Mast Cells IgE and Type 2 Immunity

The epithelium decides before the immune system does. Allergens are not intrinsically dangerous; most are proteases, and what makes them immunogenic is that they damage barrier cells. Damaged epithelium releases TSLP, IL-33 and IL-25 — the alarmins — and these act on TSLPR and ST2 on ILC2s and on dendritic cells long before a T cell has seen anything. IL-33 in particular is stored preformed in the nucleus of healthy epithelium and released on necrosis, so it functions as a pure damage signal. This is the reason atopy tracks with barrier genetics (filaggrin, for instance) rather than with anything about the allergen itself, and the reason TSLP blockade works across allergic and non-allergic asthma while IgE blockade does not.

One transcription factor, five different jobs. STAT6, downstream of IL-4Rα, and GATA3 together define the type 2 programme, and the outputs are not interchangeable. IL-4 is the class-switch and differentiation signal. IL-5 is eosinophil-specific — it does essentially nothing else. IL-13 is the effector on epithelium: mucus, remodelling, hyperresponsiveness. IL-9 is the mast-cell growth factor. Amphiregulin is the repair arm, and it is the one that makes type 2 immunity protective rather than pathological. Because they are separable, a "type 2 high" signature is not a single phenotype: an IL-5-dominant patient and an IL-13-dominant patient need different drugs, which is exactly what the biologic trials found.

The mast cell is armed long before it fires. CD40-dependent help drives CD19⁺ B cells to class-switch to IgE, and that IgE binds FcεRIα on mast cells with an affinity so high that the receptor stays occupied for weeks. Nothing happens until allergen cross-links two adjacent IgE molecules; then FcRγ ITAMs recruit SYK and the cell degranulates within seconds — tryptase and histamine from preformed granules, cysteinyl leukotrienes synthesised de novo by LTC4S over minutes. SCF through c-Kit is what keeps the mast cell alive in tissue in the first place. Two consequences follow that catch people out: sensitisation and challenge are separable in time by weeks, and a mast-cell-stabilising intervention given at challenge cannot undo arming that already happened. CD23, the low-affinity receptor, does the opposite job — it feeds back to regulate IgE production rather than to trigger.

Recruitment, remodelling and the brake. IL-13 drives CCL11 (eotaxin), which pulls CCR3⁺ eosinophils in; IL-5 acting on IL-5Rα is what keeps them alive once there, and Siglec-8 is the eosinophil-restricted brake now being targeted therapeutically. Mast-cell mediators recruit through CRTH2. Separately, IL-13 drives MUC5AC and periostin — the remodelling arm, which is why periostin became a serum biomarker of type 2 airway disease. Against all of it sits the regulatory arm: CD25⁺ regulatory T cells and IL-10, which is the mechanism allergen immunotherapy is thought to work through. The In Vivo tie-in: the functional-grade layer here is deliberately narrow and worth stating plainly — there is no In Vivo anti-IL-5, anti-IL-13, anti-IL-33 or anti-TSLP in the range, and this map does not pretend otherwise. What it does have is the arm that decides whether the response happens at all: anti-CD4, anti-IL-4 (11B11), anti-CD40 and anti-CD19 for sensitisation and class switching, and anti-CD25 and anti-IL-10 for the brake. The IL-5, IL-13, IL-4Rα and TSLP nodes link instead to the biosimilar antibodies matching mepolizumab, lebrikizumab and dupilumab, which are detection- and characterisation-grade rather than functional. For research use only; not for use in diagnostic or therapeutic procedures.