Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Haptoglobin and Sickle Cell Polymorphisms and Risk of Active Trachoma in Gambian Children

  • Mathilde Savy,

    Affiliation MRC International Nutrition Group, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Branwen J. Hennig,

    Affiliation MRC International Nutrition Group, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Conor P. Doherty,

    Affiliation MRC Laboratories, Fajara, The Gambia

  • Anthony J. Fulford,

    Affiliation MRC International Nutrition Group, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Robin Bailey,

    Affiliations MRC Laboratories, Fajara, The Gambia, Department of Infectious Tropical Diseases, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Martin J. Holland,

    Affiliations MRC Laboratories, Fajara, The Gambia, Department of Infectious Tropical Diseases, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Giorgio Sirugo,

    Affiliations MRC Laboratories, Fajara, The Gambia, Department of Medical Genetics, San Pietro Hospital, Rome, Italy

  • Kirk A. Rockett,

    Affiliation Wellcome Trust Centre for Human Genetics, Oxford, United Kingdom

  • Dominic P. Kwiatkowski,

    Affiliations Wellcome Trust Centre for Human Genetics, Oxford, United Kingdom, Malaria Programme, Wellcome Trust Sanger Institute, Hinxton, United Kingdom

  • Andrew M. Prentice,

    Affiliation MRC International Nutrition Group, London School of Hygiene & Tropical Medicine, London, United Kingdom

  • Sharon E. Cox

    Affiliation MRC International Nutrition Group, London School of Hygiene & Tropical Medicine, London, United Kingdom

Haptoglobin and Sickle Cell Polymorphisms and Risk of Active Trachoma in Gambian Children

  • Mathilde Savy, 
  • Branwen J. Hennig, 
  • Conor P. Doherty, 
  • Anthony J. Fulford, 
  • Robin Bailey, 
  • Martin J. Holland, 
  • Giorgio Sirugo, 
  • Kirk A. Rockett, 
  • Dominic P. Kwiatkowski, 
  • Andrew M. Prentice



Susceptibility and resistance to trachoma, the leading infectious cause of blindness, have been associated with a range of host genetic factors. In vitro studies of the causative organism, Chlamydia trachomatis, demonstrate that iron availability regulates its growth, suggesting that host genes involved in regulating iron status and/or availability may modulate the risk of trachoma. The objective was to investigate whether haptoglobin (Hp) haplotypes constructed from the functional polymorphism (Hp1/Hp2) plus the functional promoter SNPs -61A-C (rs5471) and -101C-G (rs5470), or sickle cell trait (HbAS, rs334) were associated with risk of active trachoma when stratified by age and sex, in rural Gambian children.

Methodology and Principal Findings

In two cross sectional surveys of children aged 6–78 months (n = 836), the prevalence of the clinical signs of active trachoma was 21.4%. Within boys, haplotype E (-101G, -61A, Hp1), containing the variant allele of the -101C-G promoter SNP, was associated with a two-fold increased risk of active trachoma (OR = 2.0 [1.17–3.44]). Within girls, an opposite association was non-significant (OR = 0.58 [0.32–1.04]; P = 0.07) and the interaction by sex was statistically significant (P = 0.001). There was no association between trachoma and HbAS.


These data indicate that genetic variation in Hp may affect susceptibility to active trachoma differentially by sex in The Gambia.


Trachoma, a chronic keratoconjunctivitis caused by the intracellular bacterium Chlamydia trachomatis, is the leading infectious cause of blindness worldwide [1]. Repeated episodes of infection cause intense conjunctival inflammation (active trachoma), which can lead to chronic infection and conjunctival scarring, corneal scarring, opacification and, ultimately, blindness [2]. Trachoma disease progression is variable and incidence varies by age and sex [3], [4]. Some individuals experience prolonged infection, while others clear their infections in a few weeks. Differences in the inflammatory response to infection have been proposed as a determinant of progressive disease [5], [6], [7]. Although interventions have considerably reduced the number of people with blinding trachoma over the past decades, current estimates indicate that active trachoma still affects some 80 million people worldwide and about 8 million people are visually impaired [8].

