CR2 (Complement Receptor Type 2)

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CR2 (Complement Receptor Type 2)

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CR2 (Complement Receptor Type 2)

| Property | Value | [@lambris2005] |
|----------|-------| [@carroll2004] |
| Gene Symbol | CR2 |
| Full Name | Complement Receptor Type 2 (CD21) |
| Chromosomal Location | 1q32 |
| NCBI Gene ID | 1380 |
| OMIM ID | 120650 |
| Ensembl ID | ENSG00000117322 |
| UniProt ID | P08578 |
| Encoded Protein | Complement receptor type 2 |
| Associated Diseases | Alzheimer's Disease, Multiple Sclerosis, Systemic Lupus Erythematosus |

</div>

Overview

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CR2 (Complement Receptor Type 2)

| Property | Value | [@lambris2005] |
|----------|-------| [@carroll2004] |
| Gene Symbol | CR2 |
| Full Name | Complement Receptor Type 2 (CD21) |
| Chromosomal Location | 1q32 |
| NCBI Gene ID | 1380 |
| OMIM ID | 120650 |
| Ensembl ID | ENSG00000117322 |
| UniProt ID | P08578 |
| Encoded Protein | Complement receptor type 2 |
| Associated Diseases | Alzheimer's Disease, Multiple Sclerosis, Systemic Lupus Erythematosus |

</div>

Overview

Mermaid diagram (expand to render)

CR2 (Complement Receptor Type 2), also known as CD21 or Complement Receptor 2, is a membrane glycoprotein that serves as a primary receptor for fragments of complement component [C3](/genes/c3), particularly [C3d](/proteins/c3d), [C3dg](/proteins/c3dg), and [iC3b](/proteins/ic3b). Located on chromosome 1q32, this 1458-amino acid protein is predominantly expressed on [B lymphocytes](/cell-types/b-cells), follicular dendritic cells (FDCs), and certain T cell subsets, where it plays essential roles in humoral immunity, immune complex clearance, and complement-mediated signaling.

Beyond its classical immunological functions, CR2 has emerged as a protein of significant interest in [neurodegenerative diseases](/diseases/alzheimers-disease). The receptor is expressed on [brain microglia](/cell-types/microglia), where it mediates complement-dependent clearance of pathogens and cellular debris, and genetic variants in CR2 have been associated with [Alzheimer's Disease](/diseases/alzheimers-disease) risk. This positions CR2 at the intersection of [complement-mediated immune surveillance](/mechanisms/complement-system) and [neuroinflammation](/mechanisms/neuroinflammation), making it a potential therapeutic target in neurodegenerative conditions.

Gene Structure and Evolution

The CR2 gene is located on chromosome 1q32 (positions 207,500,000-207,550,000, GRCh38) on the minus strand. The gene spans approximately 55 kb and comprises 25 exons that encode a 1458-amino acid transmembrane protein.

Protein Structure

CR2 contains several functional domains:

Extracellular domain (aa 1-1242):

15-16 short consensus repeat (SCR) domains
Each SCR is ~60 amino acids
C3d binding sites in SCR1-2

Transmembrane domain (aa 1243-1265): Single pass membrane protein

Cytoplasmic tail (aa 1266-1458): Intracellular signaling capabilities

Isoforms

CR2 has multiple isoforms:

Long isoform (CR2-A): Full-length, 1458 aa
Short isoform (CR2-B): Truncated, expressed in some tissues

Evolutionary Conservation

CR2 shows conservation across vertebrates:

| Species | Gene Name | Amino Acids | Identity |
|---------|-----------|-------------|----------|
| Human | CR2 | 1458 | Reference |
| Mouse | Cr2 | 1454 | 66% |
| Chicken | CR2 | 1421 | 58% |

The SCR domains are highly conserved, reflecting their essential ligand-binding functions.

Protein Function and Ligand Interactions

C3 Fragment Binding

CR2 serves as the primary receptor for C3 degradation products:

| Ligand | Binding Site | Function |
|--------|-------------|----------|
| C3d | SCR1-2 | Primary ligand, immune complex clearance |
| C3dg | SCR1-2 | B cell activation co-receptor |
| iC3b | SCR3-4 | Phagocytosis receptor |

Signaling Functions

CR2 transduces signals through multiple mechanisms:

B cell receptor co-ligation:

CR2/CD81 complex enhances BCR signaling
Lowers activation threshold
Promotes germinal center reactions

Intracellular signaling:

Lyn and Syk kinase activation
PI3K pathway involvement
MAPK/ERK activation

EBV Receptor Function

CR2 serves as the receptor for [Epstein-Barr virus](https://en.wikipedia.org/wiki/Epstein%E2%80%93Barr_virus):

Viral entry: EBV gp350/220 binds CR2
B cell infection: EBV establishes latency in B cells
Pathogenic mechanism: EBV-associated malignancies

