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CR2 / Complement receptor type 2 · IHC design guide
Design Immunohistochemistry for CR2
Plan chromogenic CR2 IHC in paraffin sections using lymph node or tonsil germinal centers and spleen white pulp as positive regions (HPA tissue IHC). This guide covers the observed cytoplasmic pattern (HPA tissue IHC), membrane topology (UniProt), and fixation and epitope checks.
Evidence assembled Oct 2026 · For research use; verify linked source records and product datasheet before use
CR2 Immunohistochemistry Experimental Design Guide
Expected localisation, validated protocols, controls and antibodies — the at-a-glance facts below, then the full design guide.
Must know before staining
| Expected localisation | Cytoplasmic in lymphoid tissue (HPA tissue IHC); cell membrane (UniProt) | |
| Staining pattern | Cytoplasmic staining in germinal center and white pulp cells (HPA tissue IHC) | |
| Antigen retrieval | Citrate pH 6.0 HIER, 95–98 °C, 20 min (rule: cytoplasmic / membrane antigen) | |
| Positive control | Appendix+3 more · see all | |
| Negative control | Adipose tissue+4 more · see all |
Important caveats
Reasons your staining may differ from the expected pattern.
| Fixation | Keep fixation consistent across paraffin sections. (standard IHC practice; not target-specific) | |
| Caveat | Tissue staining is cytoplasmic despite membrane topology (HPA tissue IHC; UniProt) | |
| Regulation | High in lymphoid cell subsets (HPA tissue IHC) | |
| Isoform / epitope | 4 isoforms; check whether the epitope is extracellular or cytoplasmic (UniProt) |
Section 1
Recommended CR2 IHC & IF Protocols
The catalog antibody’s IHC-P protocol is paired with a published CR2 immunohistochemistry protocol for human tissues (PMC12328216 methods).
| Sample | Tissue sections; selected-image fixative not specified (standard IHC workflow) |
| Fixation | Image fixative and duration unreported (datasheet M01632-2); verify before use. |
| Sectioning | 4–5 µm sections on charged slides (standard) |
| Deparaffinisation | Xylene, graded ethanol series to water (standard) |
| Antigen retrieval | Heat-induced epitope retrieval in citrate buffer, pH 6.0, 20 min at 95–98 °C (standard rule: cytoplasmic / membrane antigen) |
| Peroxidase block | 3% H2O2, 10 min, room temperature (standard) |
| Blocking | 10% normal serum of the secondary host, 30 min, room temperature (standard) |
| Primary antibody | Rabbit anti-CR2, 1:1000-1:4000 (datasheet M01632-2) |
| Primary incubation | Overnight at 4 °C (standard) |
| Detection | HRP-polymer secondary, DAB chromogen 5–10 min (standard) |
| Counterstain | Hematoxylin, blue, dehydrate and mount (standard) |
| Expected result | CR2-positive staining in lymphoid tissue of appendix (HPA tissue IHC: High). HPA tissue profile: Cytoplasmic expression in lymphoid tissues. No signal in the no-primary control. |
💡Decision noteStart with citrate pH 6.0 retrieval at 95–98 °C for 20 min (page antigen-retrieval rule; UniProt P20023: cell membrane).
Section 2
What Is the Expected CR2 Staining Pattern?
CR2 is a cell-membrane receptor with an extracellular region spanning residues 21–971 and one transmembrane segment (UniProt P20023 topology). In paraffin-section IHC, look for staining in lymphoid tissue, especially germinal center cells and splenic white-pulp cells (HPA: High in lymph node and tonsil germinal center cells; High in spleen white pulp). HPA describes the observed tissue profile as cytoplasmic and rates its IHC evidence Enhanced (HPA: tissue IHC).
