Complement receptor 1
Complement receptor 1 (CR1), also known as CD35, is a large, multifunctional type I transmembrane glycoprotein and a key member of the regulators of complement activation (RCA) family that plays a central role in innate immunity by regulating the complement system.[1] Expressed primarily on erythrocytes, monocytes, neutrophils, B lymphocytes, and glomerular podocytes, CR1 binds complement opsonins such as C3b and C4b to facilitate the clearance of immune complexes and pathogens from circulation while preventing excessive complement-mediated damage to host tissues.[2] It also exists in a soluble form (sCR1) in plasma, derived from proteolytic shedding, which contributes to systemic complement control.[3] Structurally, CR1 is encoded by the CD35 gene on chromosome 1q32 and features an extracellular domain comprising approximately 30 short consensus repeats (SCRs)—each about 60 amino acids long—organized into four long homologous repeats (LHR-A to LHR-D), a single transmembrane helix, and a short cytoplasmic tail of 43 residues.[1] The most prevalent allele, CR1*1 (or CR1-A), contains four LHRs with a molecular weight of around 190–250 kDa due to glycosylation, while polymorphisms result in variants like CR1-B (220 kDa) or rarer forms with additional LHRs that influence binding affinity and expression levels.[3] LHR-A (SCR1–7) primarily binds C4b, LHR-B and LHR-C (SCR8–21) target C3b with high affinity for dimeric ligands (1–40 nM), and LHR-D interacts with C1q and mannose-binding lectin (MBL).[2] This modular architecture enables CR1's flexible, extended conformation for bivalent ligand engagement and synergistic inhibition.[2] In terms of function, CR1 inhibits all three complement activation pathways (classical, lectin, and alternative) through decay-accelerating activity (DAA), which dissociates C2a or factor Bb from C3/C5 convertases, and cofactor activity (CFA), which promotes factor I-mediated proteolysis of C3b and C4b into inactive forms.[2] On erythrocytes, it mediates immune adherence by transporting opsonized particles to the liver and spleen for phagocytosis, while on leukocytes, it enhances phagocytic uptake and modulates adaptive immunity by regulating B-cell activation and T-cell responses.[3] CR1 also binds additional ligands like L-ficolin and contributes to anti-inflammatory effects by limiting anaphylatoxin release.[2] Dysregulation of CR1 is associated with several diseases, including systemic lupus erythematosus (SLE), where low erythrocyte CR1 levels correlate with impaired immune complex clearance and disease flares; falciparum malaria, via rosetting of infected red blood cells to uninfected ones; and glomerulonephritides, due to podocyte expression defects.[1] Genetic polymorphisms in CD35, such as the low-expression Knops blood group antigens, further link CR1 to susceptibility in autoimmune, infectious, and neurodegenerative conditions like Alzheimer's disease.[1] Therapeutically, recombinant sCR1 (e.g., TP10) has demonstrated efficacy in phase I/II trials for reducing ischemia-reperfusion injury and inflammation in conditions like myocardial infarction and rheumatoid arthritis, with emerging strategies including CR1-fusion proteins and gene therapies with mini-CR1 for targeted complement inhibition as of 2025.[1][4]Genetics
Gene Location
The CR1 gene, also designated CD35, is located on the long (q) arm of human chromosome 1 at cytogenetic band q32.2.[5] This locus spans approximately 146 kb of genomic DNA, extending from nucleotide position 207,496,147 to 207,641,776 on the GRCh38.p14 primary assembly.[6] The gene structure encompasses over 40 exons; the most prevalent F allotype includes 39 exons, while the S allotype incorporates up to 47 exons owing to insertions of additional repeat-encoding exons.[7] As a member of the regulators of complement activation (RCA) gene cluster, CR1 resides in close genomic proximity to fellow RCA family members, including the CR2 gene (encoding complement receptor 2) approximately 150 kb upstream and C4-binding protein genes (C4BPA and C4BPB), with the factor H gene (CFH) positioned nearby at 1q31.3.[8][9][10] The CR1 gene demonstrates evolutionary conservation across mammalian species, reflected in orthologs such as murine Cr1/Cr2, yet features human-specific expansions within its long homologous repeat (LHR) regions that underlie allotypic variations in repeat copy number.[11] Note that the reference genome (GRCh38.p14) represents an allele lacking eight alternate exons compared to the S isoform, and the common F allotype is not directly represented.[5]Gene Structure and Isoforms
