Natural Rubber Latex Adhesive Issues
July 1, 2008
It is well known that natural rubber latex adhesives, extracted from the rubber tree of Hevea Brasiliensis, contain antigenic proteins that may cause adverse immune responses in some individuals, such as latex allergy.
Tapping a rubber tree involves cutting channels into the bark (controlled wounding) and bleeding out the milky white latex extract. The latex is produced by special cells called laticifiers and is thought to be a defense against insect pathogens and possibly a site for the depositing of metabolic waste of the tree.
Hevea Brasiliensis latex contains about 2-5% by weight protein. Analysis indicates about 200 dissimilar proteins therein, about 50-60 of which are suspected allergens. The World Health Organization - International Union of Immunological Societies has assigned names to about 13 of these allergens (e.g., Hev-b 1-13).
Repeated exposure to these allergens through dermal absorption has been shown to cause an increased incidence of sensitization, adverse allergic reactions and death through anaphylactic shock. Research has shown that cysteine residues in some Hev-b proteins play a major role in allergenicity. For example, research has shown that recombinant NRL allergens wherein the cysteine residues are replaced with alanine form peptide variants that have markedly decreased IgE binding. Furthermore, basophil activation testing has shown decreased activation with successive cysteine exclusion.1
A study that evaluated percutaneous penetration of natural rubber latex proteins concluded that the skin is not only a plausible route for latex sensitization, but can be a major exposure route when the integument has been compromised.2
In the medical industry, efforts continue to substantially eliminate natural rubber latex in the health care environment. For example, John Hopkins Hospital recently announced that it will no longer use nearly all medical latex products. Studies show some 15% of healthcare workers are allergic to latex.
It is important to reduce natural rubber exposure in that the frequency of exposure to the Hev-b proteins may influence the prevalence of an allergy. For example, a primary challenge to the Hev-b proteins may induce the activation of virgin T & B cells, and long-lived memory T & B cells. Thereafter, repeated exposure to the Hev-b proteins activate existing memory T & B cells and stimulate further production of T & B cells (i.e., virgin cells and memory cells).
It is understood that repeated exposure to the Hev-b proteins often increases the specific-antibody population (i.e., IgE primed mast-cells) affecting the intensity and duration of the immune response. Alternatively, exclusion of the Hev-b proteins inhibit re-activation of associated T & B cells, allowing such cells to substantially decay, dampening the immune response. For example, in a follow-up study on latex allergy in children, researchers concluded that strict compliance with latex avoidance should be practiced. Furthermore, greater emphasis should be placed on reducing latex exposure in the home and school environments to reduce contact which could maintain positive IgE-antibody levels.3
Allergic responses to foodstuff proteins have also been associated with latex allergy.4-5 What can be perplexing is natural rubber latex allergy can go into remission, from reduced exposure, while cross-react allergies (e.g., food allergies) may remain persistent based on repeated exposure to such food proteins. Thus, the after effects of latex allergy may continue to stress immunity long after the latex allergy has gone into remission. Consumer groups are calling for warning labels on food packaging containing latex, saying the substance poses a potential threat to people with allergic sensitivities.6
Most recently, it has been proposed in the Journal of Medical Hypotheses that increased Hev-b protein exposure may have affected the incidence of autism spectrum disorders. The timing, frequency, intensity, and type of exposure to such proteins may have increased atopy and allergy induced autism.7
Natural rubber latex adhesives formed from Hevea Brasiliensis are not required by law to meet the minimum standard of protein content based on The American Society for Testing and Materials method ASTM D1076-06 (Category 4) that defines latex containing less than 200 micrograms total protein per gram dry weight of latex. Although this may be moot in that the threshold of sensitivity to the Hev-b proteins is unknown.
The adhesive industry has been responsive to the health and safety issues associated with Hevea Brasiliensis-based products for the medical industry. For example, adhesive companies have developed hypoallergenic formulations, which are free of natural rubber latex, for medical-packaging applications.
Although the price/performance relationship between natural rubber latex and synthetic rubber continues to give life to Hevea Brasiliensis-based adhesives, research efforts continue to develop alternative water-based formulations that have an effective combination of properties including strong wet-tack and reduced dry-tack allowing them to compete with natural rubber latex.