Host genetic factors play a major role in susceptibility or resistance to many infectious diseases [9], [10], including C. trachomatis [11], [12]. Genetic association studies so far, have focussed on trachoma scarring or trachomatous trichiasis and human polymorphisms within loci involved in immunity and inflammation [13] and include several within the TNF locus including the -308G-A TNF-α promoter single nucleotide polymorphism (SNP) [14], [15]; the -1082A-G IL-10 SNP [16], [17]; chemokine and cytokine clusters in chromosomes 4q & 5q31 and HLA class I alleles [18], reviewed in [12]. Active trachoma in Gambian children has also been associated with the IL-10 -3917-G allele [19]. Acquisition of iron is a fundamental step in the development of a pathogen within its host. Evidence from in vitro studies suggests that low iron availability leads to impaired growth and infectivity of C. trachomatis [20], [21] but may also contribute to persistence or re-activation of dormant infection [20], [22], [23], whilst the anti-chlamydial action of some compounds is reversed in the presence of an iron source including holo-transferrin [24]. The accumulation of transferrin and its receptors around chlamydial inclusions in low-iron environments may suggest the mechanism of iron acquisition by C. trachomatis [20], [25]. In combination these and other studies suggest that variation in host genes involved in modulating immune, particularly inflammatory type responses and in regulating iron status and/or iron availability may affect the outcome of trachoma infection.

Haptoglobin (Hp), an acute phase protein, encoded by two major co-dominant alleles, Hp1 and Hp2, results in three functionally distinct phenotypes, Hp11, Hp12 and Hp22. Promoter polymorphisms -61A-C (rs5471) and -101C-G (rs5470) have been associated with ahaptoglobinaemia and hypohaptoglobinaemia, respectively [26], whilst reporter gene assays also demonstrated that -61C promoter constructs had significantly decreased transcriptional activity [27]. Haptoglobin binds circulating, toxic, free hemoglobin (Hb) released during intravascular haemolysis – such as occurs during malarial infections. The resultant complex is taken up by CD163 on circulating monocytes and macrophages leading both to an altered cytokine secretory profile [28], [29] and the eventual recycling of the iron component of haem for erythropoiesis. Plasma concentrations of haptoglobin, and the binding affinities of Hp for free hemoglobin and of the Hp-Hb complex for CD163 vary by phenotype. Hp22 has been associated with evidence of increased oxidant stress and iron delocalisation [30], [31], [32], as well as an increased risk of malaria-associated anemia [33], [34]. Hp is also thought to affect immune regulation, including the balance of Th1∶Th2 cytokine responses [35] and several studies have associated haptoglobin phenotypes and genotypes with a range of conditions including cardiovascular disease [36], diabetes [37], [38], HIV infection [39], [40], [41] and susceptibility to malaria [42], [43], [44], [45] including a protective association of the Hp haplotype containing the Hp2 and the C allele for the promoter -61A-C (rs5471) SNP in Gambian children [46]. A protective effect of the genetic variant sickle cell trait (HbAS) against malaria has been well documented [47], [48], [49], [50], [51]. However, few studies have assessed whether HbAS may be protective against other conditions [50]. There is evidence that alpha thalasaemia, which is protective against severe malaria, is also protective against severe morbidity from other infections, perhaps due to an interaction between malaria and risk of other infections [52].

Therefore we investigated whether Hp haplotypes constructed from the functional Hp allele (Hp1 or Hp2) plus the functional promoter SNPs -61A-C (rs5471) and -101C-G (rs5470) and HbAS were associated with the risk of active trachoma in Gambian children.


Design and sampling

Two cross sectional surveys of clinical signs of acute trachoma of children aged 6–78 months were conducted in September 2003 & January 2005 in eight rural Mandinka villages in the West Kiang district of The Gambia. As some children were visited in both years and others only once, only the first visit of each child was considered in the analysis. Children known to have sickle cell disease (HbSS) (n = 4) were excluded from analysis.

Grading of Chlamydia trachomatis infection

Clinical signs of active trachoma were assessed using a ×2 binocular loupe and pen torch by single grader. Both eyelids were everted and scored according to the WHO simplified trachoma grading system [53]. Active trachoma was defined as the presence of grades TF (follicular trachoma) or TI (intense inflammatory trachoma) in one or both eyes, with the eye with the greatest clinical severity being used for the outcome result. Eight children were found to have trachomatous scarring (TS) but none with trichiasis (TT). These eight children were excluded from the current analysis for risk of active trachoma. All children with active trachoma were offered treatment with tetracycline eye ointment as per Gambian national eye care programme guidelines at the time. Finally, following further structured surveys as part of a national programme [54], these communities were treated with 3 annual rounds of oral azithromycin, completed in 2009 as per WHO and the International Council of Ophthalmology guidelines for community control strategies.