The CR1 gene, located on chromosome 1q32.2, spans approximately 146 kb and comprises at least 42 exons that encode the protein's structural elements.[12][6] Each short consensus repeat (SCR) module in the extracellular domain is predominantly encoded by a single exon, though certain SCRs—particularly those in the ligand-binding regions—are split across two exons to facilitate specialized functions.[12] The gene's organization features long homologous repeats (LHRs), with the number varying by isoform: four in the common F isoform (designated A, B, C, and D from 5' to 3'), and five in the S isoform (A, B/A (variable), B, C, and D); each LHR typically encompasses seven SCRs, and variations in LHR-C and LHR-D repeat units arise from genomic duplications and deletions.[12][13] Structural polymorphisms and allele-specific alternative splicing generate principal isoforms that differ in LHR composition and total SCR count, ranging from 23 to 44 modules overall.[13] The most prevalent F isoform (also denoted CR11) includes four LHRs with 30 SCRs and a molecular weight of 190–220 kDa, whereas the S isoform (CR12) incorporates an additional LHR for 40 SCRs and ~250 kDa; rarer forms include one with ~24 SCRs and another with 44 SCRs at ~280 kDa.[13] These isoforms exhibit tissue-specific preferences, with the F variant predominating on erythrocytes to support immune adherence.[14] Expression of CR1 isoforms is modulated by regulatory elements within the regulator of complement activation (RCA) gene cluster, including a TATA-less promoter and enhancers that respond to inflammatory signals, influencing splice variant selection and quantitative output.[5] Polymorphisms in the HindIII restriction fragments between LHR-C and LHR-D further contribute to isoform abundance by altering splicing efficiency.[12]Protein Structure
Overall Architecture
Complement receptor 1 (CR1), also known as CD35, is a single-pass type I transmembrane glycoprotein featuring a large extracellular domain at the N-terminus, a short transmembrane helix, and a brief cytoplasmic tail at the C-terminus.[15] The extracellular region dominates the protein's architecture, extending approximately 55 nm in length as determined by small-angle X-ray scattering, while the transmembrane domain spans about 25 amino acids to anchor the protein in the lipid bilayer.[15] The cytoplasmic tail consists of roughly 43 amino acids, lacking prominent signaling motifs.[16] The total amino acid length of CR1 varies across isoforms due to differences in the number of repeated domains, generally ranging from 1900 to 2700 residues, with the predominant isoform (CR1*1) comprising 2039 amino acids. This variation arises from allelic polymorphisms and alternative splicing, resulting in proteins with 23 to 44 complement control protein (CCP) modules in the extracellular domain.[15] As a glycoprotein, CR1 contains multiple N-linked glycosylation sites—up to 21 in the common isoform—that significantly contribute to its post-translational modification and stability.[17] The calculated molecular mass without modifications is approximately 223 kDa, but observed masses range from 190 to 280 kDa depending on the isoform, glycosylation extent, and whether the sample is reduced or unreduced during analysis, reflecting the addition of 20–60 kDa from carbohydrate moieties.[16] On cell surfaces, CR1 exhibits potential for oligomerization, forming dimers or higher-order clusters that are enhanced by ligand binding, thereby increasing avidity for multivalent targets.[16] This dynamic assembly influences the receptor's presentation and interaction efficiency without altering its core monomeric architecture.Functional Domains
The extracellular region of complement receptor 1 (CR1) is composed of 30 short consensus repeats (SCRs) arranged into four long homologous repeats (LHRs: LHR-A, LHR-B, LHR-C, and LHR-D), with each LHR containing seven SCRs for a total of 28 SCRs in these modules, plus two additional C-terminal SCRs.[18][19] This modular organization contributes to the protein's flexibility and ligand interaction capabilities, with variations in total length arising from genetic polymorphisms that can add or remove LHR units in different isoforms.[20] Each SCR domain folds into a compact β-sandwich structure consisting of two antiparallel β-sheets packed together, typically spanning 60–70 amino acids and stabilized by two intramolecular disulfide bonds formed between four conserved cysteine residues.[21][22] These structural features are characteristic of the regulators of complement activation (RCA) family and enable the SCRs to serve as independent folding units within the elongated CR1 ectodomain.[23] LHR-A serves as the leader region primarily associated with C4b binding, while LHR-B and LHR-C are linked to C3b binding, and LHR-D exhibits variability in its structural and functional contributions across isoforms.[2] The membrane-proximal region includes a single transmembrane helix of approximately 25 hydrophobic amino acids, followed by a 43-amino-acid cytoplasmic domain that lacks prominent signaling motifs but facilitates endocytosis.[24][25]Expression and Distribution
