Benzalkonium chloride

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Benzalkonium chloride
Benzalkonium chloride Structure V.1.svg
Identifiers
CAS number 68424-85-1 N
UNII F5UM2KM3W7 YesY
EC number 264-151-6
KEGG D00857 YesY
ChEMBL CHEMBL502109 N
ATC code D08AJ01D09AA11 (dressing), R02AA16
Properties
Molecular formula variable
Molar mass variable
Appearance 100% is white or yellow powder; gelatinous lumps; Solutions BC50 (50%) & BC80 (80%) are colorless to pale yellow solutions
Density 0.98 g/cm3
Solubility in water very soluble
Hazards
EU classification C, N [1]
R-phrases R21/22, R34, R50 [1]
S-phrases S36/37/39, S45, S61 [1]
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 250 °C (if solvent based)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Benzalkonium chloride, also known as BZK, BKC, alkyldimethylbenzylammonium chloride and ADBAC, is a cationic surface-acting agent belonging to the quaternary ammonium group. It has three main categories of use: as a biocide, a cationic surfactant, and phase transfer agent in the chemical industry. The chemical is a nitrogenous mixture of alkylbenzyldimethylammonium chlorides of various even-numbered[2] alkyl chain lengths.

Properties[edit]

Benzalkonium chloride is readily soluble in ethanol and acetone. Although dissolution in water is slow, aqueous solutions are easier to handle and are preferred. Solutions should be neutral to slightly alkaline, with colour ranging from colourless to a pale yellow. Solutions foam profusely when shaken, have a bitter taste and a faint almond-like odour which is only detectable in concentrated solutions.

Availability[edit]

Standard concentrates are manufactured as 50% and 80% w/w solutions, and sold under trade names such as BC50, BC80, BAC50, BAC80, etc. The 50% solution is purely aqueous, while more concentrated solutions require incorporation of rheology modifiers (alcohols, polyethylene glycols, etc.) to prevent increases in viscosity or gel formation under low temperature conditions.

Applications[edit]

The applications of benzalkonium chloride are extremely wide ranging,[3] from disinfectant formulations, such as being an active ingredient in Dettol and Lysol brand products, to microbial corrosion inhibition in the oilfield sector, and a multi-surface mould, algae and moss remover.

It is used in:

  • Skin antiseptics, such as Bactine, to protect scrapes and cuts
  • Pharmaceuticals such as throat lozenges[4] & various leave-on skin antiseptics
  • Hand sanitizers
  • Preservative in pharmaceuticals and personal care products such as eye, ear and nasal drops, as a preservative
  • Hygienic towelettes and wet wipes
  • Cleaners for floor and hard surfaces as a disinfectant
  • Soak solutions for surgical/dental instruments prior to high-level sterilisation
  • Spray disinfectants for hard surface sanitisation
  • Over-the-counter single-application treatments for herpes, cold-sores, and fever blisters, such as RELEEV and Viroxyn
  • Algaecide for clearing of algae, moss, lichens from paths, roof tiles, swimming pools, masonry and in horticultural greenhouse disinfection

Hand sanitizers based on benzalkonium chloride are more effective due to better residual activity and less irritant than alcohol gels. As an antiseptic, it has the advantage of not burning when put on a wound, which is not the case with ethanol-based antiseptics or hydrogen peroxide.

Biological activity[edit]

The greatest biocidal activity is associated with the C12 dodecyl & C14 myristyl alkyl derivatives. The mechanism of bactericidal/microbicidal action is thought to be due to disruption of intermolecular interactions. This can cause dissociation of cellular membrane lipid bilayers, which compromises cellular permeability controls and induces leakage of cellular contents. Other biomolecular complexes within the bacterial cell can also undergo dissociation. Enzymes, which finely control a wide range of respiratory and metabolic cellular activities, are particularly susceptible to deactivation. Critical intermolecular interactions and tertiary structures in such highly specific biochemical systems can be readily disrupted by cationic surfactants.

