The factors that lead one to develop allergies while another does not are still being investigated. However, there is research showing that it appears to be related to the interaction between genetics, the microbial environment (see The Hygiene Hypothesis), nutrition, pollution and climate (see Climate Change and Allergy) in addition to other factors.
The composition of one’s genetics (DNA) is known as the Genotype. In the case of humans and other mammals, one’s genotype comes from a random combination of the genotypes of both parents. But how one actually develops in the world is more complex than one’s genes. The physical manifestation of an organism, known as the Phenotype, is based on how these genes are actually expressed, which depends largely on how these genes interact with the environment.
For much of the last century, a false dichotomy was set up between the importance of one’s genes and one’s environment in terms of behavior and disease. This was known as the Nature vs. Nurture debate. More recent research has demonstrated that the factors that influence one internally (nature) are in a continuum with the factors that influence one externally (nurture), so it is difficult to draw a line where one ends and the other begins. The primary area of research into the interaction between genes and environment is known as Epigenetics.
Epigenetics refers to the patterns of gene expression. One may have many genes, but the impact these genes will have depends on the pattern and timing of how they are expressed. Two genetically identical organisms may actually show different phenotypes if the first expresses a gene that is repressed in the other. This difference in expression happens because of the impact of the environment.
DNA, and the genes it creates, do not float randomly around the cell but are tightly controlled with various regulators. Factors from the environment, such as nutrition, pollution, stress and inflammation can alter the way these regulators control the genes and thus can alter the way one responds to the environment.
It should be cautioned that this is an area of early research so claims should be taken with a grain of salt. However, there is evidence to suggest, for example, that certain prenatal exposures to pollution or nutrition can alter the way one’s genes are expressed. And these epigenetic changes have been shown in animal studies to be capable of being passed from parent to child. The implications of this are that evolutionary changes, in terms of how an organism adapts its phenotype to a changing environment, can happen very quickly, in some cases within a generation.
So one’s genetics reveal what one is predisposed to but this is not necessarily one’s destiny.
In the case of allergy, treatments such as immunotherapy appear to alter the way the body responds to allergen exposure and may provide long term relief. Whether or not this causes epigenetic changes that can be passed between generations has not been evaluated and cannot be claimed. However, there is much evidence that the immunological response does shift and this has been shown to decrease allergic symptoms in some sensitive individuals.