| Distinct staining in germinal center cells or splenic white-pulp cells, with staining at cell boundaries and possible cytoplasmic color. | This fits the strongest reported tissue sites (HPA: High in lymph node and tonsil germinal center cells; High in spleen white pulp). Boundary staining agrees with CR2 topology (UniProt P20023); a cytoplasmic appearance also occurs in HPA tissue IHC (HPA: cytoplasmic expression in lymphoid tissues). |
| Predominantly nuclear staining, without a recognizable lymphoid-cell pattern. | Treat nuclear-only staining as suspect: CR2 is assigned to the cell membrane (UniProt P20023 topology; HPA: subcellular summary, Membrane). Check the positive control and detection controls before interpreting this compartment as CR2; neither source establishes a nuclear pattern. |
| Strong color in adipocytes or respiratory epithelial cells, especially when expected lymphoid cells do not stain. | These sampled cell types were not detected in their respective tissues (HPA: adipose adipocytes; bronchus respiratory epithelial cells). Consider antibody cross-reactivity or endogenous detection activity, then compare with a no-primary control. A negative HPA observation for one cell type does not define every cell in that tissue. |
| Diffuse color across tissue structures or color in the no-primary control. | This lacks the cell-restricted distribution reported for CR2 (HPA: High in germinal center cells and splenic white-pulp cells). In routine IHC, background or endogenous detection activity can mimic a weak positive. Reassess blocking, detection chemistry and wash conditions using matched controls. |
| No signal in germinal centers of lymph node or tonsil, including the positive-control section. | The result is uninterpretable as a biological negative until the assay works in known-positive material (HPA: High in lymph node and tonsil germinal center cells). Check that the antibody is IHC validated, then review retrieval, antibody concentration and detection as general IHC variables. |
💡Expected CR2 appearanceCall positive when staining is concentrated in lymphoid cells, particularly germinal center or splenic white-pulp cells, with appreciable color at cell boundaries; HPA also reports a cytoplasmic tissue-IHC appearance (HPA: High in these cells; HPA: cytoplasmic expression; UniProt P20023 topology). Isolated nuclear or widespread control-positive color is suspect.
| Where the epitope can lie | Most of CR2 is extracellular, followed by a transmembrane segment at 972–999 and a cytoplasmic tail at 1000–1033 (UniProt P20023 topology). Interpret staining with the antibody's documented epitope, if available; this payload gives none. |
| Choice of tissue control | Lymph node or tonsil germinal centers and spleen white pulp provide reported high-staining compartments (HPA: tissue IHC, High). Adipose adipocytes and bronchial respiratory epithelial cells are reported as not detected (HPA: tissue IHC); compare the specified cells, not whole tissues indiscriminately. |
| Strength of IHC evidence | HPA rates the tissue pattern Enhanced, describing high consistency between antibody staining and RNA expression (HPA: tissue IHC reliability). Three listed antibodies have Enhanced IHC status (HPA: HPA052942, HPA060715, CAB002659). This supports the reported pattern without guaranteeing that every run or reagent will reproduce it. |
| Isoforms and antigen processing | UniProt lists 4 isoforms and a cleaved signal peptide at residues 1–20 (UniProt P20023). The supplied evidence does not assign an antibody epitope or an isoform-specific IHC pattern, so staining cannot identify an isoform or establish shedding. |
| IF/ICC: what pattern is documented? | A membrane location is summarized (HPA: subcellular summary, Membrane; UniProt P20023 topology). HPA supplies no ICC/IF image-bearing cell line or antibody ICC validation here. Use the separate IF/ICC guide for assay design. |
| Situation | Likely cause | Next action |
|---|---|---|
| Known-positive germinal centers are blank. | The assay may have failed; these cells are reported High in lymph node and tonsil (HPA: tissue IHC). The payload does not identify a CR2-specific fixation or retrieval failure. | Run a matched positive-control section and review the catalog antibody's IHC instructions. As general IHC checks, review retrieval, primary-antibody concentration and detection before calling the specimen negative. |
| Only nuclei are colored. | The compartment conflicts with the reported membrane assignment (UniProt P20023 topology; HPA: subcellular summary, Membrane). Nuclear color alone does not establish CR2. | Inspect the no-primary control, confirm the expected lymphoid-cell distribution, and repeat with the documented IHC conditions if the control or pattern is inconsistent (general IHC practice). |
