Mercury as a hapten: A review of the role of toxicant-induced brain autoantibodies in autism and possible treatment considerations

BACKGROUND

Mercury has many direct and well-recognized neurotoxic effects. However, its immune effects causing secondary neurotoxicity are less well-recognized. Mercury exposure can induce immunologic changes in the brain indicative of autoimmune dysfunction, including the production of highly specific brain autoantibodies. Mercury, and in particular, Thimerosal, can combine with a larger carrier, such as an endogenous protein, thereby acting as a hapten, and this new molecule can then elicit the production of antibodies.

METHODS

A comprehensive search using PubMed and Google Scholar for original studies and reviews related to autism, mercury, autoantibodies, autoimmune dysfunction, and haptens was undertaken. All articles providing relevant information from 1985 to date were examined. Twenty-three studies were identified showing autoantibodies in the brains of individuals diagnosed with autism and all were included and discussed in this review.

RESULTS

Research shows mercury exposure can result in an autoimmune reaction that may be causal or contributory to autism, especially in children with a family history of autoimmunity. The autoimmune pathogenesis in autism is demonstrated by the presence of brain autoantibodies (neuroantibodies), which include autoantibodies to: (1) human neuronal progenitor cells; (2) myelin basic protein (MBP); (3) neuron-axon filament protein (NAFP); (4) brain endothelial cells; (5) serotonin receptors; (6) glial fibrillary acidic protein (GFAP); (7) brain derived neurotrophic factor (BDNF); (8) myelin associated glycoprotein (MAG); and (9) various brain proteins in the cerebellum, hypothalamus, prefrontal cortex, cingulate gyrus, caudate putamen, cerebral cortex and caudate nucleus.

CONCLUSION

Recent evidence suggests a relationship between mercury exposure and brain autoantibodies in individuals diagnosed with autism. Moreover, brain autoantibody levels in autism are found to correlate with both autism severity and blood mercury levels. Treatments to reduce mercury levels and/or brain autoantibody formation should be considered in autism.

Skin reactivity to thimerosal and phenol-preserved Montenegro antigen in Brazil

A randomized double-blind trial was performed to determine the frequency of positive reactions to the Montenegro antigen (leishmanin) preserved in thimerosal (Merthiolate) 1:10,000 or phenol 0.4%. The respective products were tested separately in 400 young healthy individuals from a non-endemic area for Leishmaniases. Each volunteer received one of the following reagents: merthiolated antigen, phenolated antigen, merthiolated saline, or phenolated saline. The frequency of positive responses to each reagent after the first application was as follows: 0% (phenolated saline), 9.2% (merthiolated saline), 34.6% (antigen in phenolated saline), and 41.1% (antigen in merthiolated saline). After 1 week, volunteers who had tested positive for merthiolated or phenolated antigen were retested with the respective preservative, while negatives were retested with the preservative they had not received during the first test. In all, 331 volunteers who received merthiolated saline during the study, of whom 41 (12.4%) tested positive. Meanwhile, 326 volunteers who received phenolated saline, 4 (1.2%) tested positive. Positive reactions in each group were similar in relation to gross appearance skin reactions. Considering the high frequency of hypersensitivity to thimerosal in the study population, it is recommended that this compound should be replaced as a preservative of the leishmanin antigen. Almost 30% of positive reactions to Montenegro antigen in what is considered a non-endemic region was surprising and will be the object of future studies.

Dose and Hg species determine the T-helper cell activation in murine autoimmunity