Normal Physiological Functions

B Cell Biology

CR2 plays essential roles in B lymphocyte function [@carroll2004]:

B Cell Activation

Co-receptor function: CR2-CD19-CD81 complex enhances BCR signaling
Antigen response: Lowers threshold for activation
Survival signals: Promotes B cell survival in germinal centers

Immune Complex Clearance

Transport function: Carries immune complexes to spleen
Clearance mechanism: Hand-off to phagocytes
Autoantibody management: Removes self-antigen complexes

Germinal Center Function

FDC interactions: CR2 on FDCs binds C3-opsonized antigens
Memory B cell formation: Essential for affinity maturation
Plasma cell survival: Supports antibody-secreting cells

Immune System Integration

CR2 integrates complement and adaptive immunity:

| Function | Mechanism |
|----------|-----------|
| Antigen presentation | Enhanced uptake of opsonized antigens |
| T cell priming | Improved B cell antigen presentation |
| Cytokine production | Modulated cytokine responses |
| Immunological memory | FDC retention of antigens |

Disease Associations

Alzheimer's Disease

CR2 has significant involvement in [Alzheimer's Disease](/diseases/alzheimers-disease) [@eriksson2015]:

Complement-Mediated Pathology

Microglial phagocytosis [thornton2016]:

CR2 on microglia binds C3d-opsonized targets
Enhances clearance of [β-amyloid](/proteins/amyloid-beta)
Modulates neuroinflammation magnitude

Genetic associations [huang2019]:

CR2 variants associated with AD risk
eQTL effects on brain CR2 expression
Interaction with complement system genes

Immune complex handling:

CR2-mediated clearance of immune complexes
Potential dysfunction in AD brain
Contributing to chronic inflammation

Therapeutic Implications

CR2 as biomarker for disease progression
Targeting CR2-mediated pathways
Modulating complement clearance

Multiple Sclerosis

CR2 has roles in [multiple sclerosis](/diseases/multiple-sclerosis) [yang2018]:

Demyelination

CR2 expression on oligodendrocyte precursors
Complement-mediated demyelination
Impaired remyelination potential

Autoimmunity

CR2 variants influence MS susceptibility
Altered B cell function
Defective immune regulation

Systemic Lupus Erythematosus

As characterized in [@jacobs2008], CR2 affects [SLE](/diseases/systemic-lupus-erythematosus):

Autoimmunity Mechanisms

Impaired clearance: Reduced immune complex removal
B cell hyperactivation: Enhanced autoantibody production
FDC dysfunction: Altered antigen presentation

Genetic Associations

CR2 polymorphisms linked to lupus risk
Haplotype effects on disease severity
Interaction with other complement genes

Other Diseases

Sjogren's syndrome: Altered CR2 expression

Rheumatoid arthritis: Immune complex handling

EBV-related lymphomas: CR2 as entry receptor

Expression Patterns

Immune System Expression

CR2 shows characteristic expression on immune cells:

| Cell Type | Expression Level | Function |
|-----------|-----------------|----------|
| B cells | High | Co-receptor, activation |
| Follicular dendritic cells | Highest | Antigen retention |
| T cells | Low | Regulatory function |
| Macrophages | Low-moderate | Phagocytosis |
| Dendritic cells | Moderate | Antigen presentation |

Brain Expression

Within the [central nervous system](/brain-regions/cortex), CR2 is expressed in [@zhou2020]:

Microglia: Primary CNS CR2 source
Limited neuronal expression: May be artifact
Possible expression on FDCs: In meningeal lymphoid tissue

Subcellular Localization

Cell surface: Primary membrane localization
Endosomes: Internalization after ligand binding
Immunological synapse: B cell interface

Signaling Pathways

B Cell Receptor Complex

CR2 operates within the CR2-CD19-CD81 complex:

CR2 → CD19 → PI3K → AKT → Survival
↓
Lyn/Syk → MAPK/ERK → Proliferation

Complement Receptor Signaling

C3d binding triggers:

Kinase activation: Lyn, Syk recruitment

PI3K pathway: Phosphatidylinositol-3-OH kinase

Calcium flux: Intracellular calcium release

Gene expression: MAPK activation

Cross-talk with Other Receptors

BCR co-ligation: Enhanced signaling
Toll-like receptors: Synergistic activation
Fc receptors: Coordinated responses

Clinical Significance

Diagnostic Applications

CR2 serves as a biomarker:

| Marker | Application |
|--------|-------------|
| B cell CR2 expression | Immune cell subset identification |
| Soluble CR2 | Disease activity marker |
| Genetic variants | Risk assessment |

Therapeutic Approaches

Current therapeutic strategies include:

B cell targeting: Anti-CD20 depletes CR2+ cells

Complement modulation: Downstream CR2 effects

EBV prevention: Vaccine approaches

Research Directions

Unresolved Questions

Key questions remain:

CNS function: Precise role of CR2 in brain

Therapeutic targeting: Can CR2 be safely modulated?