| Color appears throughout adipocytes or bronchial respiratory epithelium. | HPA reports CR2 as not detected in those specified cells (HPA: tissue IHC). Cross-reactivity or endogenous detection activity is possible; the image alone cannot distinguish them. | Compare matched no-primary and positive-tissue controls, then address background in blocking or detection as indicated (general IHC practice). Score the named cell compartment rather than assuming an entire tissue is negative. |
| The whole section has a weak brown haze. | A uniform haze obscures the restricted high-staining compartments reported by HPA (HPA: germinal center and white-pulp cells, High). Routine IHC background may arise from detection or washing conditions. | Check a no-primary section and review blocking, wash steps and chromogen development (general IHC practice). Interpret CR2 only where cellular staining remains distinct from the surrounding background. |
| Cytoplasmic color is present but cell boundaries are unclear. | HPA describes cytoplasmic expression in lymphoid-tissue IHC, while CR2 has a membrane topology (HPA: tissue profile; UniProt P20023 topology). The two descriptions alone cannot establish whether this slide's color is specific. | Compare the stained cells with HPA's positive compartments and a matched negative control. Assess whether cell-restricted color exceeds background before scoring; avoid calling all cytoplasmic color an artefact. |
| Positive tissue stains unevenly across sections or runs. | HPA's Enhanced rating describes consistency of its antibody staining with RNA data, not guaranteed performance in this run (HPA: tissue IHC reliability). The supplied sources establish no CR2-specific fixation sensitivity. | Use the same positive-control tissue in each run and review section processing, retrieval, antibody application and detection as general IHC variables. Document any run-to-run change before comparing biological samples. |
Experiment evidence
IHC & IF images showing CR2 staining
M01632-2 · IHCRat spleen was stained with anti-CD21 rabbit antibody
M01632-2 · IHCHuman spleen was stained with anti-CD21 rabbit antibody
M01632 · IHCImmunohistochemical analysis of paraffin-embedded human spleen, using CD21 Antibody.
HPA · Cerebral cortexHuman Protein Atlas IHC, Cerebral cortex. Source: https://www.proteinatlas.org/ENSG00000117322-CR2/tissue/cerebral+cortex#imid_17240171
HPA · ColonHuman Protein Atlas IHC, Colon. Source: https://www.proteinatlas.org/ENSG00000117322-CR2/tissue/colon#imid_17240105
HPA · LiverHuman Protein Atlas IHC, Liver. Source: https://www.proteinatlas.org/ENSG00000117322-CR2/tissue/liver#imid_17240098
HPA · KidneyHuman Protein Atlas IHC, Kidney. Source: https://www.proteinatlas.org/ENSG00000117322-CR2/tissue/kidney#imid_17240099
← swipe / scroll for more images →
Real IHC / IF data behind the staining-pattern expectations in this section.
Sample controls for CR2 IHC & IF
🧪Run lymph node first and look for CR2 staining in germinal center cells (HPA: High in lymph-node germinal center cells); run adipose tissue as the negative, focusing on adipocytes (HPA: Not detected in adipocytes). On the positive slide, use adjacent cells outside the germinal center as an internal background comparison; they should lack the specific membrane pattern seen in positive cells (HPA: germinal center cells High; UniProt P20023: cell membrane).
Positive control tissue: Appendix (Lymphoid tissue, HPA High)
Negative control tissue: Adipose tissue (HPA Not detected)
ICC-IF cell lines (HPA subcellular resource): HPA carries no ICC-IF cell line for CR2; derive a cell-line control from the positive tissue's cell type (Lymphoid tissue) and confirm it by RNA or western blot first.
Technical controls: Include a no-primary, secondary-only control, a rabbit IgG isotype control matched to the catalog antibody where applicable, and a CR2-knockout biological negative (selected-SKU caption: rabbit antibody; standard IHC practice). Block endogenous peroxidase in the lymph-node section for chromogenic detection, and check endogenous biotin if using a biotin-based detection system (standard IHC practice).
⚠️Feasibility: The supplied evidence reports no CR2-specific fixation window or retrieval effect; the M01632-2 rat-spleen caption does not state a fixative (selected-SKU caption). Optimize antigen retrieval empirically for paraffin IHC; these data do not establish whether frozen sections or IF are easier (standard IHC practice). Dense lymph-node follicles can complicate cell-by-cell scoring, so assess the expected membrane pattern against the adjacent background (HPA: High in lymph-node germinal center cells; UniProt P20023: cell membrane).