Genotyping and haplotype construction

DNA was extracted from peripheral blood leukocytes using a standard salting out method [55]. Aliquots of DNA were shipped to the Wellcome Trust Centre for Human Genetics, Oxford, UK, where all genotyping was conducted. Haptoglobin was genotyped by allele-specific PCR adapted from a method published by Koch et al. [56]. This method determines the Hp1 and Hp2 alleles but does not distinguish between the “F” and “S” subtypes of the Hp1 and Hp2 alleles. It therefore avoids potential misclassification based on the presence of these sub-types in different populations [57]. PCR products were resolved by agarose gel electrophoresis and visualised under UV light. Details of primers and cycling conditions are provided in the supplementary material (Table S1). Sequenom® MassARRAY® (Sequenom®, Hamburg Germany) was used to genotype the haptoglobin promoter -61A-C (rs5471), -101C-G (rs5470) SNPs and HbS (rs334: hemoglobin – sickle) according to manufacturer's instructions.

Genotyping accuracy was verified by sequencing (eurofins MWG operon) in both directions in 56 samples with different Hp haplotypes. PCR products for sequencing were generated using primers Hp-Ex-U (5′-GCA GTG TGA AAA TCC TCC AAG ATA A-3′) and Hp-Ex1-L (5′-AAT TTA GCC CAT TTG CCC GTT TCT T-3′) under standard conditions.

Haplotypes for the two Hp promoter SNPs and the Hp gene alleles (Hp1 and Hp2) were constructed using SNPHAP [58].

Ethical permission

The study was approved by the joint Medical Research Council/Gambian Government Ethics Committee. The Gambian National DNA Collection Guidelines were followed regarding the handling of genetic material and information [59]. Parental written informed consent was obtained for all study participants.

Data analyses

Data analyses were performed using SAS version 9.1 (SAS Institute, Cary NC, USA). Differences in the prevalence of active trachoma between villages and by age (in years) and sex of children were tested using logistic regression. We have previously observed age-dependent effects of Hp variation on malaria risk [46] [45] within a sub-set of these children, whilst iron status can differ by sex in young children [60]. Therefore we analysed associations between genotypes and active trachoma using logistic regression models stratified by age group (greater or less than the median age of 36 months, to reduce the number of groups tested and increase precision of the estimates of effect) and by sex, and subsequently tested for interactions by comparison of models with and without inclusion of the interaction terms gene*age (greater or less than 36 months) or gene*sex in models. Village was included in all models a priori, whilst in models stratified by age, sex was included a priori and age (in years) was included a priori in models stratified by sex. Hp haplotypes were fitted as categorical variable with 0, 1 or 2 copies. As children who were found to be HbSS were excluded from analyses, sickle genotype was fitted as a simple binary variable (HbAS vs. HbAA).


Demographic and genetic characteristics of study subjects

A total of 1155 children were examined for clinical signs of trachoma. Of these, 235 (20.3%) had active trachoma, 11 (0.95%) had signs of trachomatous inflammation (TI) and 8 (0.69%) with trachomatous scarring (TS). There was no evidence of a difference by sex (Table 1). DNA samples were available from 1089 of these children. Complete genetic, clinical and socio-demographic data were available for 836 children with complete Hp haplotypes, and for 807 children screened for HbS and were included in the current analyses. The mean age of children included in the analysis was 35.6±18.1 months. The distribution of covariates and genotypes for children included in the analysis versus those not included is presented in Table 1. With the exception of village of residence (P = 0.01) there were no statistically significant difference in the distribution of covariates between the included/excluded groups, including the prevalence of active trachoma. There were no differences in the distribution of genotypes between the two groups, except for the Hp-61A-C promoter SNP for which fewer AA homozygotes and more heterozygotes were detected in the children included in the analysis (P = 0.02). Genotype frequencies did not differ by village of residence, except for HbAS (P = 0.0004) (data not shown).

