In a thread "Re: When the FBI Guys Come Knocking..." I claimed (from memory) that cats can decrease allergies. I was unable to find the _Science_ ref but I found a few refs to the original research reported in The Lancet 357:752-56 (2001). Reproduced below. I don't make this stuff up. http://www.niaid.nih.gov/newsroom/focuson/asthma01/research.htm#cats Contrary to popular belief, high levels of cat allergen in the home can sometimes decrease the risk of a child developing asthma, says grantee Thomas A. Platts-Mills, M.D., Ph.D., of the University of Virginia. Apparently, the presence of a cat can alter the immune system in a manner similar to allergy shots, he reports. For other allergens that trigger asthma, such as the dust mite and cockroach, the higher the exposure level, the more likely it is that a child will produce "allergic" antibodies, called immunoglobulin-E or Ig-E antibodies, against them. This high exposure increases the child's risk of becoming allergic and developing asthma. But with cats, high exposure actually can confer protection -- at least in some children, Dr. Platts-Mills says. He and colleagues measured the levels of allergic antibodies to cat allergen in 226 children, aged 12 to 14 years, and tested the children for asthma. They also measured the amount of cat allergen in the children's homes and discovered that low-to-moderate amounts of cat allergen seemed to trigger allergy, but high amounts -- greater than 20 micrograms per gram of house dust -- reduced both IgE antibodies and the likelihood of asthma. "This result alters the advice we give patients," says Dr. Platts-Mills. "I would not recommend that all parents get rid of their cat because they are concerned their child might develop asthma. High exposure to cat allergen appears to be protective for some children and a risk factor for others. If the child is wheezing and has a positive skin test to cat allergen, then you should get rid of your cat." The high levels of cat allergen prompted the children's immune systems to make mostly a particular subtype of immunoglobulin G (IgG), called IgG4 antibody, rather than IgE, Dr. Platts-Mills explains. Allergy shots are believed to produce a similar effect. "This research sheds more light on the relationship between allergen exposure and asthma," he says. "When we further understand this process, it might lead to new treatments for asthma." Reference: T Platts-Mills et al. Sensitisation, asthma, and a modified Th2 response in children exposed to cat allergen: a populations-based cross-sectional study. The Lancet 357:752-56 (2001). Sensitisation, asthma, and a modified Th2 response in children exposed to cat allergen: a population-based cross-sectional study. Platts-Mills T, Vaughan J, Squillace S, Woodfolk J, Sporik R. University of Virginia Asthma and Allergic Diseases Center, University of Virginia Department of Medicine, Charlottesville, USA. tap2z@virginia.edu BACKGROUND: Although asthma is strongly associated with immediate hypersensitivity to indoor allergens, several studies have suggested that a cat in the house can decrease the risk of asthma. We investigated the immune response to cat and mite allergens, and asthma among children with a wide range of allergen exposure. METHODS: We did a population-based cross-sectional study of children (aged 12-14 years), some of whom had symptoms of asthma and bronchial hyper-reactivity. Antibodies to mite (Der f 1) and cat (Fel d 1) allergens measured by isotype (IgG and IgG4) specific radioimmunoprecipitation assays were compared with sensitisation and allergen concentrations in house dust. FINDINGS: 226 children were recruited, 47 of whom had symptoms of asthma and bronchial hyper-reactivity. Increasing exposure to mite was associated with increased prevalence of sensitisation and IgG antibody to Der f 1. By contrast, the highest exposure to cat was associated with decreased sensitisation, but a higher prevalence of IgG antibody to Fel d 1. Thus, among children with high exposure, the odds of sensitisation to mite rather than cat was 4.0 (99% CI 1.49-10.00). Furthermore, 31 of 76 children with 23 microg Fel d 1 at home, who were not sensitised to cat allergen had >125 units of IgG antibody to Fel d 1. Antibodies to Fel d 1 of the IgG4 isotype were strongly correlated with IgG antibody in both allergic and non-allergic children (r=0.84 and r=0.66, respectively). Sensitisation to mite or cat allergens was the strongest independent risk factor for asthma (p<0.001). INTERPRETATION: Exposure to cat allergen can produce an IgG and IgG4 antibody response without sensitisation or risk of asthma. This modified T-helper-2 cell response should be regarded as a form of tolerance and may be the correct objective of immunotherapy. The results may also explain the observation that animals in the house can decrease the risk of asthma. PMID: 11253969 [PubMed - indexed for MEDLINE]