HPA tissue IHC evidence for CR2
Comprehensive Human Protein Atlas IHC scoring per tissue (reliability: Enhanced — High consistency between antibody staining and RNA expression data.). Rows are taken directly from the HPA tissue chart — click any row's HPA link to view the source.
Positive expression · recommended positive controls
| Tissue | Cell type | Level | Evidence | Source |
|---|---|---|---|---|
| Appendix | Lymphoid tissue | High | Protein (IHC) | HPA → |
| Lymph node | Germinal center cells | High | Protein (IHC) | HPA → |
| Spleen | Cells in white pulp | High | Protein (IHC) | HPA → |
| Tonsil | Germinal center cells | High | Protein (IHC) | HPA → |
Undetected expression · recommended negative controls
| Tissue | Cell type | Level | Evidence | Source |
|---|---|---|---|---|
| Adipose tissue | Adipocytes | Not detected | Protein (IHC) | HPA → |
| Adrenal gland | Glandular cells | Not detected | Protein (IHC) | HPA → |
| Bone marrow | Hematopoietic cells | Not detected | Protein (IHC) | HPA → |
| Breast | Adipocytes | Not detected | Protein (IHC) | HPA → |
| Bronchus | Respiratory epithelial cells | Not detected | Protein (IHC) | HPA → |
Section 3
Advanced CR2 IHC Tips
Troubleshoot CR2 staining by checking retrieval, cellular distribution, controls and scoring against the antibody’s documented application and the tissue’s expected expression.
How should I retrieve CR2 in paraffin sections with weak staining?
Start with citrate buffer at pH 6.0, heated to 95–98 °C for 20 min (page retrieval rule: cytoplasmic / membrane antigen). Let sections cool in buffer, then compare staining with an identically processed positive control from lymphoid tissue (standard IHC practice; HPA: lymphoid tissue expression). Check whether signal outlines cells or appears within germinal center cells and white pulp, rather than judging intensity alone (UniProt P20023: cell membrane; HPA: lymph node, tonsil and spleen staining). If staining remains weak, optimise retrieval time on adjacent sections while keeping antibody and detection conditions constant (standard IHC practice). Record tissue preservation alongside signal intensity, because damaged morphology makes a stronger stain difficult to interpret (standard IHC practice).
Could fixation explain variable CR2 staining between sections?
CR2-specific sensitivity to fixation is unknown from the supplied evidence; the rat spleen image caption does not state a fixative (caption: M01632-2). Compare sections with documented fixation and processing histories, using the same citrate pH 6.0 retrieval at 95–98 °C for 20 min (standard IHC practice; page retrieval rule). Include a lymphoid positive control in each staining run so loss of signal can be distinguished from a run-wide failure (HPA: high staining in lymphoid sites; standard IHC practice). Examine morphology and staining together before changing retrieval conditions (standard IHC practice). Do not assign a CR2-specific fixation effect from the tissue expression pattern or receptor topology (HPA: tissue IHC profile; UniProt P20023: topology).
Should CR2 staining be membranous or cytoplasmic?
Expect a membrane-associated pattern because CR2 has an extracellular region at residues 21–971, a transmembrane segment at 972–999 and a short cytoplasmic tail (UniProt P20023 topology). HPA nevertheless describes its tissue IHC profile as cytoplasmic expression in lymphoid tissues, so assess the actual compartment alongside tissue architecture (HPA: tissue IHC profile). Compare staining in germinal center cells and splenic white pulp with adjacent cells and a negative control (HPA: lymph node, tonsil and spleen staining; standard IHC practice). Diffuse colour outside intact cells warrants checking section damage and detection background before assigning localisation (standard IHC practice). Document membranous and cytoplasmic staining separately if both occur, without assuming they represent distinct CR2 isoforms (UniProt P20023: four isoforms; standard IHC practice).
Could epitope location or CR2 isoforms explain discordant staining?
