Eosinophils, IL-5 and the Type 2 Granulocyte Axis
The eosinophil is not recruited by the allergen. It is recruited by the epithelium that met the allergen. Damage or irritation makes airway and gut epithelium release IL-33, TSLP and IL-25 — alarmins, not antigens — and those three switch on ILC2 and Th2 cells that had no part in recognising anything. The type 2 cytokines that follow divide the labour cleanly: IL-5 owns the eosinophil, IL-4 and IL-13 own the tissue. That division is why mepolizumab empties the blood of eosinophils and yet helps only a subset of patients, and why the successful drugs in this space are aimed either at the very top of the cascade or at one precise branch of it. Click any protein for the matching Assay Genie ELISA kit or In Vivo antibody.
Alarmins start it, and that is why the newest drug works highest up. IL-33, TSLP and IL-25 are released by epithelium in response to damage, protease activity, viruses or smoke — no antigen recognition required. They act on ST2, TSLPR and IL-17RB on ILC2 cells, which then make type 2 cytokines with no T-cell receptor involvement at all. Tezepelumab targets TSLP for exactly this reason: block one alarmin at the top and you reduce eosinophils, IgE, FeNO and exacerbations together, including in patients whose blood eosinophil count was never high. Every downstream drug on this map is, by contrast, a single-branch drug.
The type 2 cytokines are not interchangeable, and the clinic proves it. GATA3 licenses the whole locus, but its products divide the work. IL-5 is the eosinophil cytokine — it drives production in the marrow, survival in the tissue, and nothing much else. IL-4 and IL-13 share IL-4Rα and own the tissue phenotype: IgE class-switching, MUC5AC and goblet-cell metaplasia, smooth-muscle change and periostin. So mepolizumab (anti-IL-5) and benralizumab (anti-IL-5Rα) empty the eosinophil compartment while leaving mucus and remodelling largely intact, whereas dupilumab (anti-IL-4Rα) does the reverse and can transiently raise blood eosinophils because it blocks their exit into tissue rather than their production. Two drugs, opposite eosinophil counts, both working. IL-3 and GM-CSF matter because they share the βc receptor chain with IL-5 and partially cover for it.
Recruitment is a two-signal problem, and only one signal is a chemokine. The eotaxins — CCL11, CCL24, CCL26 — all act through CCR3, which is why blocking one eotaxin does very little and why CCR3 looked like the obvious target it never became. Before an eosinophil can follow a chemokine gradient it has to arrest, and it does that using CD49d (VLA-4) against VCAM-1 on IL-4-activated endothelium, with CD62L handling the initial tether and integrin β7 directing the gut-tropic pool. In humans CCL26 becomes the dominant eotaxin in chronic disease while CCL11 dominates acutely — a detail worth knowing before choosing which one to measure.
The granule proteins are the damage, and they are also the assay. An activated eosinophil releases EPX, ECP, MBP and EDN, all of which are directly toxic to epithelium, plus galectin-10, which forms the Charcot-Leyden crystals now recognised as an inflammatory stimulus in their own right rather than a histological curiosity. These are the analytes that tell you the cell degranulated, which a blood count cannot. Siglec-8 is the counterweight — engaging it induces eosinophil apoptosis, which is the mechanism lirentelimab was built on. The In Vivo tie-in: seven of 37 nodes carry a functional-grade antibody — IL-4 (clone 11B11), GM-CSF, CD62L, CD49d, integrin β7, pan-TGF-β and IL-10 — which is the adhesion and shared-cytokine layer rather than the IL-5 axis itself. The IL-5 arm is covered here by ELISA and by research-grade mepolizumab and benralizumab biosimilars, and the map is honest about that split: measure the type 2 cytokines, block the trafficking. For research use only; not for use in diagnostic or therapeutic procedures.
Every protein node links to a product — ELISA kit, In Vivo antibody or research antibody.