IL1 is produced mainly by macrophages (most important source) as well as some other cell types. Locally, IL1 from stimulated macrophages stimulates cytokine and cytokine receptor production by T-cells as well as stimulating B-cell proliferation.

There are 2 forms of IL1: IL1alpha and IL1beta , both with the same activities but different structures. IL1alpha appears to be primarily membrane-associated while IL1beta can circulate. Both forms bind to the same receptors, of which there there are two types found on different cell types, for example T- and B-cells, macrophages, neutrophils , bone marrow cells, fat cells, bone cells (osteoclasts), brain cells, cells of the adrenal gland , vascular endothelial cells and smooth muscle cells (amongst others).

These secretions are involved in inflammation , accounting in part for such inflammatory characteristics as vasodilation , cramps, fever and can produce a wasting syndrome called cachexia . Inducing adhesion molecule expression on vascular endothelium is part of the inflammation syndrome - IL1 induces ICAM 1 expression, while TNFalpha induces ELAM 1 expression on endothelium.

Corticosteroids , produced by the adrenals in response to signals from the brain (cortisone), inhibit the action of IL1 and are therefore used to reduce inflammation.