Cellular Expression
Complement receptor 1 (CR1, CD35) is primarily expressed on the surface of various immune cells and erythrocytes, where it functions as a membrane-bound glycoprotein. It is highly expressed on erythrocytes, with approximately 500–1,000 copies per cell in most individuals, though this number can vary up to 10-fold due to genetic polymorphisms. CR1 is also prominently expressed on B cells, monocytes, neutrophils, dendritic cells, and subsets of T cells, including CD4+ and CD8+ lymphocytes. These expression patterns position CR1 as a key player in immune complex handling and complement regulation on circulating and tissue-resident immune cells.[26][1][27] Expression levels differ across cell types, with neutrophils displaying notably higher densities, around 57,000 molecules per cell, compared to the lower counts on erythrocytes. Monocytes and macrophages of the myeloid lineage exhibit moderate to high surface expression, though generally lower than on neutrophils but sufficient for phagocytic functions. Glomerular podocytes show lower levels of CR1 expression, primarily localized to their membrane, contributing to local complement control in the kidney. Eosinophils and Langerhans cells also bear CR1, albeit at variable densities depending on activation state.[27][1][27] The predominant form of CR1 is membrane-bound, anchored via a transmembrane domain, but soluble variants (sCR1) are generated primarily through proteolytic shedding from leukocyte surfaces, with minor contributions from alternative splicing in certain contexts. Soluble CR1 circulates in plasma and can be detected in urine associated with podocyte-derived vesicles. Expression is developmentally regulated; for instance, CR1 levels increase during monocyte differentiation into macrophages, enhancing their capacity for complement-mediated phagocytosis in tissues like the spleen. Isoform preferences may vary slightly by cell type, such as longer forms on erythrocytes.[28][1]Tissue Distribution and Regulation
Complement receptor 1 (CR1) is expressed in various non-hematopoietic tissues, contributing to local complement regulation. In the brain, CR1 protein is present on microglia and astrocytes but absent from neurons, as demonstrated by immunofluorescence and western blot analyses of human brain tissue and iPSC-derived cells. Expression levels are low in healthy brain regions but can increase under pathological conditions; notably, CR1 is also detected on choroid plexus epithelium, where it helps modulate complement activity at the blood-cerebrospinal fluid interface.[29][30] In the kidney, CR1 mRNA and protein are localized to podocytes, the specialized epithelial cells of the glomerulus, supporting complement inhibition on the filtration barrier. This expression is evident from early glomerular development in fetal kidneys and persists in adults, aiding in the protection against complement-mediated damage. In the liver, CR1 is primarily associated with resident macrophages (Kupffer cells), though its presence on non-hematopoietic cells like hepatocytes or endothelial cells is minimal or undetectable in normal conditions. Placental expression of CR1 occurs on trophoblast cells and other epithelial layers, facilitating immune tolerance at the maternal-fetal interface by regulating complement deposition.[31][32][33] A soluble form of CR1 (sCR1) circulates in plasma at low concentrations, approximately 50 ng/mL, and is generated through proteolytic cleavage of the membrane-bound receptor, primarily from leukocytes such as polymorphonuclear cells. This shedding process involves enzymatic release near the transmembrane domain, producing a functional extracellular fragment that retains complement regulatory activity in the bloodstream.[34][35] Transcriptional regulation of CR1 expression is influenced by cytokines, with interferon-gamma (IFN-γ) acting as a potent upregulator; exposure to IFN-γ increases CR1 mRNA levels in neutrophils from both healthy individuals and patients, in a dose- and time-dependent manner. Epigenetic mechanisms, including promoter methylation, also modulate CR1 gene expression, with hypermethylation associated with reduced transcription in certain contexts, though baseline patterns in healthy tissues maintain steady-state levels.[36][37] Post-translational modifications, particularly sialylation of N-linked glycans on CR1, significantly impact protein stability and shedding rates. Higher sialylation levels enhance circulatory half-life by reducing clearance via the asialoglycoprotein receptor in the liver, while desialylation promotes proteolytic release and faster degradation; this dynamic influences both membrane-bound and soluble CR1 pools.[38]Biological Functions
Complement Regulation
