Combined Actions and Interactions of Chemicals in Mixtures

tested separately. This is a phenomenon also known from testing with other
mixtures such as rubber allergens (Menné et al 1992) and preservatives (Menné
and Hjort 1988).
These results have often been interpreted as a false-positive patch test to the mix.
“False-positive” patch tests may have several explanations: cumulative irritancy of
allergens in combination, false-negative reactions to the constituents, unequal
absorption of the allergens, generation of a new hapten as a reaction product, etc.
(Johansen and Menné 1995). However, despite a number of changes carried out in
the composition of the fragrance mix to overcome problems related to irritancy and
unequal absorption of allergens, the frequency of false-positive patch tests has
remained stable over the years (Krasteva et al 1996).
7.7.4 Experimental studies on the elicitation of allergic response to
mixtures of chemicals.
Fourteen patients with positive patch tests to two unrelated allergens were studied
and the response to those two allergens was measured when tested singly or in
combination, using 10 different paired combinations from 15 common allergens.
Two-fold serial dilutions of the allergens were used for patch tests and change in
skin thickness used as the objective measure of response.
Subthreshold doses: The threshold concentration at which a response occurred was
established. Two further subthreshold dilutions were made for each of the allergens
and when a subthreshold dose of each allergen was combined on one patch, a
measurable response was produced. The threshold at which a response occurred to
the combination of allergens was significantly lower than the threshold for a
response to the individual allergens. It was stated that at subthreshold doses a
purely additive effect would be indistinguishable from synergy.
Linear part of dose-response curve: 2-6 antigen concentrations were used, which
produced a patch-test response to the individual allergen on the linear part of the
dose-response curve. A greater response was elicited by the dilution of allergens in
combination than to either of the individual allergens. The response to the mixture
of the allergens was the same as the arithmetical sum of the responses to the
individual allergens as measured by change in skin fold thickness. Thus the authors
conclude that the increase in response to the mixture of allergens is additive
(McLelland and Shuster 1990).
Female Balb/C mice were experimentally sensitised to the two contact sensitisers
2,4-dinitroflurobenzene (DNFB) and 4-ethoxy-methylene-2-phenyloxasol-5-one
(oxazolone). The sensitisation was measured by applying the chemicals to the ear
and measuring increase in ear thickness. When optimal challenge concentration of
the single chemical and the chemicals in combination were used there was no
significant difference in the ear swelling between the single chemicals and the
mixture. As the two single chemicals were used in a concentration giving a
maximum response reaching the plateau of the dose response curve it is not
possible to induce a further reaction by mixing the two chemicals. When the mice
were challenged with suboptimal or subthreshold concentrations of the two
chemicals in combination strong positive responses were induced. The authors
conclude that the results suggest that a “false-positive” reaction to a mixture of
allergens may reveal a genuine sensitisation to the constituents (Krasteva et al
1996).
Two groups of patients with allergic contact dermatitis to perfume ingredients were
included in the study, 18 subjects with a contact allergy to two fragrance
128