CD125 Antibody Selection: Resolving IL-5Rα Nomenclature and Validation Before You Order
CD125 appears in eosinophil, asthma and type 2 inflammation literature often enough that most laboratories treat the name as settled. It is not. The designation has been attached to more than one molecule, and the annotations do not always agree with each other. An antibody that binds the wrong arm of a shared receptor complex will still produce a clean histogram, which is precisely why the confusion survives. This article sets out the naming problem, the biology behind both candidate molecules, the three technical difficulties that detection raises, and the reference parameters for one biotin-conjugated reagent.
The Naming Problem Comes First
One Designation, Two Molecules
The antigen name listed for the reagent discussed here is CD125, with the molecular alias IL5RA. That resolves to the interleukin-5 receptor α subunit, a membrane protein and the private subunit of the IL-5 receptor. IL-5Rα governs eosinophil differentiation, survival and activation, and it is the target that allergy and asthma researchers know best.
The background text on the same product page describes a different protein. It describes the interleukin-4 receptor α subunit: a type I transmembrane protein that serves as a shared signalling component of both type I and type II IL-4 receptors. That subunit is also the primary binding component for IL-4 and IL-13, and it signals through the JAK-STAT route, specifically JAK1 and STAT6.
Two annotations, two molecules. The datasheet that ships with the lot takes precedence over any descriptive text.
Why the Confusion Persists
IL-5Rα and IL-4Rα both sit on the type 2 inflammation axis, just at different positions in its division of labour. IL-5Rα controls the eosinophil compartment: production, survival and degranulation. IL-4Rα controls IgE class switching, mucus production and alternative macrophage activation. A review that discusses "the type 2 receptor" without naming the subunit leaves room for both readings.
That overlap is probably why mixed usage has survived in the literature. It is also why a search strategy built on a single alias will miss papers that used another one.
What IL-5Rα Does on Eosinophils
The IL-5 receptor is a heterodimer. The α subunit binds IL-5 with specificity, and the βc subunit carries the signalling output shared with the IL-3 and GM-CSF receptors. Engagement activates JAK2, then STAT3 and STAT5, and the result is survival signalling rather than immediate effector release.
The functional consequence is a shift in population size. IL-5 extends eosinophil lifespan in tissue, so a change in receptor occupancy changes how long the cells persist rather than how quickly they fire. This matters for study design: an intervention that blocks IL-5Rα may lower tissue eosinophil numbers over days without altering acute activation markers.
Disease Associations and the Drug Landscape
The axis is central to Th2-mediated inflammation, allergic reactions and asthma. Downstream activation produces IgE class switching, eosinophil recruitment, mucus production and alternative macrophage activation. Those four features define the pathological core of allergic disease.
Monoclonal antibodies that block this axis are already approved for moderate-to-severe atopic dermatitis, asthma, and chronic rhinosinusitis with nasal polyps. This is the most crowded segment of type 2 inflammation drug development, which means reagent performance is compared across programmes constantly.
The product documentation provides background and validation data only. It contains no reference bibliography. When starting a target project, work from three documents. Read the datasheet, the lot-specific certificate of analysis, and the sequence record under UniProt accession Q01344. Then search the primary literature independently.
Three Practical Difficulties in Detection
Nomenclature Drift in Literature Searches
CD125, IL5RA and IL-5Rα are not annotated consistently across databases, and the background text on this page points at IL-4Rα. Search all aliases, and state explicitly in your own writing which molecule you mean. If the designations do not line up, every downstream comparison is built on sand.
Low-Frequency Target Cells
The validation figure uses human peripheral blood cells, and the positive reference is the relevant population within that sample. Such cells are a small fraction of peripheral blood. When event acquisition is insufficient, weakly positive populations read as negative. Set an acquisition ceiling before the experiment, not after.
Two-Step Biotin Development
The reagent is biotin-conjugated and needs a streptavidin-APC probe. The concentration and incubation time of that second step require separate titration. If either is off, background and signal shift together, and the isotype control no longer mirrors the test well.
Reference Parameters for the Biotin Conjugate
The table transcribes the documented specifications. Price and lead time are omitted deliberately.
| Parameter | Specification |
|---|---|
| Host and antibody type | Mouse; Mouse mAb |
| Clone number | S-4480 |
| Antigen name | CD125 |
| Molecular aliases | IL5RA; IL5R; CDw125; IL-5R-alpha |
| Cellular localization | Membrane |
| UniProt accession | Q01344 |
| Antibody isotype | IgG1, κ |
| Isotype control | S0B5431 |
| Reactive species | Human |
| Positive sample | Human peripheral blood cells |
| Purification and concentration | Protein G; 0.2 mg/ml |
| Conjugate and form | Biotin; liquid |
| Buffer system | PBS pH 7.4, 0.09% sodium azide |
| Storage | 12 months from receipt; 2 to 8 °C |
| Application and dilution | FCM; 5 µl per million cells in 100 µl |
Two formats are available, 25T and 100T. The isotype-matched control listed in the table is a biotin conjugate as well, so the second development step is identical between the control well and the test well. That symmetry is the point of specifying a matched control rather than a generic IgG.
Reading the Validation Panel
Paired flow cytometry plots of human peripheral blood cells stained with a biotin mouse IgG1 isotype control and with biotin anti-human CD125, both developed with streptavidin-APC
The figure shows two plots side by side: isotype control on the left, target antibody on the right. Both axes are logarithmic. The right panel contains a discrete population in the 10⁴ fluorescence decade that is absent from the left panel, which is the signal being claimed.
This is a single-colour experiment without a co-stained lineage marker. There is no gating antibody to define the parent population, so the positive cluster must be justified by the isotype control and by light scatter together. That is a weaker position than a two-colour design, and it should be read as such.
The isotype match does hold. Host, isotype and conjugate all correspond, and the left panel is flat in the same region where the right panel shows events. The shift is therefore credible as specific binding, even though the identity of the positive cells is not independently established.
No Western blot, immunohistochemistry or immunocytochemistry data appear on the product page. That absence matters if your application is tissue-based: the validation here supports flow cytometry on peripheral blood, and nothing beyond it.
Choosing Between the Two Listed Reagents
The article references two products: the anti-CD125 conjugate and its matched isotype control. Ordering both is not optional. A biotinylated isotype control carries the same secondary development behaviour as the test reagent, so any streptavidin-related background appears in both wells and cancels out during comparison.
Check three things before the order is placed. Confirm that the alias on the vial matches the molecule your panel targets. Confirm that the isotype control shares host and isotype with the test antibody. Confirm that the application on the datasheet covers the sample type you intend to stain.
Summary
The naming history of CD125 is a reminder that a designation is an index, not a definition. The same label can point at different molecules in different papers, and a reagent datasheet can carry both readings at once. Experimental success depends on internal agreement. The antigen name, the aliases and the accession number must all point at the same molecule. The validation data must also cover the application you plan to run. A supplier that states those boundaries clearly is worth more than one that quotes the lowest price.
Product Information
| Product Name | Catalog No. | Conjugate | Application |
|---|---|---|---|
| Biotin Mouse Anti-Human CD125 Antibody (S-4480) | S0B80812 | Biotin | FCM |
| Biotin Mouse IgG1, κ Isotype Control Antibody (S-844-79) | S0B5431 | Biotin | FCM |
For research use only. Not intended for diagnostic or therapeutic procedures.
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