Complement receptor 1 (CR1) plays a pivotal role in regulating the complement system by binding to the opsonins C3b and C4b, thereby preventing excessive activation and promoting their inactivation. This binding facilitates CR1's function as a cofactor for the serine protease Factor I, which cleaves C3b into iC3b and C3dg, and C4b into C4c and C4d, limiting the amplification of the complement cascade.[2][28] These activities ensure controlled complement deposition on immune complexes and pathogens, reducing potential tissue damage from uncontrolled inflammation. In addition to its cofactor activity, CR1 exhibits decay-accelerating activity (DAA) by promoting the dissociation of key convertase enzymes, such as the alternative pathway's C3bBb and the classical/lectin pathways' C4b2a, as well as C5 convertases. This DAA disrupts the enzymatic complexes responsible for C3 and C5 cleavage, thereby halting the progression to membrane attack complex formation. CR1's dual regulatory mechanisms—cofactor and decay-accelerating—act synergistically to inhibit complement amplification across all three activation pathways (classical, alternative, and lectin) at the C3/C5 convertase stage.[39][40][2] The specificity of CR1's regulatory functions is mediated by distinct short consensus repeat (SCR) domains within its extracellular region. SCRs 8–11 (LHR-B) and 15–18 (LHR-C) primarily bind C3b with high affinity, facilitating regulation of the alternative pathway, while SCRs 1–4 (LHR-A) bind C4b, supporting regulation in the classical and lectin pathways. Sites in LHR-B and C also contribute to C4b binding with lower affinity. These domain-specific interactions, often involving multiple SCRs for enhanced affinity, underscore CR1's versatility as a central complement inhibitor expressed on various immune cells.[28][41][2]Immune Complex Clearance and Phagocytosis
Complement receptor 1 (CR1), expressed abundantly on erythrocytes, plays a pivotal role in the clearance of immune complexes by binding to C3b- and C4b-opsonized particles, thereby facilitating their transport from the circulation to fixed macrophages in the liver and spleen.[42] Erythrocytes, which account for approximately 95% of total peripheral blood CR1, capture these soluble immune complexes on their surface without triggering erythrocyte phagocytosis, allowing safe shuttling to hepatic Kupffer cells.[43] Upon reaching the liver sinusoids, the immune complexes are transferred to macrophages, where they are engulfed primarily through interactions with complement receptor 3 (CR3) and Fcγ receptors on the macrophage surface, leading to lysosomal degradation and preventing recirculation of potentially harmful complexes.[44] This mechanism is particularly efficient in primates, where erythrocyte CR1 density correlates with clearance capacity.[42] On professional phagocytes such as neutrophils, monocytes, and macrophages, CR1 directly contributes to the engulfment of complement-opsonized targets, including bacteria, apoptotic cells, and cellular debris. CR1 binds C3b-coated particles, promoting their attachment to the phagocyte membrane, which facilitates internalization into phagosomes for subsequent fusion with lysosomes and destruction.[44] This process is enhanced when CR1 acts as a bridging receptor, linking multiple opsonins on the target surface to cluster and activate downstream signaling for efficient particle uptake.[45] For instance, CR1-mediated binding is crucial for the phagocytosis of opsonized microorganisms and debris, ensuring rapid removal from tissues.[46] CR1 exhibits synergy with other receptors to amplify phagocytosis efficiency, particularly with CR3, which recognizes iC3b degradation fragments. While CR1 primarily handles initial attachment via C3b, CR3 drives ingestion of iC3b-opsonized targets, and their cooperative clustering on the phagocyte surface lowers the threshold for activation, enabling enhanced engulfment of complex particles like bacteria or apoptotic bodies.[45] This partnership, often requiring additional Fcγ receptor engagement for full internalization, ensures robust clearance without isolated receptor activity leading merely to transient binding.[47] Such interactions are vital for handling diverse opsonized threats in inflammatory environments.[48] By promoting the sequestration and degradation of immune complexes, CR1 limits their persistence in circulation, thereby mitigating excessive complement activation and subsequent inflammatory responses that could lead to tissue damage.[44] This clearance function is essential in preventing the deposition of complexes in vascular beds or organs, reducing the risk of chronic inflammation associated with conditions like systemic lupus erythematosus.[45]Rosetting