Genotypes were not in Hardy Weinberg equilibrium (Table 1) for Hp1/2 as well as the -61A-C polymorphism, for which, as previously reported [46], no CC homozygotes were detected. Sequencing 56 samples in both directions confirmed the absence of -61C-C homozygotes and genotypes determined by sequencing correlated 100% with those determined by hME Sequenom typing for both Hp promoter SNPs.

Four common (frequency >10%) and two less common (frequency ≤1%) Hp haplotypes were detected (Table 2). As previously reported [26] the variant -61C allele appeared much more frequently in conjunction with the Hp2 allele (haplotype D), whilst the -101G allele was found more often with the Hp1 allele (haplotype E). No haplotypes containing the variant alleles for both of the promoter SNPs were detected.

Table 2. Prevalence of haplotypes constructed from main haptoglobin alleles (Hp1 and Hp2) and Hp promoter SNPs -101C-G (rs5470) and-61A-C (rs5471).

Associations between active trachoma and haptoglobin haplotypes and sickle cell trait

The prevalence of active trachoma did not vary by age or sex of the study participants, but did by village of residence, ranging from 7.1 to 34.7% (P = 0.0004) (Table 3).

Table 3. Active trachoma as a function of children age, sex and village of residence included in the current analysis (n = 836).

When stratified by sex. The Hp haplotype E (-101G, -61A, Hp1), containing the variant allele of the -101C-G promoter SNP, was associated with a two-fold increased risk of active trachoma in boys, but with a non-significantly decreased risk in girls (P = 0.07) (Table 4). This interaction was statistically significant (P = 0.001). There was no evidence of an association of the prevalence of active trachoma with the other Hp haplotypes or with HbAS vs HbAA, in either sex or either age group (data not shown by age group).

Table 4. Associations between active trachoma and haplotypes or genotypes by sex.


The prevalence of active trachoma was high, an average of 20%, compared to 6% in a smaller survey of 140 children aged 1–9 years in West Kiang District, conducted in 2006 [54]. The prevalence of active trachoma varied significantly by village, similar to other observations in The Gambia [54]. We did not find any difference in the rate of active trachoma between age groups or sex, contrary to some reports in which older children (3–5 years old) and females are more likely to have trachoma [3], [4]. However, it is unclear how much of previously observed sex and age differences relate to inherent responses to infection or to cultural practices leading to differences in exposure [4].

Here we report a sex-specific effect of the Hp haplotype E (-101G, -61A, Hp1) and susceptibility to active trachoma in Gambian children. The small number of individuals with two copies of the Hp haplotype E did not allow us to assess whether there might be a dose effect of carriage of this haplotype on risk of trachoma. In 257 Gambian children with no malaria infection, haplotype E (-101G, -61A, Hp1) was highly associated with a decreased risk of having non-detectable levels of plasma Hp (OR = 0.41, p = 0.002) [46]. It is possible that an effect of this Hp haplotype on risk of active trachoma is mediated though increased levels of Hp and differential inflammatory responses from interactions between Hp1 or Hp2 protein with macrophages [35], T cells [61], or other immune-modulatory effects [62] including possible effects on adaptive immunity [63]. We cannot currently explain why the effects of this Hp haplotype differ by sex; however, sex differences in disease patterns and in responses to vaccination are well known, and it is increasingly recognised that many immune functions may differ by sex even in young children [64], [65]. Sex specific effects of Hp phenotype and markers of iron status have been documented in some [66], but not other studies [67] for reasons that are not clear. Data on iron status was available for a small number of individuals (N<250), preventing us from carrying out meaningful analysis of possible interactions between Hp haplotype and iron status.

As we have previously reported in a sub-set of this population [46], we did not detect any C-C homozygotes for Hp -61A-C. To confirm that the absence of -61C-C homozygotes was not due to technical issues the accuracy of the genotyping method for the two Hp promoter SNPs was confirmed by sequencing 56 samples, selected to represent each haplotype. Indeed we have replicated these results again employing another hME Sequenom assay employing different primers taking into account two other promoter polymorphisms in the same vicinity (rs5469 3bp from -101C-G and rs5472 6pb from -61A-C; unpublished data, Rockett & Cox). The distribution of the Hp haplotypes reported by ourselves previously was confirmed in this larger sample [46], whilst the associations between the promoter SNPs with the Hp1 & Hp2 alleles has also been previously reported in West Africans from Ghana [26]. Similarly, publicly available data through the National Center for Biotechnology Information (NCBI indicates that -61C-C homozygotes are not present in populations of Caucasian, Pacific RIM, Hispanic, African American and African ancestry, with exception of the Yoruba panel where 1 out of 24 individuals was found to be homozygous for the C allele. This supports our earlier hypothesis that this polymorphism is under some selection pressure (similar to HbS), possibly from a deleterious effect of carriage of two copies of the C allele, resulting in the absence of -61C-C homozygotes.

