What you carry at this position in your DNA and what research says about it.
Asthma and Allergy Risk: 10/10 evidence, 35 publications.
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Sample genotype (not yours): C/C
Higher Asthma and Allergy Risk
Two copies of the C allele lead to impaired immune receptor trafficking, which increases susceptibility to asthma and environmental allergies. However, this same cellular deficit provides strong protection against leprosy.
What this genotype means: Increased susceptibility to childhood-onset asthma; Higher risk of allergic rhinitis and eczema; Strong protection against leprosy infection.
Sample genotype (not yours): A/A
Typical Asthma and Allergy Risk
Two copies of the A allele ensure normal immune receptor expression, protecting against asthma and allergies. However, this results in typical susceptibility to leprosy.
What this genotype means: Lower risk of asthma and environmental allergies; Typical baseline susceptibility to mycobacterial infections.
Sample genotype (not yours): C/A
Slightly Higher Asthma and Allergy Risk
One copy of the C allele results in intermediate levels of immune receptor expression, slightly raising the risk of asthma and allergies. It also offers moderate protection against leprosy.
What this genotype means: Slightly increased risk of asthma and hay fever; Moderate protection against leprosy.
Gene function and biological role
Gene Function: The TLR1 gene encodes toll-like receptor 1, a key cell-surface receptor of the innate immune system that detects bacterial lipopeptides [1]. By forming a functional heterodimer with toll-like receptor 2, it initiates inflammatory signaling cascades to defend against pathogens [2]. Genetic variations in this gene can alter receptor trafficking, surface expression, and downstream cytokine production [3]. These alterations directly influence how the body balances defense against infections with the risk of inflammatory and allergic diseases [4].
Variant Impact: This missense variant, known as p.Ser602Ile, alters the amino acid at position 602 of the TLR1 protein from serine to isoleucine. The serine-encoding C allele impairs the trafficking of the receptor to the cell membrane, resulting in drastically reduced cell-surface expression [1]. Conversely, the isoleucine-encoding A allele allows normal trafficking and robust cell-surface expression [2]. This molecular difference dictates whether the immune system reacts strongly or weakly to specific bacterial components, shaping susceptibility to both infectious and inflammatory conditions [3].
Key Insight: A genetic trade-off exists where a dysfunctional immune receptor protects against ancient infections like leprosy but increases the risk of modern allergic diseases like asthma.
Location: TLR1. Chromosome 4 · 4p14 · 38.8 Mb
Origin Story: The protective but dysfunctional serine allele is rare in Africa but became highly prevalent in European populations, suggesting strong historical natural selection by mycobacteria or other regional pathogens.
Estimated prevalence by population
Global 31.4%; European 52.7%; African 1.7%; East Asian < 0.1%; South Asian 0.9%; Americas 10.3%; Ashkenazi 21.4%; Finnish 75.5%; Middle Eastern 4.6%; Amish 43.9%.
Estimated CC genotype frequency.
Global 32.6%; European 39.8%; African 22.9%; East Asian 2.3%; South Asian 17.0%; Americas 43.6%; Ashkenazi 49.7%; Finnish 22.8%; Middle Eastern 33.7%; Amish 44.7%.
Estimated CA genotype frequency.
Global 36.0%; European 7.5%; African 75.3%; East Asian 97.7%; South Asian 82.1%; Americas 46.2%; Ashkenazi 28.9%; Finnish 1.7%; Middle Eastern 61.7%; Amish 11.4%.
Estimated AA genotype frequency.
Showing effects for C/C genotype
Organ Systems
Respiratory Health
Asthma. Evidence: 10/10. Source: PMID 30929738. ~1.25x Higher Risk.
Immunity & Inflammation
Allergy & Hypersensitivity
Allergy Susceptibility. Evidence: 8/10. Source: PMID 29083406. ~1.1x Higher Risk.
Allergic Rhinitis. Evidence: 10/10. Source: PMID 30013184. ~1.11x Higher Risk.
Infection Susceptibility & Resistance
Leprosy Susceptibility. Evidence: 4/10. Source: PMID 17548585. Strong Protection.
Showing effects for A/A genotype
Asthma. Evidence: 10/10. Source: PMID 30929738. Typical Risk.
Allergy Susceptibility. Evidence: 8/10. Source: PMID 29083406. Typical Risk.
Allergic Rhinitis. Evidence: 10/10. Source: PMID 30013184. Typical Risk.
Leprosy Susceptibility. Evidence: 4/10. Source: PMID 17548585. Typical Resistance.
Showing effects for C/A genotype
Asthma. Evidence: 10/10. Source: PMID 30929738. Slightly Higher Risk.
Allergy Susceptibility. Evidence: 8/10. Source: PMID 29083406. Slightly Higher Risk.
Allergic Rhinitis. Evidence: 10/10. Source: PMID 30013184. Slightly Higher Risk.
Leprosy Susceptibility. Evidence: 4/10. Source: PMID 17548585. Moderate Protection.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Impaired cell-surface expression of the TLR1 receptor is characteristic of this genotype, as the serine amino acid prevents efficient trafficking to the cell membrane [1]. This lack of functional surface receptors alters how the immune system interacts with environmental antigens, leading to a higher risk of developing childhood-onset asthma [2] and general allergic diseases [3]. Interestingly, this exact receptor deficiency prevents certain pathogens, such as Mycobacterium leprae, from exploiting the receptor to cause infection, thereby conferring strong resistance to leprosy [4]. In modern environments where leprosy is extremely rare, the primary health impact of this genotype is an elevated predisposition to respiratory and allergic inflammation [2] [3].
Increased susceptibility to childhood-onset asthma; Higher risk of allergic rhinitis and eczema; Strong protection against leprosy infection.
Sample genotype: AA (Homozygous)
Normal trafficking and high cell-surface expression of the TLR1 receptor are maintained by this genotype, allowing robust inflammatory signaling in response to bacterial lipopeptides [1]. This active immune surveillance is associated with a lower risk of developing childhood-onset asthma [2] and allergic diseases [3]. However, because the receptor is fully functional and present on the cell surface, pathogens like Mycobacterium leprae can readily engage it, resulting in typical susceptibility to leprosy and its associated inflammatory complications [4]. In modern, hygienic environments, this genotype is generally highly favorable due to the reduced burden of chronic allergic conditions [2] [3].
Lower risk of asthma and environmental allergies; Typical baseline susceptibility to mycobacterial infections.
Sample genotype: CA (Heterozygous)
Intermediate levels of TLR1 cell-surface expression occur with this heterozygous genotype, as only one copy of the trafficking-impaired serine allele is present [1]. This partial receptor deficiency leads to a mild increase in susceptibility to childhood-onset asthma [2] and allergic rhinitis [3]. The presence of one functional A allele maintains some level of inflammatory response, which reduces the absolute protection against mycobacterial infections compared to having two C alleles, though some moderate resistance to leprosy remains [4]. For most individuals, managing environmental triggers and monitoring respiratory health are the most practical considerations.
Slightly increased risk of asthma and hay fever; Moderate protection against leprosy.
Variants with shared genetic architecture
rs4833095 · TLR1 Gene · Chr 4 · Immune Response and Allergies
rs5743551 · TLR1 Gene · Chr 4 · Sepsis Complications Risk
rs20541 · IL13 Gene · Chr 5 · Asthma and Eczema Risk
Differential trafficking of TLR1 I602S underlies host protection against pathogenic mycobacteria.
Shared genetic origin of asthma, hay fever and eczema elucidates allergic disease biology.
Genetic Architectures of Childhood- and Adult-Onset Asthma Are Partly Distinct.
Content updated August 2026.
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