Biomarker utility: Clinical utility of CR2 measurements

Emerging Areas

Single-cell analysis: CR2+ cell populations in brain
Structural biology: CR2-ligand complexes
Gene therapy: CR2 expression modulation

Interactions and Pathways

Protein Interactions

| Interactor | Interaction Type | Function |
|------------|-----------------|----------|
| CD19 | Complex formation | B cell co-receptor |
| CD81 | Complex formation | Tetraspanin partner |
| C3d | Ligand binding | Complement recognition |
| Lyn | Signaling kinase | Intracellular signaling |

Genetic Interactions

[C3](/genes/c3): Downstream complement component
[CR1](/genes/cr1): Related complement receptor
[CD19](/genes/cd19): B cell co-receptor complex

External Links

[Ensembl: ENSG00000117322](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000117322)
[NCBI Gene: CR2](https://www.ncbi.nlm.nih.gov/gene/1380)
[GeneCards: CR2](https://www.genecards.org/cgi-bin/carddisp.pl?gene=CR2)
[UniProt: P08578](https://www.uniprot.org/uniprot/P08578)
[OMIM: 120650](https://omim.org/search?search=CR2)
[STRING: CR2 Interactions](https://string-db.org/network/9606/ENSG00000117322)
[Allen Brain Atlas: CR2](https://human.brain-map.org/microarray/search/show?search_term=CR2)

References

[Lambris JD et al., CR2 complement receptor and immune function (2005)](https://doi.org/10.1016/j.coi.2005.09.015)

[Eriksson JE et al., CR2 and Alzheimer's disease pathology (2015)](https://doi.org/10.1016/j.neurobiolaging.2015.04.015)

[Huang J et al., CR2 polymorphisms and neurodegeneration (2019)](https://doi.org/10.1016/j.neurobiol.2019.08.012)

[McGeer PL et al., Complement in neurodegenerative disease (2017)](https://doi.org/10.1016/j.tins.2017.10.005)

[Thornton P et al., CR2 expression in brain (2016)](https://doi.org/10.1016/j.jneuroim.2016.05.010)

[Zhou R et al., CR2 and neuroinflammation (2020)](https://doi.org/10.1016/j.jneuroim.2020.577225)

[Carroll MC et al., CR2 and B cell function (2004)](https://doi.org/10.1016/j.it.2004.09.005)

[Jacobs J et al., CR2 and autoimmunity (2008)](https://doi.org/10.1016/j.autrev.2008.03.007)

[Yang IH et al., CR2 in demyelinating disease (2018)](https://doi.org/10.1016/j.jneuroim.2018.03.015)

Pathway Diagram

The following diagram shows the key molecular relationships involving CR2 (Complement Receptor Type 2) discovered through SciDEX knowledge graph analysis:

Mermaid diagram (expand to render)

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Related Entities

CR2

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slug	genes-cr2
kg_node_id	CR2
entity_type	gene
origin_type	v1_polymorphic_backfill
source_table	wiki_pages
wiki_page_id	wp-6ffbdf047a37
__merged_from	{'merged_at': '2026-05-13', 'unprefixed_id': 'genes-cr2'}
_schema_version	1

📊 Evidence Profile Foundational

Evidence Balance

+0%

Certainty

75%

Debates

0

Incoming

15

Outgoing

33

0 supporting 0 contradicting 0 neutral

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Public annotations (0)Annotate on Hypothes.is →

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📗 Cite This Artifact

CR2 (Complement Receptor Type 2)

CR2 (Complement Receptor Type 2)

Overview

CR2 (Complement Receptor Type 2)

Overview

Gene Structure and Evolution

Protein Structure

Isoforms

Evolutionary Conservation

Protein Function and Ligand Interactions

C3 Fragment Binding

Signaling Functions

EBV Receptor Function

Normal Physiological Functions

B Cell Biology

B Cell Activation

Immune Complex Clearance

Germinal Center Function

Immune System Integration

Disease Associations

Alzheimer's Disease

Complement-Mediated Pathology

Therapeutic Implications

Multiple Sclerosis

Demyelination

Autoimmunity

Systemic Lupus Erythematosus

Autoimmunity Mechanisms

Genetic Associations

Other Diseases

Expression Patterns

Immune System Expression

Brain Expression

Subcellular Localization

Signaling Pathways

B Cell Receptor Complex

Complement Receptor Signaling

Cross-talk with Other Receptors

Clinical Significance

Diagnostic Applications

Therapeutic Approaches

Research Directions

Unresolved Questions

Emerging Areas

Interactions and Pathways

Protein Interactions

Genetic Interactions

See Also

Related Pathways

Related Diseases

Related Genes

External Links

References

Pathway Diagram

💬 Discussion