In summary, in a cohort of 836 children up to 6.5 years of age across 8 villages in West Kiang The Gambia, the prevalence of active trachoma was 21.4%. Our observation of an increased risk of active trachoma in children with the Hp haplotype (-101G, -61A, Hp1), in boys, with a non-significant decreased risk in girls and statistically significant interaction warrants further investigation of possible associations between Hp polymorphisms and inflammatory, immune and infectious outcomes by sex.

Supporting Information

Table S1.

Haptoglobin Hp1/Hp2 - PCR conditions and primer sequences - method adapted from Koch et al. 2003.

(0.05 MB DOC)


We would like to thank the children who took part in this study and their parents, as well as the fieldworkers for their invaluable assistance in the field. We would also like to thank Hassan Joof for assisting with data collection in the field & Mamodou Bah for assistance with the DNA isolations and genotyping.

Guarantor: Sharon E Cox.

Author Contributions

Conceived and designed the experiments: CPD RLB AP SEC. Performed the experiments: MS BJH MJH GS KR SEC. Analyzed the data: MS AJCF. Contributed reagents/materials/analysis tools: GS KR DK. Wrote the paper: MS BJH CPD RLB SEC.


  1. 1. Resnikoff S, Pascolini D, Etya'ale D, Kocur I, Pararajasegaram R, et al. (2004) Global data on visual impairment in the year 2002. Bull World Health Organ 82: 844–851.
  2. 2. Mabey DC, Solomon AW, Foster A (2003) Trachoma. Lancet 362: 223–229.
  3. 3. Abdou A, Nassirou B, Kadri B, Moussa F, Munoz BE, et al. (2007) Prevalence and risk factors for trachoma and ocular Chlamydia trachomatis infection in Niger. Br J Ophthalmol 91: 13–17.
  4. 4. Courtright P, West SK (2004) Contribution of sex-linked biology and gender roles to disparities with trachoma. Emerg Infect Dis 10: 2012–2016.
  5. 5. Stephens RS (2003) The cellular paradigm of chlamydial pathogenesis. Trends Microbiol 11: 44–51.
  6. 6. Ward M, Bailey R, Lesley A, Kajbaf M, Robertson J, et al. (1990) Persisting inapparent chlamydial infection in a trachoma endemic community in The Gambia. Scand J Infect Dis Suppl 69: 137–148.
  7. 7. West SK, Munoz B, Mkocha H, Hsieh YH, Lynch MC (2001) Progression of active trachoma to scarring in a cohort of Tanzanian children. Ophthalmic Epidemiol 8: 137–144.
  8. 8. International Trachoma Initiative (2006) Annual report 2006. New York, USA: International Trachoma Initiative. 24 p.
  9. 9. Clementi M, Di Gianantonio E (2006) Genetic susceptibility to infectious diseases. Reprod Toxicol 21: 345–349.
  10. 10. Hill AV (2006) Aspects of genetic susceptibility to human infectious diseases. Annu Rev Genet 40: 469–486.
  11. 11. Gambhir M, Basanez MG, Turner F, Kumaresan J, Grassly NC (2007) Trachoma: transmission, infection, and control. Lancet Infect Dis 7: 420–427.
  12. 12. Natividad A, Hull J, Luoni G, Holland M, Rockett K, et al. (2009) Innate immunity in ocular Chlamydia trachomatis infection: contribution of IL8 and CSF2 gene variants to risk of trachomatous scarring in Gambians. BMC Med Genet 10: 138.
  13. 13. Atik B, Skwor TA, Kandel RP, Sharma B, Adhikari HK, et al. (2008) Identification of novel single nucleotide polymorphisms in inflammatory genes as risk factors associated with trachomatous trichiasis. PLoS One 3: e3600.
  14. 14. Conway DJ, Holland MJ, Bailey RL, Campbell AE, Mahdi OS, et al. (1997) Scarring trachoma is associated with polymorphism in the tumor necrosis factor alpha (TNF-alpha) gene promoter and with elevated TNF-alpha levels in tear fluid. Infect Immun 65: 1003–1006.