Inorganic mercury (mercuric chloride--HgCl(2)) induces in mice an autoimmune syndrome (HgIA) with T cell-dependent polyclonal B cell activation and hypergammaglobulinemia, dose- and H-2-dependent production of autoantibodies targeting the 34 kDa nucleolar protein fibrillarin (AFA), and systemic immune-complex deposits. The organic mercury species methylmercury (MeHg) and ethylmercury (EtHg--in the form of thimerosal) induce AFA, while the other manifestations of HgIA seen after treatment with HgCl(2) are present to varying extent. Since these organic Hg species are converted to the autoimmunogen Hg(2+) in the body, their primary autoimmunogen potential is uncertain and the subject of this study. A moderate dose of HgCl(2) (8 mg/L drinking water--internal dose 148 micro gHg/kg body weight [bw]/day) caused the fastest AFA response, while the induction was delayed after higher (25 mg/L) and lower (1.5 and 3 mg/L) doses. The lowest dose of HgCl(2) inducing AFA was 1.5 mg/L drinking water which corresponded to a renal Hg(2+) concentration of 0.53 micro g/g. Using a dose of 8 mg HgCl(2)/L this threshold concentration was reached within 24 h, and a consistent AFA response developed after 8-10 days. The time lag for the immunological part of the reaction leading to a consistent AFA response was therefore 7-9 days. A dose of thimerosal close to the threshold dose for induction of AFA (2 mg/L drinking water--internal dose 118 micro gHg/kg bw per day), caused a renal Hg(2+) concentration of 1.8 micro g/g. The autoimmunogen effect of EtHg might therefore be entirely due to Hg(2+) formed from EtHg in the body. The effect of organic and inorganic Hg species on T-helper type 1 and type 2 cells during induction of AFA was assessed as the presence and titre of AFA of the IgG1 and IgG2a isotype, respectively. EtHg induced a persistent Th1-skewed response irrespectively of the dose and time used. A low daily dose of HgCl(2) (1.5-3 mg/L) caused a Th1-skewed AFA response, while a moderate dose (8 mg/L) after 2 weeks resulted in a balanced or even Th2-skewed response. Higher daily doses of HgCl(2) (25 mg/L) caused a balanced Th2-Th1 response already from onset. In conclusion, while metabolically formed Hg(2+) might be the main AFA-inducing factor also after treatment with EtHg, the quality of the Hg-induced AFA response is modified by the species of Hg as well as the dose.

Assessment of metallothionein and antibodies to metallothionein in normal and autistic children having exposure to vaccine-derived thimerosal

Allergic autoimmune reaction after exposure to heavy metals such as mercury may play a causal role in autism, a developmental disorder of the central nervous system. As metallothionein (MT) is the primary metal-detoxifying protein in the body, we conducted a study of the MT protein and antibodies to metallothionein (anti-MT) in normal and autistic children whose exposure to mercury was only from thimerosal-containing vaccines. Laboratory analysis by immunoassays revealed that the serum level of MT did not significantly differ between normal and autistic children. Furthermore, autistic children harboured normal levels of anti-MT, including antibodies to isoform MT-I (anti-MT-I) and MT-II (anti-MT-II), without any significant difference between normal and autistic children. Our findings indicate that because autistic children have a normal profile of MT and anti-MT, the mercury-induced autoimmunity to MT may not be implicated in the pathogenesis of autism.

Patch testing of Singapore children and adolescents: our experience over 18 years

This is a retrospective epidemiologic study of allergic contact dermatitis in children and adolescents in Singapore who had undergone patch testing from January 1, 1986 to December 31, 2003. A total of 2,340 patients below the age of 21 years had undergone patch testing. Of these, 1,063 (583 girls, 480 boys) were positive to one or more allergens. The most common allergens were: nickel (40%), thimerosal (15%), colophony (9%), lanolin (8%), cobalt (8%), fragrance mix (5%), and neomycin (4%). Each of these allergens will be discussed individually. This is the first study with over 1000 patients on this subject from Asia.

Immunosuppressive and autoimmune effects of thimerosal in mice

The possible health effects of the organic mercury compound thimerosal (ethylmercurithiosalicylate), which is rapidly metabolized to ethylmercury (EtHg), have recently been much debated and the effect of this compound on the immune system is largely unknown. We therefore studied the effect of thimerosal by treating A.SW (H-2s) mice, susceptible to induction of autoimmunity by heavy metals, with 10 mg thimerosal/L drinking water (internal dose ca 590 microg Hg/kg body weight/day) for up to 30 days. The lymph node expression of IL-2 and IL-15 mRNA was increased after 2 days, and of IL-4 and IFN-gamma mRNA after 6 and 14 days. During the first 14 days treatment, the number of splenocytes, including T and B cells as well as Ig-secreting cells decreased. A strong immunostimulation superseded after 30 days treatment with increase in splenic weight, number of splenocytes including T and B cells and Ig-secreting cells, and Th2- as well as Th-1-dependent serum immunoglobulins. Antinucleolar antibodies (ANoA) targeting the 34-kDa nucleolar protein fibrillarin, and systemic immune-complex deposits developed. The H-2s strains SJL and B10.S also responded to thimerosal treatment with ANoA. The A.TL and B10.TL strain, sharing background genes with the A.SW and B10.S strain, respectively, but with a different H-2 haplotype (t1), did not develop ANoA, linking the susceptibility to H-2. Thimerosal-treated H-2s mice homozygous for the nu mutation (SJL-nu/nu), or lacking the T-cell co-stimulatory molecule CD28 (B10.S-CD28-/-), did not develop ANoA, which showed that the autoimmune response is T-cell dependent. Using H-2s strains with targeted mutations, we found that IFN-gamma and IL-6, but not IL-4, is important for induction of ANoA by thimerosal. The maximum added renal concentration of thimerosal (EtHg) and inorganic mercury occurred after 14 days treatment and was 81 microg Hg/g. EtHg made up 59% and inorganic mercury 41% of the renal mercury. In conclusion, the organic mercury compound thimerosal (EtHg) has initial immunosuppressive effects similar to those of MeHg. However, in contrast to MeHg, thimerosal treatment leads in genetically susceptible mice to a second phase with strong immunostimulation and autoimmunity, which is T-cell dependent, H-2 linked and may at least partly be due to the inorganic mercury derived from the metabolism of ethyl mercury.