Benzalkonium chloride solutions are fast-acting biocidal agents with a moderately long duration of action. They are active against bacteria and some viruses, fungi, and protozoa. Bacterial spores are considered to be resistant. Solutions are bacteriostatic or bactericidal according to their concentration. Gram-positive bacteria are generally more susceptible than Gram-negative. Activity is not greatly affected by pH, but increases substantially at higher temperatures and prolonged exposure times. In a 1998 study utilizing the FDA protocol, a non-alcohol sanitizer utilizing the active ingredient benzalkonium chloride met the FDA performance standards, while Purell, a popular alcohol-based sanitizer, did not. The study found that a benzalkonium chloride-based sanitizer was the most favorable non-alcohol-based hand sanitizer.[5] Advancements in the quality and efficacy of benzalkonium chloride in current non-alcohol hand sanitizers has addressed the CDC concerns regarding gram negative bacteria, with the leading products being equal if not more effective against gram negative, particularly New Delhi metallo-beta-lactamase 1 and other antibiotic resistant bacteria.

Newer formulations using benzalkonium blended with various quaternary ammonium derivatives can be used to extend the biocidal spectrum and enhance the efficacy of benzalkonium based disinfection products. Formulation techniques have been used to great effect in enhancing the virucidal activity of quaternary ammonium-based disinfectants such as Virucide 100 to typical healthcare infection hazards such as hepatitis and HIV. The use of appropriate excipients can also greatly enhance the spectrum, performance and detergency, and prevent deactivation under use conditions. Formulation can also help minimise deactivation of benzalkonium solutions in the presence of organic and inorganic contamination.

Effectively formulated quaternary ammonium disinfectants are effective at very low ppm levels, and are now the disinfectants of choice for hospitals. This is on account of user and patient safety on contact with treated surfaces and the absence of harmful fumes. Benzalkonium solutions for hospital use tend to be neutral to alkaline, non-corrosive on metal surfaces, non-staining, and safe to use on all washable surfaces. Solutions are incompatible with soaps, and must not be mixed with anionic surfactants[citation needed]. Hard water salts can also reduce biocidal activity[citation needed]. As with any disinfectant, it is recommended that surfaces are free from visible dirt and interfering materials for maximal disinfection performance by quaternary ammonium products.

Safety[edit]

Benzalkonium chloride is highly toxic to fish (LC50 = 280 μg ai/L), very highly toxic to aquatic invertebrates (LC50 = 5.9 μg ai/L), moderately toxic to birds (LD50 = 136 mg/kg-bw), and slightly toxic to mammals (LD50 = 430 mg/kg-bw).[6]

Benzalkonium chloride is widely used as a preservative or biocide in eyewashes, nasal sprays, hand and face washes, mouthwashes, spermicidal creams, and in various other cleaners, sanitizers, and disinfectants.

Benzalkonium chloride formulations for consumer use are dilute solutions. Concentrated solutions are toxic to humans, causing corrosion/irritation to the skin and mucosa, and death if taken internally in sufficient volumes.[7] A 2014 case study detailing the fatal ingestion of up to 240ml of 10% benzalkonium chloride in a 78 year-old male also includes a summary of the currently published case reports of benzalkonium chloride ingestion.[8]

Benzalkonium chloride is effective at exceptionally low concentrations; contact lens solutions typically contain exceptionally low (0.002% to 0.01%) concentrations of benzalkonium chloride for effective preservative action.[9] Swan found that repeated use of benzalkonium chloride at 10-fold higher concentrations of 1:5000 (0.02%) or stronger can denature corneal protein and cause damage to the eye.[10] Intraocular benzalkonium chloride is highly toxic to the corneal endothelium in concentrations commonly used extraocularly.[11] Avoiding the use of benzalkonium chloride solutions while contact lenses are in place is discussed in the literature.[12][13] Concentrations of 1% to 2% benzalkonium chloride completely destroy the ocular anterior chamber of small animals within a week.[14] Although historically benzalkonium chloride has been ubiquitous as a preservative in ophthalmic preparations, its toxicity and irritant properties,[15] in conjunction with consumer demand, have led pharmaceutical companies to increase production of preservative-free preparations, or to use less harmful preservatives.

