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The prevalence and severity of asthma has increased in the last 20 years, and the greatest increase has been seen among children and young adults living in U.S. inner cities. The reasons for this increase are obviously complex, but include environmental exposures to allergens and pollutants, changing patterns of medication, and the psychosocial stresses of living in poor inner-city neighborhoods. This paper presents an overview of environmental, immunologic, and genetic factors associated with asthma morbidity and mortality. This overview can be used to provide a framework for designing an interdisciplinary research program to address the complexities of asthma etiology and exacerbation. The strongest epidemiologic association has been found between asthma morbidity and the exposure of immunologically sensitive asthmatic patients to airborne allergens. Our current understanding of the process of sensitization suggests that there is a strong genetic predisposition to form IgE to allergenic proteins on airborne particles. Much of this work has been conducted with animal models, but in a number of instances, specific confirmation has been reported in humans. Sensitized individuals respond to inhaled exposure with immediate mast-cell dependent inflammation that may be augmented by pollutant particles, especially diesel exhaust particles. Relatively little is known about the methods of assessing exposure to airborne pollutants, especially biologically active particulates. However, to examine the relationship of morbidity in genetically predisposed individuals, it will be important to determine the most relevant method of making this assessment.