A generalized reaction to thimerosal from an influenza vaccine

BACKGROUND

Thimerosal is a preservative commonly used in ophthalmic solutions, otic drops, and vaccines because of its bactericidal property.

OBJECTIVE

To report a case of a generalized reaction to thimerosal in a patient who received an influenza vaccine.

METHODS

We describe a patient who developed a generalized maculopapular eruption after receiving a thimerosal-containing influenza vaccine. Patch testing was performed to determine if there was an allergy to thimerosal.

RESULTS

Patch testing confirmed a T-cell-mediated sensitivity to thimerosal.

CONCLUSIONS

Physicians need to be aware that thimerosal is found in many products, including vaccinations. Clinicians should also be aware that allergic reactions occur with exposure to thimerosal even in vaccines. To our knowledge, this is the first case report in the literature of a generalized reaction to thimerosalfrom an influenza vaccine.

Detection of antinuclear and antilaminin antibodies in autistic children who received thimerosal-containing vaccines

Autism, a neurodevelopmental disorder, may involve autoimmune pathogenesis. Since mercury is potentially a risk factor for autoimmunity, we conducted a study of mercury-induced antinuclear and antilaminin antibodies in autistic and normal children who had been pre-administered with thimerosal-containing vaccines. Laboratory analysis by different immunoassays showed that the serum level of these two autoimmune markers did not significantly differ between autistic and normal children. This finding suggests that the mercury as in thimerosal-containing vaccines is likely not related to autoimmune phenomenon in autism.

Eyedrop‐Induced Allergy: Clinical Evaluation and Diagnostic Protocol

BACKGROUND

In the last few years, adverse reactions to mydriatic eyedrops have been investigated. However, is not still available a standardized protocol, capable of identify the pathogenetic mechanism. In the light of these findings we have evaluated the reliability of a protocol with well established concentration of specific allergens.

METHODS

The diagnostic method includes the application of patch test series Gruppo Italiano Ricerca Dermatiti da Contatto e Ambientali (GIRDCA), medicaments, corticosteroids, local anesthetics, main eyedrops' excipients, pure active principles and extemporaneous preparations with mydriatic eyewashes. At the same time, skin prick test with a solution of atropine sulfate at 1 per thousand and an intradermal test with injection of atropine at 0.01 per thousand were carried out.

RESULTS

After patch tests were removed, we detected seven positiveness to the phenylephrine, two to the benzalkonium chloride, one to thiomersal, one to the ethylendiamine and one to the atropine sulfate 1 per thousand. With intradermal tests we obtained three positiveness in patients who reported adverse reactions to atropine.

CONCLUSIONS

Our results show that phenylephrine is frequently responsible for allergic conjunctivitis (53.8%). In the case of atropine, even though the limited number of patients suggests to perform more extensive studies, it emerges that our diagnostic protocol is safe and might be able to screen allergic reactions in the field of ophthalmopathies.

Homozygous gene deletions of the glutathione S-transferases M1 and T1 are associated with thimerosal sensitization

OBJECTIVE

Thimerosal is an important preservative in vaccines and ophthalmologic preparations. The substance is known to be a type IV sensitizing agent. High sensitization rates were observed in contact-allergic patients and in health care workers who had been exposed to thimerosal-preserved vaccines. There is evidence for the involvement of the glutathione system in the metabolism of thimerosal or its decomposition products (organomercury alkyl compounds). Thus detoxification by polymorphically expressed glutathione S-transferases such as GSTT1 and GSTM1 might have a protective effect against sensitization by these substances.

