Interleukin 1 receptor, type II (IL1R2) also known as CD121b (Cluster of Differentiation 121b) is an interleukin receptor. The protein encoded by this gene is a cytokine receptor that belongs to the interleukin-1 receptor family.

This protein binds interleukin alpha (IL1A), interleukin beta (IL1B), and interleukin 1 receptor, type I(IL1R1/IL1RA), and acts as a decoy receptor that inhibits the activity of its ligands. Interleukin 4 (IL4) is reported to antagonize the activity of interleukin 1 by inducing the expression and release of this cytokine. This gene and three other genes form a cytokine receptor gene cluster on chromosome 2q12. Two alternatively spliced transcript variants encoding the same protein have been reported.