CR2 has four reported isoforms, A–D, but the supplied antibody evidence does not locate its epitope or establish isoform coverage (UniProt P20023: isoforms; supplied catalog evidence). The extracellular region contains 15 Sushi domains and 11 listed glycosylation sites, making epitope accessibility a consideration during retrieval optimisation (UniProt P20023: topology, domains and glycosylation; standard IHC practice). Compare adjacent sections under the same citrate pH 6.0 retrieval conditions before attributing a difference to isoforms (page retrieval rule; standard IHC practice). Record the antibody’s documented epitope if it becomes available, then check whether it lies in a region shared by the isoforms being studied (standard IHC practice). IHC pattern alone cannot establish which isoform was detected (standard IHC practice).
How can I check CR2 localisation with multiplex immunofluorescence?
Treat IF as a separate application requiring its own antibody validation; the supplied catalog image documents tissue IHC staining only (caption: M01632-2). Pair CR2 with a B-cell marker to assess expected cellular overlap, while keeping single-stain and secondary-only controls for each channel (UniProt P20023: mature B-lymphocyte expression; standard IF practice). Choose fluorophores after inspecting unstained tissue autofluorescence, reserving a brighter, spectrally distinct channel for weak CR2 signal (standard IF practice). For an extracellular epitope, start without permeabilisation; for a cytoplasmic-tail epitope, test mild permeabilisation on matched sections (UniProt P20023 topology; standard IF practice). Because the antibody epitope is unspecified, document that choice rather than treating either condition as validated for this antibody (supplied catalog evidence; standard IF practice).
What causes widespread brown staining in CR2 IHC?
First compare a no-primary control with the stained section to identify colour generated by the detection system (standard chromogenic IHC practice). Include an endogenous peroxidase block before peroxidase-based DAB detection and check whether brown deposits persist in the control (standard chromogenic IHC practice). If they do, optimise blocking and detection before changing CR2 retrieval or antibody concentration (standard IHC practice). If only the primary-antibody section is affected, compare signal in lymphoid structures with nearby negative compartments and review section edges or damaged areas (HPA: lymphoid tissue expression; standard IHC practice). Genuine CR2 interpretation should account for its membrane topology and HPA’s reported cytoplasmic tissue-IHC profile, rather than treating uniform tissue colour as specific staining (UniProt P20023 topology; HPA: tissue IHC profile).
How should I quantify CR2 staining across lymphoid sections? ⚠ ANSWER MARKED FOR VERIFICATION
Define a consistent region of interest, such as a germinal center or splenic white pulp, before scoring (HPA: high staining in lymph node and tonsil germinal center cells and splenic white pulp; standard IHC practice). Report the percentage of positive cells and staining intensity, or calculate an H-score from the proportions at each intensity (standard IHC practice). For spatial comparisons, report CR2-positive cell density per mm² of analysed tissue and exclude folds, necrosis and section edges (standard IHC practice). Normalise cell-based results to the number of evaluable cells in the same compartment and area-based results to evaluable tissue area (standard IHC practice). Apply one threshold and scoring scheme across batches, with matched controls to monitor drift (standard IHC practice).
When is a CR2-positive IHC pattern credible?
A credible result follows lymphoid architecture, including germinal center cells or splenic white pulp, and survives comparison with a no-primary control (HPA: high staining in lymph node, tonsil and spleen; standard IHC practice). Consider membrane-associated staining alongside HPA’s reported cytoplasmic tissue-IHC pattern when judging cellular localisation (UniProt P20023: cell membrane and topology; HPA: tissue IHC profile). Diffuse colour in an unexpected compartment or cell population calls for review of morphology, antibody specificity and detection background (standard IHC practice). Treat staining confined to tissue edges, necrotic regions or areas with endogenous enzyme activity as suspect until controls resolve it (standard chromogenic IHC practice). The rat spleen caption confirms a stained image for SKU M01632-2, but does not state its fixative (caption: M01632-2).
Boster reagents
Best CR2 / Complement receptor type 2 IHC Antibodies
Anti-CR2 antibodies have IHC images from human spleen and tonsil, mouse spleen, and rat spleen (catalog IHC image captions); IF is listed as an application, with no IF images supplied (catalog application and image lists).
