rs1801274 — FCGR2A H131R

FCGR2A H131R — The IgG2 Gateway in Autoimmune Disease

Every IgG antibody your immune system makes eventually gets processed through a receptor. Fc gamma receptor IIa (FcγRIIa), encoded by FCGR2A, is the principal receptor on neutrophils and macrophages for IgG2 — the subclass that handles responses to polysaccharide antigens, bacterial capsules, and immune complexes. The H131R polymorphism sits precisely at the IgG2-binding interface, and which amino acid you carry at position 131 determines whether your myeloid cells can efficiently clear antibody-coated targets or leave them to accumulate.

The H131 variant (encoded by the A allele) binds IgG2 and IgG3 with high affinity. The R131 variant (G allele) binds IgG1 and IgG3 normally but has almost no affinity for IgG2. This single amino acid change is functionally the difference between effective and negligible IgG2 phagocytosis¹¹ functionally the difference between effective and negligible IgG2 phagocytosis
R131 homozygotes can only clear IgG2-opsonized targets via other receptor pathways, which are substantially less efficient than direct FcγRIIa-mediated uptake. Because IgG2 immune complexes drive much of the end-organ damage in lupus nephritis — depositing in the glomerular basement membrane and activating complement — R131 homozygosity correlates with impaired kidney protection in SLE patients.

The Mechanism

The H131R substitution is in the second Ig-like domain²² Ig-like domain
Immunoglobulin-like domain — a protein fold shared across antibodies and many receptors; the second domain of FcγRIIa directly contacts the Fc region of bound IgG of FcγRIIa, at the precise contact point with the Fc region of IgG2. Histidine at position 131 creates a favorable electrostatic interaction with the IgG2 Fc region, enabling tight binding. Arginine at the same position disrupts this interaction. The result is dose-dependent: H/H131 homozygotes have the highest IgG2 clearance efficiency, H/R131 heterozygotes have intermediate capacity, and R/R131 homozygotes have minimal IgG2-mediated phagocytosis — confirmed in functional neutrophil assays.

The clinical consequence depends on which immune pathway is engaged. For SLE and lupus nephritis, efficient IgG2 complex clearance is protective: R131 carriers accumulate more immune complexes in tissues. For Kawasaki disease, the same high-affinity H131 receptor appears to amplify the aberrant immune activation that drives coronary arteritis — a striking example of how the same variant can be risk-increasing in one condition and protective in another.

The Evidence

A meta-analysis of 33 SLE studies³³ meta-analysis of 33 SLE studies
5,652 SLE patients and 6,322 controls across Asian, European, and North American cohorts established that the G allele (R131) significantly increases SLE risk overall (OR 1.238) and in Asian (OR 1.237) and European (OR 1.212) populations. Lupus nephritis susceptibility was also significant across all four genetic models tested. The association is most consistent in populations with higher IgG2 immune complex burden, where FcγRIIa clearance capacity is directly rate-limiting.

For Kawasaki disease, the biology inverts: the H131 variant (A allele) is the risk factor in Asian populations. A GWAS and meta-analysis⁴⁴ GWAS and meta-analysis
Including a Chinese Han case-control of 428 KD patients and 493 controls found the A allele associated with Kawasaki disease risk (OR 1.35 per allele; AA genotype OR 1.93 vs GG). This fits the mechanism: Kawasaki disease involves over-activation of myeloid IgG2 signaling, which H131 amplifies. The same receptor property that helps clear SLE immune complexes also drives the excessive inflammatory response in KD.

Pharmacogenomically, FcγRIIa genotype influences how well certain biological therapies work. In rheumatoid arthritis, the AA (H/H131) genotype predicted superior abatacept response⁵⁵ AA (H/H131) genotype predicted superior abatacept response
OR 6.62 for low disease activity at 12 months in a prospective cohort of 120 Caucasian RA patients, while GG carriers had much lower response rates. For adalimumab⁶⁶ adalimumab
An anti-TNF monoclonal antibody — its Fc region directly interacts with FcγRIIa on immune cells, so receptor affinity affects drug-mediated effector functions, the G allele was associated with non-response (P=0.022); infliximab showed the same pattern in anti-CCP-positive patients (P=0.035), while etanercept — which lacks an intact Fc region — showed no genotype association.

Practical Actions

The GG genotype (R131/R131) does not impair IgG1- or IgG3-mediated immunity, which handles the majority of viral and intracellular bacterial responses. The specific vulnerability is to pathogens and disease processes that rely on IgG2 — particularly encapsulated bacteria (pneumococcus, Haemophilus, meningococcus) and immune complex-driven tissue damage in autoimmune disease.

For GG carriers with diagnosed SLE, the reduced IgG2 clearance capacity is clinically relevant: lupus nephritis monitoring should be proactive. For those considering biologic therapy for RA, FcγRIIa genotype can inform drug selection — abatacept and adalimumab response differs substantially by genotype, providing a rationale for genotype-guided prescribing discussions with a rheumatologist.

Interactions

FCGR2A is typically studied alongside FCGR3A rs396991⁷⁷ FCGR3A rs396991
The V158F variant in Fc gamma receptor IIIa — another IgG receptor on NK cells and macrophages; V158 has higher affinity for IgG1 and IgG3, affecting antibody-dependent cellular cytotoxicity (FcγRIIIa F158V), which tags IgG1/IgG3 handling by NK cells. Compound low-affinity haplotypes (FCGR2A R131 + FCGR3A F158) show additive impairment in Fc-mediated effector function. In RA treated with Fc-containing biologics, the combined low-affinity haplotype predicts poor therapeutic response more strongly than either variant alone.

rs1050501 (FCGR2B I232T) is a related Fc gamma receptor variant on the inhibitory receptor that modulates B-cell activation thresholds; combined dysregulation of activating (FCGR2A) and inhibitory (FCGR2B) FcγR signaling is proposed as a driver of the autoimmune phenotype in lupus.