  15. 15. Natividad A, Hanchard N, Holland MJ, Mahdi OS, Diakite M, et al. (2007) Genetic variation at the TNF locus and the risk of severe sequelae of ocular Chlamydia trachomatis infection in Gambians. Genes Immun 8: 288–295.
  16. 16. Mozzato-Chamay N, Mahdi OS, Jallow O, Mabey DC, Bailey RL, et al. (2000) Polymorphisms in candidate genes and risk of scarring trachoma in a Chlamydia trachomatis-endemic population. J Infect Dis 182: 1545–1548.
  17. 17. Natividad A, Wilson J, Koch O, Holland MJ, Rockett K, et al. (2005) Risk of trachomatous scarring and trichiasis in Gambians varies with SNP haplotypes at the interferon-gamma and interleukin-10 loci. Genes Immun 6: 332–340.
  18. 18. Conway DJ, Holland MJ, Campbell AE, Bailey RL, Krausa P, et al. (1996) HLA class I and II polymorphisms and trachomatous scarring in a Chlamydia trachomatis-endemic population. J Infect Dis 174: 643–646.
  19. 19. Natividad A, Holland MJ, Rockett KA, Forton J, Faal N, et al. (2008) Susceptibility to sequelae of human ocular chlamydial infection associated with allelic variation in IL10 cis-regulation. Hum Mol Genet 17: 323–329.
  20. 20. Al-Younes HM, Rudel T, Brinkmann V, Szczepek AJ, Meyer TF (2001) Low iron availability modulates the course of Chlamydia pneumoniae infection. Cell Microbiol 3: 427–437.
  21. 21. Freidank HM, Billing H, Wiedmann-Al-Ahmad M (2001) Influence of iron restriction on Chlamydia pneumoniae and C. trachomatis. J Med Microbiol 50: 223–227.
  22. 22. Raulston JE (1997) Response of Chlamydia trachomatis serovar E to iron restriction in vitro and evidence for iron-regulated chlamydial proteins. Infect Immun 65: 4539–4547.
  23. 23. LaRue RW, Dill BD, Giles DK, Whittimore JD, Raulston JE (2007) Chlamydial Hsp60-2 is iron responsive in Chlamydia trachomatis serovar E-infected human endometrial epithelial cells in vitro. Infect Immun 75: 2374–2380.
  24. 24. Slepenkin A, Enquist PA, Hagglund U, de la Maza LM, Elofsson M, et al. (2007) Reversal of the antichlamydial activity of putative type III secretion inhibitors by iron. Infect Immun 75: 3478–3489.
  25. 25. van Ooij C, Apodaca G, Engel J (1997) Characterization of the Chlamydia trachomatis vacuole and its interaction with the host endocytic pathway in HeLa cells. Infect Immun 65: 758–766.
  26. 26. Teye K, Quaye IK, Koda Y, Soejima M, Tsuneoka M, et al. (2003) A-61C and C-101G Hp gene promoter polymorphisms are, respectively, associated with ahaptoglobinaemia and hypohaptoglobinaemia in Ghana. Clin Genet 64: 439–443.
  27. 27. Grant DJ, Maeda N (1993) A base substitution in the promoter associated with the human haptoglobin 2-1 modified phenotype decreases transcriptional activity and responsiveness to interleukin-6 in human hepatoma cells. Am J Hum Genet 52: 974–980.
  28. 28. Kristiansen M, Graversen JH, Jacobsen C, Sonne O, Hoffman HJ, et al. (2001) Identification of the haemoglobin scavenger receptor. Nature 409: 198–201.
  29. 29. Moestrup SK, Moller HJ (2004) CD163: a regulated hemoglobin scavenger receptor with a role in the anti-inflammatory response. Ann Med 36: 347–354.
  30. 30. Okazaki T, Nagai T (1997) Difference in hemoglobin-binding ability of polymers among haptoglobin phenotypes. Clin Chem 43: 2012–2013.
  31. 31. Delanghe JR, Langlois MR, De Buyzere ML, Torck MA (2007) Vitamin C deficiency and scurvy are not only a dietary problem but are codetermined by the haptoglobin polymorphism. Clin Chem 53: 1397–1400.