METHODS

To address this question, a case control study was conducted, including 91 Central European individuals with a positive patch-test reaction to thimerosal. This population was compared with 169 healthy controls and additionally with 114 individuals affected by an allergy against para-substituted aryl compounds. The latter population was included in order to test whether possible associations were due to substance-specific effects, or were a general feature connected with type IV immunological diseases. Homozygous deletions of GSTT1 and GSTM1 were determined by polymerase chain reaction.

RESULTS

Glutathione S-transferase M1 deficiency was significantly more frequent among patients sensitized to thimerosal (65.9%, P = 0.013) compared with the healthy control group (49.1%) and the "para-compound" group (48%, P = 0.034). Glutathione S-transferase T1 deficiency in the thimerosal/mercury group (19.8%) was barely elevated versus healthy controls (16.0%) and the "para-compound" group (14.0%). The combined deletion (GSTT1-/GSTM1-) was markedly more frequent among thimerosal-sensitized patients than in healthy controls (17.6% vs. 6.5%, P = 0.0093) and in the "para-compound" group (17.6% vs. 6.1%, P =0.014), revealing a synergistic effect of these enzyme deficiencies (healthy controls vs. thimerosal GSTM1 negative individuals, OR = 2.0 [CI = 1.2-3.4], GSTT1-, OR = 1.2 [CI = 0.70-2.1], GSTM1/T1-, OR = 3.1 [CI = 1.4-6.5]).

CONCLUSIONS

Since the glutathione-dependent system was repeatedly shown to be involved in the metabolism of thimerosal decomposition products, the observed association may be of functional relevance.

Mechanisms of drug-induced allergic contact dermatitis

Allergic contact dermatitis is induced by a wide variety of drugs that trigger specific immune responses following topical exposure. Identified chemical structures involved in such reactions include the mercuric and thiosalicylic acid groups of thimerosal, the diphenylketone group of the anti-inflammatory drug ketoprofen, the amide or ester structure of local anesthetics, and the side-chain and thiazolidine ring of beta-lactams. The T cell responses to such compounds involve CD4+ and CD8+ alphabeta+ T lymphocytes and also CD4 /CD8 gammadelta+ T cells. Although "T helper 2" cytokine production by drug-specific human T cells from patients with allergic contact dermatitis has been described, T helper 1-like and T cytotoxic 1-like responses clearly play key roles in this cutaneous reaction.

Mercurochrome allergy. Immediate and delayed hypersensitivity

We describe eight patients suffering from Mercurochrome allergy. Patch and prick tests were carried out with the following organic and inorganic mercury compounds: thimerosal, Mercurochrome, phenylmercuric acetate, phenylmercuric nitrate, metallic mercury, and mercuric chloride, and with sodium fluorescein. Two patients had an anaphylactic reaction a few minutes after application of Mercurochrome. The prick tests with Mercurochrome were positive and they were negative with the other tested products. All patch tests were negative. In the other six patients, the clinical picture was local eczema, and the patch tests were all positive with Mercurochrome and the inorganic mercuric derivatives. Positive patch tests with thimerosal were found only in two patients, and only one had a positive patch test with salts of phenylmercury. In four patients, the prick test with Mercurochrome, negative in immediate reading, gave a late eczematous reaction.

All these positive tests to thimerosal

By and large, positive patch tests with thimerosal lack clinical relevance and should cause concern neither to the dermatologist nor to the patient. The substance should be retained in standard test series for scientific purposes, but on clinical grounds its inclusion does not seem to be indicated. Nevertheless, the phenomenon is intriguing in its own right.

[An evaluation of the allergizing properties of merthiolate as a component of adsorbed DTP vaccine]

The study of the allergenic properties of thimerosal contained in adsorbed DPT vaccine was carried out. Two variants of this vaccine, with and without thimerosal, as well as thimerosal alone (diluted 1:10,000, which corresponds to the content of this preservative in the vaccine), were used in experiments. The phenomenon of anaphylactic shock, passive dermal anaphylaxis and skin tests were used as test-systems. The injection of each of the two variants of the vaccine induced the state of sensitization in guinea pigs. At the same time no differences in the intensity of manifestations of immediate and delayed hypersensitivity induced by preparations with and without thimerosal were registered. The solution of thimerosal alone exhibited no sensitizing properties.