Rosettes form when erythrocytes adhere to immune complex-coated targets through the interaction between complement receptor 1 (CR1) on the erythrocyte surface and C3b (or C4b) opsonins deposited on the complexes during complement activation.[1] This immune adherence phenomenon clusters multiple erythrocytes around the opsonized particle, effectively binding it to the red blood cell membrane without internalization.[49] The binding affinity is enhanced for dimeric or multimeric C3b ligands compared to monomeric forms, promoting stable rosette assembly.[2] The rosetting assay exploits this CR1-C3b interaction to assess CR1 function and density on erythrocytes. In this method, indicator cells such as sheep erythrocytes sensitized with IgM antibody and human complement (forming EAC3b or EAC14b complexes) are mixed with test erythrocytes; the percentage of rosette-forming erythrocytes correlates directly with CR1 expression levels. This quantitative technique has been instrumental in evaluating CR1-mediated adherence in various physiological contexts.[50] Physiologically, CR1-mediated rosetting enables erythrocytes to act as carriers, shuttling opsonized immune complexes through the circulation to the spleen and liver for phagocytic clearance by macrophages.[51] This process prevents systemic accumulation of immune complexes and supports efficient complement-mediated immune surveillance.[52] Rosetting efficiency varies with the copy number of CR1 on individual erythrocytes, as higher receptor density facilitates more robust binding and rosette stability.Genetic Variations
Alleles and Polymorphisms
Complement receptor 1 (CR1) is characterized by several genetic variants, including alleles associated with the Knops blood group system, single nucleotide polymorphisms (SNPs), and copy number variations (CNVs) that influence its structure and expression.[53] The Knops blood group system antigens are encoded by polymorphisms in the CR1 gene, primarily within the long homologous repeat D (LHR-D) region. Common alleles include the F allele, which expresses the Sl^a (Sl(a+)) antigen, and the S allele, which lacks it (Sl(a-)), resulting from a nucleotide substitution that alters the amino acid sequence and affects protein conformation.[54] These alleles are part of a broader set of Knops variants, such as McC^b and Kp, defined by specific single nucleotide changes in the extracellular domains.[55] The Sl(a-) variant has a high frequency in African populations (up to 80%), while Sl(a+) predominates in Caucasians (nearly 100%). Similar disparities exist for other Knops polymorphisms, such as higher McC^b frequencies in African groups.[56][57] Notable SNPs in CR1 include rs6656401, an intronic variant that tags the CR1*2 structural isoform (long form with 37 SCRs), and rs4844609, which encodes a serine-to-threonine change (p.Ser1610Thr) in the LHR-D region. These SNPs are associated with variations in CR1 expression levels, with the minor alleles correlating to modestly elevated soluble CR1 (sCR1) in plasma.[58] The minor allele frequency for rs4844609 is approximately 0.02 in European populations.[59] CR1 also features a CNV arising from duplication of low-copy repeats in the genomic region encoding the LHR-C modules, which consist of short consensus repeats (SCRs). This variation produces four co-dominant alleles: CR11 (F allele) with two LHR-C copies (total of 30 SCRs), CR12 (S allele) with three LHR-C copies (37 SCRs), CR13 (F' allele) with one LHR-C copy (23 SCRs), and CR14 (D allele) with four LHR-C copies (44 SCRs). These isoforms differ in the number of complement-binding sites due to the variable SCR count in the C-terminal region.[60][61] Note that the structural CNV is distinct from Knops polymorphisms; the CR1*2 (S) allele has a frequency of approximately 0.02–0.11 in Europeans and up to 0.20 in some African populations, while higher frequencies (0.5–0.7) in Africans refer to Knops variants like Sl(a-). Additionally, the HindIII restriction fragment length polymorphism (e.g., low-expression H allele) independently affects CR1 density on erythrocytes, with the H allele more common in Africans (~0.4–0.6) and linked to reduced immune complex clearance.[62]Functional and Phenotypic Impacts
Genetic variations in the complement receptor 1 (CR1) gene lead to structural differences in the protein, particularly in the number of short consensus repeats (SCRs), which directly influence its binding affinity for complement components such as C3b and C4b. The F allele, characterized by 30 SCRs organized into four long homologous repeats (LHRs), exhibits efficient ligand binding but with fewer sites compared to the S allele (CR12), which includes an additional LHR and 37 SCRs, thereby increasing the overall capacity for multivalent ligand interactions and enhancing complement decay-accelerating activity through cofactor function for factor I-mediated cleavage.[63][64] These SCR variations modulate the rate of complement inactivation on immune complexes, with the S allele promoting faster decay rates due to additional binding domains that stabilize the cofactor complex.