Interleukin
Interleukins are a numbered but structurally unrelated set of cytokines that carry signals between leukocytes, several of which are the targets of approved monoclonal antibody therapies.
Interleukin is a naming convention rather than a protein family. Numbers were assigned in order of discovery as each new leukocyte-derived signalling molecule was purified, so IL-1, IL-6 and IL-10 share no structural relationship and no common receptor architecture. More than forty have been designated. What they share is a role: soluble mediators active at picomolar to nanomolar concentrations that carry instructions between immune cells and tissue. Some are pro-inflammatory, some restrain inflammation, and several do both depending on the receptor complex engaged.
The therapeutic record is where the specifics live. Anakinra blocks the IL-1 receptor and canakinumab neutralises IL-1 beta. Tocilizumab blocks the IL-6 receptor and is used in rheumatoid arthritis and cytokine release syndrome. Secukinumab neutralises IL-17A for psoriasis, ustekinumab the shared p40 subunit of IL-12 and IL-23, and dupilumab the IL-4 receptor alpha chain for atopic dermatitis.
That success is also the argument against treating interleukins interchangeably. Blocking one helps only where that cytokine is dominant in that disease: IL-17A blockade transformed plaque psoriasis, yet secukinumab tested in Crohn's disease performed worse than placebo, because IL-17 helps maintain the gut barrier. Cytokine, tissue and disease all have to line up before a blockade is useful.
The common misreading is the serum cytokine panel. Circulating IL-6 or IL-1 beta in a healthy person often sits at or below the detection limit of a standard immunoassay, so small between-group differences are as often noise as signal, and tissue concentrations can be orders of magnitude higher than plasma. A peptide that lowers a serum interleukin in a rodent given endotoxin has blunted one acute response, not treated an inflammatory disease.