Th1/Th2 Balance

The Th1 and Th2 scheme describes how a naive CD4 T cell commits to a cytokine programme according to the signals present when it is activated. Interleukin-12 and the factor T-bet drive a Th1 programme producing interferon gamma, supporting macrophage activation and defence against intracellular pathogens. Interleukin-4 and GATA3 drive a Th2 programme producing IL-4, IL-5 and IL-13, which underlies defence against helminths and, when misdirected, allergic disease. The two programmes cross-inhibit, and that reciprocity is what gives the model its balance metaphor.

The scheme has been extended rather than replaced. Th17 cells produce IL-17 and defend mucosal surfaces against extracellular bacteria and fungi; they are the axis secukinumab targets in psoriasis. Follicular helper cells support germinal centre antibody responses, and regulatory T cells restrain them all. Dupilumab, which blocks the shared IL-4 receptor alpha chain and so neutralises both IL-4 and IL-13, is the clearest demonstration that the Th2 axis is real and drug-addressable.

As a decision tool the model is a teaching abstraction with limits. Human helper T cells are plastic, can co-express lineage factors, and shift programme with the local environment. The hygiene hypothesis rested on a strict reciprocity that could not explain the parallel rise of both allergic and Th1-mediated autoimmune disease.

The claim to reject is rebalancing offered as a product benefit. No state of correct balance is defined for a healthy person, no validated test measures it, and no systemic push toward either pole is obviously desirable: more Th1 is not more immunity, and less Th2 in someone without allergy is not an improvement. A ratio of interferon gamma to IL-4 from stimulated whole blood is a laboratory number, not a diagnosis.