There has been concern that long-term use of benzalkonium as a preservative in nasal sprays may cause swelling of mucosa and lead to rhinitis medicamentosa. Although some studies have found no correlation between use of benzalkonium chloride in nasal sprays and rhinitis medicamentosa,[16] others have found benzalkonium chloride in oxymetazoline nasal spray to worsen rhinitis medicamentosa in healthy volunteers after both long-term use[17] and short-term use.[18] A review of the literature to 2004 by Marple et al.[16] revealed very limited data that demonstrated statistically significant safety concerns for benzalkonium chloride concentrations at or below 0.1%. Graf's review of the literature in 2001 was more balanced in its evaluation of studies finding adverse clinical effects on human nasal tissue, advocating that "without conclusive data regarding BKC and the possibility of harmful effects, the use of nasal formulations without BKC might be a reasonable alternative".[19] This view supports Berg et al., who in 1997 demonstrated that degenerative morphological changes in the nasal mucosa of rats were directly attributed to the benzalkonium chloride in topical nasal steroid sprays. They concluded that the deleterious effects of benzalkonium chloride in topical nasal steroid sprays outweighed any benefit, and that “rhinosinusitis can be effectively treated without it”.[20]

The ototoxicity of benzalkonium chloride was established by Aursnes in 1982 when it was found that solutions of 0.1% benzalkonium chloride instilled into the ears of guinea pigs produced damage to the vestibulum and cochlear. In addition, pathological changes to the tympanic cavity and the perilymphatic spaces of vestibulum and cochlea were found.[21]

As with antibiotics, the use of biocides at sub-inhibitory concentrations can potentially result in resistant organisms, and should be used at recommended dilutions and extended contact time to ensure effective disinfection. While resistance is rarely linked to disinfectant usage at low concentrations, a 2009 study suggested that when used in less than lethal concentrations, benzalkonium chloride solutions could result in increased resistance Pseudomonas aeruginosa, and an increase in resistance of the bacteria to the ciprofloxacin antibiotic, even though the bacterial colonies had not been previously exposed to the antibiotic.[22]

Several human case studies have identified allergic and irritant reactions to benzalkonium chloride, including asthma,[23] contact dermatitis[24][25][26][27][28] and ocular hypersensitivity.[29] Animal studies support the toxicity of benzalkonium chloride on the lung,[30][31][32] including the induction of bronchoconstriction.[33]

Several larger studies, including epidemiological studies, have identified a strong link between the occupational use of quaternary ammonium compounds, specifically benzalkonium chloride, with an increased incidence of asthma[34][35][36][37][38][39][40][41] and rhinitis,[42] especially amongst nurses, cleaners and farmers. Whether these are irritant or immune-mediated responses is not clear, though the hypothesis that bronchial hyperresponsiveness is secondary to epithelial cell damage is supported by the literature.[43] More recently, biopsy results on mucosal epithelium suggest that neurogenic inflammation as a result of epithelial injury may be responsible for irritant-induced asthma and rhinitis.[44][45] Studies on exposure of atopic subjects to ozone[46] and to diesel exhaust particles (DEPs)[47] have identified that co-exposure to irritants and allergens leads to allergic sensitization.

A 2012 study reported that simultaneous ocular exposure to an inert antigen and benzalkonium chloride leads to a significant change in the systemic immune response to the administered antigen in mice, thus suggesting that the preservative's effects could be more profound than the local disruption of the epithelial barrier integrity and raising the question of whether benzalkonium chloride has a role in the induction of allergy.[48]

See also[edit]

References[edit]

