to cat allergen ≥7%.
8. Peripheral blood basophils > two million per 100 ml of blood (criterion only applicable in the ancillary study)

Exclusion Criteria:

1. Asthma with baseline FEV1 < 80% predicted, and/or moderate to severe asthma classification per the 2007 NAEPP guidelines.
2. Individuals with total serum IgE levels less than 30 IU/mL or greater than 700 IU/mL at the time of enrollment.
3. Individuals with reduced hematocrit (< 32%), WBC count (2400/microliter), platelet count (< 75000/microliter), and increased creatinine (> 141.4 micromolar/L), or AST (> 100 IU/L).
4. Individuals with body weight less than 30 kg or greater than 150 kg.
5. Pregnant females or females with plans to become pregnant or breastfeed during the duration of the study.
6. Individuals with perforated nasal septum, structural nasal defects, large nasal polyps causing obstruction, evidence of acute or chronic sinusitis.
7. History of malignancy, anaphylaxis or bleeding disorder.
8. Mental illness or history of drug or alcohol abuse that, in the opinion of the investigator, would interfere with the participant's ability to comply with study requirements.
9. Inability or unwillingness of a participant to give written informed consent or comply with study protocol.
10. Use of any investigational drugs within 8 weeks of participation.
11. Contraindications to omalizumab including patients with a previous hypersensitivity to omalizumab.
12. Recent recipient of any licensed or investigational live attenuated vaccine(s) within two months of study initiation such as flu mist.
13. Any prior use of omalizumab.
14. Frequent episodes of acute sinusitis (>2 documented episodes per year) or active sinusitis within 2 weeks prior to enrollment
15. Use of aeroallergen immunotherapy within 5 years prior to enrollment
16. Current or within 4 weeks prior to enrollment use of nasal steroids, nasal cromolyn or oral steroids
17. Past or current medical problems or findings from physical examination or laboratory testing that are not listed above, which, in the opinion of the investigator, may pose additional risks from participation in the study, may interfere with the participant's ability to comply with study requirements or may compromise the quality