Piroxicam has at least two epitopes for contact photoallergy

We have demonstrated previously in guinea pigs that the induction of photocontact sensitivity to piroxicam (PXM) also induces a state of cross-reactive contact hypersensitivity to two compounds having structurally related elements, thimerosal (TMS) and thiosalicylate (TOS). The present study was conducted to determine whether oral administration of TOS would desensitize guinea pigs previously photosensitized with PXM. At the same time, the spectrum of reactivities against these compounds and against tenoxicam (TXM) which resembles only piroxicam was assessed by appropriate sensitizing and eliciting protocols. As expected, animals photosensitized to PXM developed reactivities against all four compounds, PXM and TXM (photosensitivity) and TMS and TOS (contact sensitivity). By contrast, photosensitization with TXM induced cross-reactivity only against PXM. Moreover, the induction of contact sensitivity against TMS or TOS induced photosensitive cross-reactivity to PXM, but not to TXM. Finally, the oral administration of TOS produced a transient desensitization only for TMS and TOS. These results suggest that photosensitization with PXM induces two distinct reactivities. The first reactivity cross-reacts with TMS and TOS and is suppressible with orally administered TOS. The second cross-reacts only with TXM and is not suppressible with oral TOS. We conclude that PXM acquires at least two distinct immunogenic epitopes when exposed to UVA irradiation.

Ocular hypersensitivity to thimerosal in rabbits

Rabbits were immunized to thimerosal conjugates and challenged with specific antigen-sensitized contact lenses. The symptoms of acute ocular hypersensitivity observed included corneal edema, corneal infiltration and erosion, infiltration of the anterior chamber, iritis, conjunctival edema and hyperemia, and a significant increase in mucous production. Occasional evidence of corneal neovascularization and giant conjunctival papillae were seen. All these parameters were assessed on a five-point scale (0 to +4) and tabulated as an ocular index. The index of ocular hypersensitivity correlated with the titers of the serum antibodies to thimerosal. The major class of serum antibodies consisted of immunoglobulin (Ig) G, with IgA compromising approximately 5% of serum antibodies. During the ocular challenge, the total and IgG tear antibody titers increased as a result of increased vascular permeability. The tear IgA titers increased to a lesser extent than IgG. The influx of serum proteins directly into the tear film was confirmed by a protein-dye tracer technique. Histologic analysis showed that the ocular inflammatory response was accompanied by both polymorphonuclear and mononuclear cell infiltrates into the cornea and conjunctiva. In conclusion, thimerosal-immunized rabbits show an exquisite sensitivity to the minute quantities of thimerosal conjugates adsorbed to contact lenses. Both serum and tear antibodies correlate with the severity of the ocular inflammatory response. This model would appear to simulate an antibody-mediated immune complex or Arthus type of ocular hypersensitivity commonly seen to foreign antigens including preservatives.

Piroxicam-induced photosensitivity and contact sensitivity to thiosalicylic acid

A photocontact dermatitis developed in three patients after the application of gel containing 0.5% piroxicam. Patch tests were positive to thiomersal and thiosalicylic acid. Photopatch tests with piroxicam at several concentrations were positive in the three patients but negative in 62 normal volunteer subjects. Patch tests performed on 14 patients with proved systemic photosensitivity to piroxicam were positive for thiomersal and thiosalicylic acid. Nine of 12 patients previously sensitized to thiosalicylic acid and with no history of exposure to piroxicam showed positive photopatch test reactions to this chemical. These results support a relation between piroxicam-induced photosensitivity and contact sensitivity to thiosalicylic acid. Contact allergic sensitivity to the latter is a marker for patients with a high risk of developing photosensitivity reactions to piroxicam. These reactions may be due to photoproducts of the drug rather than metabolites.

Reactions to thimerosal in hepatitis B vaccines

Hypersensitivity to thimerosal in vaccines has been reported to induce persistent local reactions, urticarial and generalized exanthematic eruptions, and, in the case of the hepatitis B vaccine, urticaria with asthma. The authors describe two cases of extensive reactions, one in a patient who did not form antibodies to the principal vaccine antigen. Although not all thimerosal-sensitive patients develop adverse reactions to vaccines containing this material, there is a potential risk, and the reactions can be very long lasting.