  1. ^ a b c Record of Quaternary ammonium compounds, benzyl-C8–18-alkyldimethyl, chlorides in the European chemical Substances Information System ESIS
  2. ^ U.S. Environmental Protection Agency: Reregistration Eligibility Decision for Alkyl Dimethyl Benzyl Ammonium Chloride (ADBAC)
  3. ^ [1]
  4. ^ Bradosol
  5. ^ David Dyer Kenneth Gerenraich Peter Whams (1998). Testing a New Alcohol-Free Hand Sanitizer to Combat Infection. AORN Journal Vol 68 Issue 2. pp. 239–251. 
  6. ^ Frank T. Sanders, ed. (August 2006). Reregistration Eligibility Decision for Alkyl Dimethyl Benzyl Ammonium Chloride (ADBAC) (Report). U.S. Environmental Protection Agency Office of Prevention, Pesticides, and Toxic Substances. pp. 114. http://www.epa.gov/oppsrrd1/REDs/adbac_red.pdf. Retrieved 2009-03-31.
  7. ^ Seymour Stanton Block (2001). Disinfection, sterilization, and preservation (5, illustrated ed.). Lippincott Williams & Wilkins. p. 311. ISBN 0-683-30740-1. 
  8. ^ Spiller, Henry A (2014). "A Case of Fatal Ingestion of a 10% Benzalkonium Chloride Solution". J Forensic Toxicol Pharmacol 3 (1). doi:10.4172/2325-9841.1000113. 
  9. ^ U.S. Patent 5,725,887, column 2, line 8
  10. ^ Swan, K. C., "Reactivity of the Ocular Tissues to Wetting Agents", Am. J. Ophthalmol., 27, 118 (1944),
  11. ^ Liu H, Routley I, Teichmann KD (2001). "Toxic endothelial cell destruction from intraocular benzalkonium chloride". J Cataract Refract Surg 27 (11): 1746–50. doi:10.1016/S0886-3350(01)01067-7. PMID 11709246. 
  12. ^ Otten, Mary; Szabocsik, John M. (1976). "Measurement of Preservative Binding with SOFLENS (polymacon) Contact Lens". Clinical and Experimental Optometry 59 (8): 277. doi:10.1111/j.1444-0938.1976.tb01445.x. 
  13. ^ M Akers, "Consideration in selecting antimicrobial preservative agents for parenteral product development", Pharmaceutical Technology, May, p. 36 (1984).
  14. ^ Hoang-Xuan, Thanh; Baudouin, Christophe; Creuzot-Garcher, Catherine (2001). Inflammatory Diseases of the Conjunctiva (1, illustrated ed.). Thieme. p. 141. ISBN 978-3-131-25871-7. 
  15. ^ Baudouin, C; Labbé, A; Liang, H; Pauly, A; Brignole-Baudouin, F (2010). "Preservatives in eyedrops: the good, the bad and the ugly". Prog Retin Eye Res 29 (4): 312–34. doi:10.1016/j.preteyeres.2010.03.001. PMID 20302969. 
  16. ^ a b Marple, B; Roland, P; Benninger, M (2004). "Safety review of benzalkonium chloride used as a preservative in intranasal solutions: an overview of conflicting data and opinions.". Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery 130 (1): 131–41. doi:10.1016/j.otohns.2003.07.005. PMID 14726922. 
  17. ^ Graf, P (2005). "Rhinitis medicamentosa: a review of causes and treatment.". Treatments in respiratory medicine 4 (1): 21–9. doi:10.2165/00151829-200504010-00003. PMID 15725047. 
  18. ^ Graf, P; Enerdal, J; Hallén, H (1999). "Ten days' use of oxymetazoline nasal spray with or without benzalkonium chloride in patients with vasomotor rhinitis.". Archives of otolaryngology--head & neck surgery 125 (10): 1128–32. doi:10.1001/archotol.125.10.1128. PMID 10522506. 
  19. ^ Graf P (2001). "Benzalkonium chloride as a preservative in nasal solutions: re-examining the data". Respir Med 95 (9): 728–33. doi:10.1053/rmed.2001.1127. PMID 11575893. 
  20. ^ Berg, Ø. H.; Lie, K; Steinsvåg, S. K. (1997). "The effects of topical nasal steroids on rat respiratory mucosa in vivo, with special reference to benzalkonium chloride". Allergy 52 (6): 627–32. doi:10.1111/j.1398-9995.1997.tb01041.x. PMID 9226056. 
