Allergy often begins with a subtle and/or transient T cell defect. This defect is first responsible for an IgA deficiency. The normal function of IgA is immune exclusion. In its absence, allergens can pass through the mucosa and stimulate the immunocompetent cells. The T cell defect may also be implied by the synthesis of IgE directed against the allergens which passed through. Clinical, biological and immunological findings (T cell defect in allergic disease, low range of IgA in the early life of atopics) are in agreement. The genetic factor for pollinosis and house dust allergy are segregated. In ragweed allergy there is an Ir gene coding for antigen-specific Ig of different classes and a group of non-linked major histocompatibility complex alleles coding for non antigen-specific IgE. There are some links with HLA. In house dust allergy the Ir gene is very common and almost everyone can produce an allergy under some conditions (T cell defect). Whatever the immunologic and genetic factors are, they need allergens and environmental factors to induce allergy. Allergy is a complex state in which several mechanisms, often associated and sometimes unclear, are involved. Some of them may be an abnormality of the autonomic nervous system, and/or an increase in the mucous membrane permeability, and/or a subtle immunodeficiency. All these mechanisms are regulated by genetic factors and modulated by environmental ones.