  32. 32. Melamed-Frank M, Lache O, Enav BI, Szafranek T, Levy NS, et al. (2001) Structure-function analysis of the antioxidant properties of haptoglobin. Blood 98: 3693–3698.
  33. 33. Atkinson SH, Rockett K, Sirugo G, Bejon PA, Fulford A, et al. (2006) Seasonal childhood anaemia in West Africa is associated with the haptoglobin 2-2 genotype. PLoS Med 3: e172.
  34. 34. Cox SE, Doherty CP, Atkinson SH, Nweneka CV, Fulford A, et al. (2008) Haptoglobin Genotype, Anaemia and Malaria in Gambian Children. Trop Med Int Health.
  35. 35. Guetta J, Strauss M, Levy NS, Fahoum L, Levy AP (2006) Haptoglobin genotype modulates the balance of Th1/Th2 cytokines produced by macrophages exposed to free hemoglobin. Atherosclerosis.
  36. 36. Asleh R, Marsh S, Shilkrut M, Binah O, Guetta J, et al. (2003) Genetically determined heterogeneity in hemoglobin scavenging and susceptibility to diabetic cardiovascular disease. Circ Res 92: 1193–1200.
  37. 37. Mustafa S, Vukovich T, Prikoszovich T, Winzer C, Schneider B, et al. (2004) Haptoglobin phenotype and gestational diabetes. Diabetes Care 27: 2103–2107.
  38. 38. Quaye IK, Ababio G, Amoah AG (2006) Haptoglobin 2-2 phenotype is a risk factor for type 2 diabetes in Ghana. J Atheroscler Thromb 13: 90–94.
  39. 39. Delanghe JR, Langlois MR, Boelaert JR, Van Acker J, Van Wanzeele F, et al. (1998) Haptoglobin polymorphism, iron metabolism and mortality in HIV infection. Aids 12: 1027–1032.
  40. 40. Friis H, Gomo E, Nyazema N, Ndhlovu P, Krarup H, et al. (2003) Iron, haptoglobin phenotype, and HIV-1 viral load: a cross-sectional study among pregnant Zimbabwean women. J Acquir Immune Defic Syndr 33: 74–81.
  41. 41. Quaye IK, Ekuban FA, Brandful JA, Gyan BA, Akanmori BD, et al. (2000) Haptoglobin phenotypes in HIV-1-seropositive patients in Ghana: decreased risk for Hp0 individuals. Hum Hered 50: 382–383.
  42. 42. Elagib AA, Kider AO, Akerstrom B, Elbashir MI (1998) Association of the haptoglobin phenotype (1-1) with falciparum malaria in Sudan. Trans R Soc Trop Med Hyg 92: 309–311.
  43. 43. Minang JT, Gyan BA, Anchang JK, Troye-Blomberg M, Perlmann H, et al. (2004) Haptoglobin phenotypes and malaria infection in pregnant women at delivery in western Cameroon. Acta Trop 90: 107–114.
  44. 44. Singh IP, Walter H, Bhasin MK, Bhardwaj V, Sudhakar K (1986) Genetic markers and malaria. Observations in Gujarat, India. Hum Hered 36: 31–36.
  45. 45. Atkinson SH, Mwangi TW, Uyoga SM, Ogada E, Macharia AW, et al. (2007) The haptoglobin 2-2 genotype is associated with a reduced incidence of Plasmodium falciparum malaria in children on the coast of Kenya. Clin Infect Dis 44: 802–809.
  46. 46. Cox SE, Doherty C, Atkinson SH, Nweneka CV, Fulford AJ, et al. (2007) Haplotype association between haptoglobin (Hp2) and Hp promoter SNP (A-61C) may explain previous controversy of haptoglobin and malaria protection. PLoS ONE 2: e362.
  47. 47. Aidoo M, Terlouw DJ, Kolczak MS, McElroy PD, ter Kuile FO, et al. (2002) Protective effects of the sickle cell gene against malaria morbidity and mortality. Lancet 359: 1311–1312.
  48. 48. Cabrera G, Cot M, Migot-Nabias F, Kremsner PG, Deloron P, et al. (2005) The sickle cell trait is associated with enhanced immunoglobulin G antibody responses to Plasmodium falciparum variant surface antigens. J Infect Dis 191: 1631–1638.