Real IHC data
Anti-CD21 Rabbit Monoclonal Antibody
Cat # M01632-2
Real IHC data
Anti-CD21 CR2 Rabbit Monoclonal Antibody
Cat # M01632
M01632 has an IHC image of paraffin-embedded human spleen and lists human and mouse reactivity plus IHC and IF/ICC applications (M01632 image caption; catalog application and reactivity lists). M01632-2 has IHC images of rat, human, and mouse spleen and human tonsil, and lists IHC and IF applications (M01632-2 image captions; catalog application list).
Which to pick: For human paraffin-section IHC, choose M01632: its own image shows paraffin-embedded human spleen; the fixative is unreported (M01632 image caption). For IF/ICC, choose M01632 because both applications are listed, although no IF image is supplied (M01632 application and image lists). For IHC across human, mouse, and rat, choose M01632-2: all three species appear in its own spleen IHC captions; processing and fixative are unreported in those captions (M01632-2 image captions).
Each figure is that product's own IHC / IF validation image from its datasheet.
References
- UniProt Consortium. UniProt entry P20023 (CR2_HUMAN, Complement receptor type 2).
- Human Protein Atlas. CR2 tissue IHC expression (reliability: Enhanced).
- Human Protein Atlas. CR2 subcellular location (ICC-IF): Membrane.
- Human Protein Atlas. CR2 antibody validation summary (3 antibodies).
- Encapsulated Cell Technology-Based Delivery of a Complement Inhibitor Reduces Choroidal Neovascularization in a Mouse Model. Translational vision science & technology 2018 — PMC5846441.
- The use of Matrigel combined with encapsulated cell technology to deliver a complement inhibitor in a mouse model of choroidal neovascularization. Molecular vision 2020 — PMC7245607.
- R9AP is a common receptor for EBV infection in epithelial cells and B cells. Nature 2025 — PMC12328216.
- Dissecting the complement pathway in hepatic injury and regeneration with a novel protective strategy. The Journal of experimental medicine 2014 — PMC4144741.
- PubMed PMID:2563370 — UniProt-cited evidence.
- PubMed PMID:2832506 — UniProt-cited evidence.
- PubMed PMID:2827171 — UniProt-cited evidence.
Immunohistochemistry protocol sheet for CR2 (Complement receptor type 2). Expected localisation Cytoplasmic in lymphoid tissue (HPA tissue IHC); cell membrane (UniProt). Antigen retrieval: Citrate pH 6.0 HIER, 95–98 °C, 20 min (rule: cytoplasmic / membrane antigen). Validated IHC antibody: M01632-2 at 1:1000-1:4000 (the product's own IHC validation image is shown). Controls (HPA): positive Appendix (Lymphoid tissue); negative Adipose tissue. IHC workflow; specimen preparation unspecified. Protocol steps: 1. Sample: Tissue sections; selected-image fixative not specified (standard IHC workflow); 2. Fixation: Image fixative and duration unreported (datasheet M01632-2); verify before use.; 3. Sectioning: 4–5 µm sections on charged slides (standard); 4. Deparaffinisation: Xylene, graded ethanol series to water (standard); 5. Antigen retrieval: Heat-induced epitope retrieval in citrate buffer, pH 6.0 , 20 min at 95–98 °C…; 6. Peroxidase block: 3% H2O2, 10 min , room temperature (standard); 7. Blocking: 10% normal serum of the secondary host, 30 min , room temperature (standard); 8. Primary antibody: Rabbit anti-CR2, 1:1000-1:4000 (datasheet M01632-2); 9. Primary incubation: Overnight at 4 °C (standard). Watch for: Fixation: Keep fixation consistent across paraffin sections. (standard IHC practice; not target-specific); Caveat: Tissue staining is cytoplasmic despite membrane topology (HPA tissue IHC; UniProt); Isoform / epitope: 4 isoforms; check whether the epitope is extracellular or cytoplasmic (UniProt); Regulation: High in lymphoid cell subsets (HPA tissue IHC). Evidence: UniProt P20023, HPA tissue IHC · Enhanced, HPA ICC-IF · Membrane, Datasheet M01632-2, PMC12328216. QR code links to https://www.bosterbio.com/bosterbio-gene-info-cards/CR2/immunohistochemistry-ihc-guide.