  21. ^ Aursnes, J (1982). "Ototoxic effect of quaternary ammonium compounds". Acta Otolaryngol. 93 (5–6): 421–33. PMID 6125078. 
  22. ^ Mc Cay, P. H.; Ocampo-Sosa, A. A.; Fleming, G. T. A. (2009). "Effect of sub-inhibitory concentrations of benzalkonium chloride on the competitiveness of Pseudomonas aeruginosa grown in continuous culture". Microbiology 156 (Pt 1): 30–8. doi:10.1099/mic.0.029751-0. PMID 19815578. 
  23. ^ Purohit A, Kopferschmitt-Kubler MC, Moreau C, Popin E, Blaumeiser M, Pauli G (2000). "Quaternary ammonium compounds and occupational asthma". Int Arch Occup Environ Health 73 (6): 423–7. doi:10.1007/s004200000162. PMID 11007347. 
  24. ^ Park HJ, Kang HA, Lee JY, Kim HO (2000). "Allergic contact dermatitis from benzalkonium chloride in an antifungal solution". Contact Derm. 42 (5): 306–7. PMID 10789868. 
  25. ^ Wong DA, Watson AB (2001). "Allergic contact dermatitis due to benzalkonium chloride in plaster of Paris". Australasian J. Dermatology 42 (1): 33–5. doi:10.1046/j.1440-0960.2001.00469.x. PMID 11233718. 
  26. ^ Kanerva L, Jolanki R, Estlander T (2000). "Occupational allergic contact dermatitis from benzalkonium chloride". Contact Derm. 42 (6): 357–8. PMID 10871106. 
  27. ^ Oiso N, Fukai K, Ishii M (2005). "Irritant contact dermatitis from benzalkonium chloride in shampoo". Contact Derm. 52 (1): 54. doi:10.1111/j.0105-1873.2005.0483j.x. PMID 15701139. 
  28. ^ Basketter DA, Marriott M, Gilmour NJ, White IR (2004). "Strong irritants masquerading as skin allergens: the case of benzalkonium chloride". Contact Derm. 50 (4): 213–7. doi:10.1111/j.0105-1873.2004.00331.x. PMID 15186375. 
  29. ^ Chiambaretta F, Pouliquen P, Rigal D (1997). "[Allergy and preservatives. Apropos of 3 cases of allergy to benzalkonium chloride]". J Fr Ophtalmol (in French) 20 (1): 8–16. PMID 9099278. 
  30. ^ Xue, Y; Hieda, Y; Kimura, K; Takayama, K; Fujihara, J; Tsujino, Y (2004). "Kinetic characteristics and toxic effects of benzalkonium chloride following intravascular and oral administration in rats". J. Chromatogr. B Analyt. Technol. Biomed. Life Sci. 811 (1): 53–8. PMID 15458722. 
  31. ^ Swiercz, R; Hałatek, T; Wasowicz, W; Kur, B; Grzelińska, Z; Majcherek, W (2008). "Pulmonary irritation after inhalation exposure to benzalkonium chloride in rats". Int J Occup Med Environ Health 21 (2): 157–63. doi:10.2478/v10001-008-0020-1. PMID 18715840. 
  32. ^ Swiercz, R; Hałatek, T; Stetkiewicz, J; Wąsowicz, W; Kur, B; Grzelińska, Z; Majcherek, W (2013). "Toxic effect in the lungs of rats after inhalation exposure to benzalkonium chloride". Int J Occup Med Environ Health 26 (4): 647–56. doi:10.2478/s13382-013-0137-8. PMID 24288119. 
  33. ^ Larsen, ST; Verder, H; Nielsen, GD (2012). "Airway effects of inhaled quaternary ammonium compounds in mice". Basic Clin. Pharmacol. Toxicol. 110 (6): 537–43. doi:10.1111/j.1742-7843.2011.00851.x. PMID 22188809. 
  34. ^ Preller L, Doekes G, Heederik D, Vermeulen R, Vogelzang PF, Boleij JS (1996). "Disinfectant use as a risk factor for atopic sensitization and symptoms consistent with asthma: an epidemiological study". Eur. Respir. J. 9 (7): 1407–13. doi:10.1183/09031936.96.09071407. PMID 8836651. 
  35. ^ Vogelzang PF, van der Gulden JW, Preller L, Tielen MJ, van Schayck CP, Folgering H (1997). "Bronchial hyperresponsiveness and exposure in pig farmers". Int Arch Occup Environ Health 70 (5): 327–33. doi:10.1007/s004200050226. PMID 9352336. 