Thimerosal: an ophthalmic preservative which acts as a hapten to elicit specific antibodies and cell mediated immunity

A rabbit model for the study of hypersensitivity to thimerosal was established in order to develop better techniques for screening patient sera and tears for specific antibodies to lens care re-agents. Thimerosal (sodium ethylmercury thiosalicylatelate) was successfully coupled to several protein carriers using a water soluble carbodiimide which linked the carboxyl group of thimerosal to free amino groups of the carrier proteins. Thimerosal was also shown to spontaneously react with proteins as detected by the irreversible binding of mercury to the protein carrier. Immunization of rabbits with the chemically coupled thimerosal resulted in the production of antibodies which specifically reacted with thimerosal. The rabbits also manifested delayed and immediate forms of hypersensitivity to the thimerosal conjugates. The ELISA assay for specific serum antibodies was found to be a sensitive, reliable and specific screening tool However, there was no immunological cross reactivity between the chemically coupled thimerosal and the spontaneously coupled thimerosal. Therefore, the epitopes produced by these two reaction mechanisms were probably immunochemically different even though both contained detectable thimerosal derived mercury.

Thiomersal allergy and vaccination reactions

Thiomersal is the preservative in all toxoid vaccines routinely administered to children in the UK, but exposure from other sources is uncommon. Delayed hypersensitivity to thiomersal was demonstrated in 1% of individuals attending the Contact Dermatitis Investigation Unit, and 50 of these patients with positive patch tests to thiomersal were studied. Cross-reaction with other mercurials occurred in 17 of 29 patients tested (59%). 31 of the patients replied to a questionnaire regarding vaccination reactions, and were compared with case-controls matched for age, sex, and site of dermatitis. 4 patients in each group reported reactions to vaccines which contained thiomersal, suggesting that thiomersal hypersensitivity was not associated with an increased risk of vaccination reactions. However, individual cases of severe reactions to thiomersal demonstrate a need for vaccines with an alternative preservative.

Allergy to non-toxoid constituents of vaccines and implications for patch testing

We report 3 patients with persistent symptoms at vaccination sites. All were allergic to aluminium and one to thiomersal and neomycin too. Aluminium allergy causes false positive patch test reactions and we propose methods of patch testing patients with symptoms at vaccination sites in order to avoid this problem. The practical relevance of allergy to non-toxoid constituents of vaccines is discussed.

Two further examples of IgG thimerosal-dependent antibodies and their serological characteristics

Two further examples of IgG thimerosal-dependent antibodies have been found by routine immunohematological tests carried out in our laboratory. Both antibodies fixed the complement and had no apparent blood group specificity. Their reactivity was inhibited by adding thimerosal to the serum prior to the addition of red blood cells. If cross matching is performed accordingly, thimerosal-dependent antibodies should not any longer represent a blood bank problem.

The adjuvant effect of tuberculin in experimental sensitization to merthiolate

When guinea pigs were sensitized to tuberculin by exposure to complete Freund's adjuvant the size of the intradermal PPD test was positively correlated to the body weight of the animal. Old Tuberculin showed stimulating effect in a subsequent sensitization to merthiolate (thimerosal). The high frequency of delayed cutaneous allergy to merthiolate in Swedish population is probably explained by sensitization through intradermal testing with tuberculin containing merthiolate as a preservative.

Effects of vehicles and elicitation concentration in contact dermatitis testing. I. Experimental contact sensitization in humans

A study was made to evaluate the effects of vehicle and challenge concentration on response of human subjects to potential allergens. In the vehicle studies the modified Draize test was used to test the response of subjects to cinnamic aldehyde and to costus oil, administered at two skin sites, in petrolatum and in 95% ethyl alcohol. In two tests of costus oil, alcohol proved to be more effective in eliciting a response than petrolatum; on the other hand, in one test with cinnamic aldehyde, no difference in results was obtained with these two vehicles. In the concentration studies, subjects known to be sensitive to the test substance were tested by the Al test with costus oil (three concentrations), chloracetamide (four concentrations), or thimerosal (three concentrations); petrolatum was used as the vehicle in each case. Results of the vehicle test showed no compelling reason for the selection of one vehicle rather than another. Results of the concentration tests indicated that concentration does have an effect on the intensity and frequency of reactions to potential allergens.