Focal infection theory and bacterial allergy
As mentioned previously, the PHM hypothesis has some similarity to the
early version of the focal infection theory, which also included the
concept of bacterial allergy[2]. Desensitization to bacterial
allergens was tested in several clinical trials and appeared to be
successful in treating several CIDs, including chronic colitis[178],
allergic rhinitis[179] and asthma[180,181]. Research found that
autologous bacteria induced chemotaxis of peripheral blood mononuclear
cells in non-atopic asthmatics but not controls[182], suggesting a
role for hypersensitivity to colonizing bacteria. A review[181]
concluded that bacterial vaccines (immunotherapy) were useful in at
least a proportion of asthma cases.
However, a later review concluded that the evidence did not sufficiently
support immunotherapy for bacterial allergens in asthma[183]. The
discrepancy might be explained by an increasing effect of diverse, low
abundance microbes, leading to decreasing effectiveness of approaches
that focus on only a few bacterial species.
The PHM hypothesis suggests that earlier researchers using bacterial
vaccines may have been increasing the ability of the immune system to
reduce bacterial colonization, thus reducing symptoms. Similarly, one
might speculate that some forms of current allergen-specific
immunotherapy may be stimulating the immune system to eliminate at least
some PHMs that cross-react with the known inhalant or food allergens.
The elimination of the colonizing microbes by the immune system might be
what leads to desensitization and reduction or elimination of symptoms
in a proportion of patients.
The variation in severity of allergic reactions might reflect the levels
of colonizing PHMs. Asymptomatic sensitization to allergens might be due
to PHM colonization levels that are below the threshold needed for a
detectable reaction.