  36. ^ Vandenplas O, D'Alpaos V, Evrard G, Jamart J, Thimpont J, Huaux F, Renauld JC (2013). "Asthma related to cleaning agents: a clinical insight". BMJ Open 3 (9): e003568. doi:10.1136/bmjopen-2013-003568. PMID 24056489. 
  37. ^ Gonzalez M, Jégu J, Kopferschmitt MC, Donnay C, Hedelin G, Matzinger F, Velten M, Guilloux L, Cantineau A, de Blay F (2014). "Asthma among workers in healthcare settings: role of disinfection with quaternary ammonium compounds". Clin. Exp. Allergy 44 (3): 393–406. doi:10.1111/cea.12215. PMID 24128009. 
  38. ^ Quirce S, Barranco P (2010). "Cleaning agents and asthma". J Investig Allergol Clin Immunol 20 (7): 542–50. PMID 21313993. 
  39. ^ Paris C, Ngatchou-Wandji J, Luc A, McNamee R, Bensefa-Colas L, Larabi L, Telle-Lamberton M, Herin F, Bergeret A, Bonneterre V, Brochard P, Choudat D, Dupas D, Garnier R, Pairon JC, Agius RM, Ameille J; Members of the RNV3P. (2012). "Work-related asthma in France: recent trends for the period 2001-2009". Occup Environ Med 69 (6): 391–7. doi:10.1136/oemed-2011-100487. PMID 22383588. 
  40. ^ Bernstein JA, Stauder T, Bernstein DI, Bernstein IL (1994). "A combined respiratory and cutaneous hypersensitivity syndrome induced by work exposure to quaternary amines". J. Allergy Clin. Immunol. 94 (2 Part 1): 257–9. doi:10.1053/ai.1994.v94.a52646. PMID 8064078. 
  41. ^ Anila Bello, Margaret M Quinn, Melissa J Perry, and Donald K Milton (2009). "Characterization of occupational exposures to cleaning products used for common cleaning tasks--a pilot study of hospital cleaners". Environ Health 8 (11): 11. doi:10.1186/1476-069X-8-11. PMID 19327131. 
  42. ^ Smit LA, Zuurbier M, Doekes G, Wouters IM, Heederik D, Douwes J (2007). "Hay fever and asthma symptoms in conventional and organic farmers in The Netherlands". Occup Environ Med 64 (2): 101–7. doi:10.1136/oem.2006.028167. PMC 2078430. PMID 17018582. 
  43. ^ Wardlaw AJ, Dunnette S, Gleich GJ, Collins JV, Kay AB (1988). "Eosinophils and mast cells in bronchoalveolar lavage in subjects with mild asthma. Relationship to bronchial hyperreactivity". Am. Rev. Respir. Dis. 137 (1): 62–9. doi:10.1164/ajrccm/137.1.62. PMID 2447813. 
  44. ^ Meggs WJ (1997). "Hypothesis for induction and propagation of chemical sensitivity based on biopsy studies". Environ. Health Perspect. 105 Suppl 2 (March): 473–8. doi:10.1289/ehp.97105s2473. PMC 1469810. PMID 9167982. 
  45. ^ Meggs WJ (1994). "RADS and RUDS--the toxic induction of asthma and rhinitis". J. Toxicol. Clin. Toxicol. 32 (5): 487–501. doi:10.3109/15563659409011053. PMID 7932908. 
  46. ^ Molfino NA, Wright SC, Katz I, Tarlo S, Silverman F, McClean PA, Szalai JP, Raizenne M, Slutsky AS, Zamel N (1991). "Effect of low concentrations of ozone on inhaled allergen responses in asthmatic subjects". Lancet 338 (8761): 199–203. doi:10.1016/0140-6736(91)90346-q. PMID 1676776. 
  47. ^ Diaz-Sanchez D, Garcia MP, Wang M, Jyrala M, Saxon A (1999). "Nasal challenge with diesel exhaust particles can induce sensitization to a neoallergen in the human mucosa". J. Allergy Clin. Immunol. 104 (6): 1183–8. doi:10.1016/s0091-6749(99)70011-4. PMID 10588999. 
  48. ^ Galletti, J. G.; Gabelloni, M. L.; Morande, P. E.; Sabbione, F.; Vermeulen, M. E.; Trevani, A. S.; Giordano, M. N. (2012). "Benzalkonium chloride breaks down conjunctival immunological tolerance in a murine model". Mucosal Immunology. [Epub ahead of print] (1). doi:10.1038/mi.2012.44. PMID 22692451. 

Further reading[edit]

External links[edit]