  49. 49. Williams TN, Mwangi TW, Roberts DJ, Alexander ND, Weatherall DJ, et al. (2005) An immune basis for malaria protection by the sickle cell trait. PLoS Med 2: e128.
  50. 50. Williams TN, Mwangi TW, Wambua S, Alexander ND, Kortok M, et al. (2005) Sickle cell trait and the risk of Plasmodium falciparum malaria and other childhood diseases. J Infect Dis 192: 178–186.
  51. 51. Jallow M, Teo YY, Small KS, Rockett KA, Deloukas P, et al. (2009) Genome-wide and fine-resolution association analysis of malaria in West Africa. Nat Genet.
  52. 52. Allen SJ, O'Donnell A, Alexander ND, Alpers MP, Peto TE, et al. (1997) alpha+-Thalassemia protects children against disease caused by other infections as well as malaria. Proc Natl Acad Sci U S A 94: 14736–14741.
  53. 53. Thylefors B, Dawson CR, Jones BR, West SK, Taylor HR (1987) A simple system for the assessment of trachoma and its complications. Bull World Health Organ 65: 477–483.
  54. 54. Harding-Esch EM, Edwards T, Sillah A, Sarr I, Roberts CH, et al. (2009) Active trachoma and ocular Chlamydia trachomatis infection in two Gambian regions: on course for elimination by 2020? PLoS Negl Trop Dis 3: e573.
  55. 55. Miller SA, Dykes DD, Polesky HF (1988) A simple salting out procedure for extracting DNA from human nucleated cells. Nucleic Acids Res 16: 1215.
  56. 56. Koch W, Latz W, Eichinger M, Roguin A, Levy AP, et al. (2002) Genotyping of the common haptoglobin Hp 1/2 polymorphism based on PCR. Clin Chem 48: 1377–1382.
  57. 57. Koch W, Latz W, Eichinger M, Gschwendner C, Teige B, et al. (2003) Haptoglobin gene subtyping by restriction enzyme analysis. Clin Chem 49: 1937–1940.
  58. 58. Clayton D SNPHAP: a program for estimating frequencies of large haplotypes of SNPs (Version 1.0). 1.0 ed. Cambridge: Institute for Medical Research.
  59. 59. Sirugo G, Schim van der Loeff M, Sam O, Nyan O, Pinder M, et al. (2004) A national DNA bank in The Gambia, West Africa, and genomic research in developing countries. Nat Genet 36: 785–786.
  60. 60. Wieringa FT, Berger J, Dijkhuizen MA, Hidayat A, Ninh NX, et al. (2007) Sex differences in prevalence of anaemia and iron deficiency in infancy in a large multi-country trial in South-East Asia. Br J Nutr 98: 1070–1076.
  61. 61. Arredouani M, Matthijs P, Van Hoeyveld E, Kasran A, Baumann H, et al. (2003) Haptoglobin directly affects T cells and suppresses T helper cell type 2 cytokine release. Immunology 108: 144–151.
  62. 62. Quaye IK (2008) Haptoglobin, inflammation and disease. Trans R Soc Trop Med Hyg 102: 735–742.
  63. 63. Langlois M, Delanghe J, Philippe J, Ouyang J, Bernard D, et al. (1997) Distribution of lymphocyte subsets in bone marrow and peripheral blood is associated with haptoglobin type. Binding of haptoglobin to the B-cell lectin CD22. Eur J Clin Chem Clin Biochem 35: 199–205.
  64. 64. Fish EN (2008) The X-files in immunity: sex-based differences predispose immune responses. Nat Rev Immunol 8: 737–744.
  65. 65. Klein SL (2004) Hormonal and immunological mechanisms mediating sex differences in parasite infection. Parasite Immunol 26: 247–264.
  66. 66. Langlois MR, Martin ME, Boelaert JR, Beaumont C, Taes YE, et al. (2000) The haptoglobin 2-2 phenotype affects serum markers of iron status in healthy males. Clin Chem 46: 1619–1625.
  67. 67. Kasvosve I, Gordeuk VR, Delanghe JR, Gomo ZA, Gangaidzo IT, et al. (2002) Iron status in black persons is not influenced by haptoglobin polymorphism. Clin Chem Lab Med 40: 810–813.