[19] The rarer short isoform (CR13, 23 SCRs) has reduced binding capacity. Certain single nucleotide polymorphisms (SNPs), such as rs6656401, are associated with reduced CR1 surface density on erythrocytes and leukocytes, leading to diminished efficiency in immune complex clearance. The minor A allele of rs6656401 tags the CR1*2 structural variant that results in the long isoform, but is linked to lower CR1 expression levels in some contexts, impairing the receptor's ability to bind and transport opsonized particles to phagocytic cells in the liver and spleen, which can prolong complement activation and increase inflammation.[65][60] This reduction in surface expression compromises the receptor's role in preventing excessive complement deposition, as evidenced by functional assays showing decreased uptake of C3b-coated targets in carriers of low-density variants.[58] Phenotypically, these genetic variations manifest in interindividual differences in rosetting and phagocytosis rates, critical for immune adherence and pathogen clearance. Individuals with low-CR1-expression polymorphisms, including those linked to rs6656401 and the HindIII H allele, display reduced rosette formation between uninfected erythrocytes and Plasmodium falciparum-infected cells, as the lower receptor density limits adhesion mediated by parasite proteins like PfEMP1.[56] Similarly, phagocytosis of complement-opsonized particles varies, with homozygous carriers of low-expression alleles showing up to 50% reduced uptake by neutrophils and monocytes compared to wild-type, highlighting CR1's dose-dependent role in innate immune responses.[5][66] From an evolutionary perspective, certain CR1 alleles conferring low receptor expression have been positively selected in malaria-endemic regions as adaptations for resistance to severe disease. For instance, the low-CR1 polymorphism prevalent in up to 80% of populations in Papua New Guinea reduces rosetting by parasitized erythrocytes, thereby decreasing microvascular obstruction and cerebral malaria risk by approximately one-third in heterozygous carriers.[56][67] This selective pressure is evident from genomic signatures of adaptation in African and Oceanian cohorts, where reduced CR1 levels balance infection susceptibility with protection against life-threatening complications.[68][69]Role in Blood Groups
Knops System Association
The Knops blood group system (KNS, ISBT number 022) comprises antigens expressed on the complement receptor 1 (CR1) glycoprotein, a key regulator of the complement system located on chromosome 1q32.2. These antigens arise from single nucleotide polymorphisms (SNPs) in the CR1 gene (CD35), primarily within short consensus repeats (SCRs) 25–29 of the long homologous repeat D (LHR-D) domain, which influence the protein's surface expression on erythrocytes and other cells.[70][55] The system currently includes 14 recognized antigens, though four principal antithetical pairs—Swain-Langley (Sl^a/Sl^b), McCoy (McC^a/McC^b), Kna/Knb, and others like York (Yk^a)—account for the majority of serological variation and are defined by specific amino acid substitutions in the CR1 extracellular domain. Recent additions include DACY (KN12), YCAD (KN13), KNMB (KN14), and KNEH (KN15) located in LHR-C and other regions.[53][71] The molecular basis of these antigens involves missense mutations that alter the CR1 protein sequence, typically without significantly impacting complement regulatory function but affecting antigenicity. For instance, the high-prevalence Sl^a (KN4) antigen is the wild-type form, while the low-prevalence Sl^b (KN7) results from an arginine-to-glycine substitution at position 1601 (R1601G, c.4801A>G); similarly, McC^b (KN6) arises from a lysine-to-glutamic acid change at position 1590 (K1590E, c.4768A>G), and Kn^b (KN5) from a valine-to-methionine substitution at position 1561 (V1561M, c.4681G>A).[72] These polymorphisms are often in linkage disequilibrium, forming haplotypes that vary by population; for example, the Sl^b and McC^b alleles are more frequent in individuals of African ancestry (up to 20–30% allele frequency) compared to Caucasians (<5%).[72] Additional antigens, such as Sl3 (KN8, S1610T) and KCAM (KN9, I1615V), stem from nearby substitutions in the same SCR region.[73]| Antigen Pair | ISBT Number | Nucleotide Change | Amino Acid Substitution | Prevalence Notes |
|---|---|---|---|---|
| Sl^a / Sl^b | KN4 / KN7 | c.4801A>G | R1601G | Sl^a high (97–99%); Sl^b low, higher in Africans |
| McC^a / McC^b | KN2 / KN6 | c.4768A>G | K1590E | McC^a high (98–99%); McC^b low, higher in Africans |
| Kn^a / Kn^b | KN1 / KN5 | c.4681G>A | V1561M | Kn^a high (>99%); Kn^b very low (<1%) |
| Yk^a / Yk- | KN10 / - | c.4223C>T | T1408M | Yk^a high (99%); null rare |