The aim of this project is to understand the biological differences between two distinct subtypes of depression, patients with and without inflammation as defined by c-reactive protein (CRP). Using a double-blinded, parallel group, placebo-controlled design, participants will be phenotyped before and after a low-dose lipopolysaccharide (LPS) challenge designed to perturb the immune system and trigger a transient, mild inflammatory response. This experimental design will allow for the delineation of the homeostatic mechanisms underlying sensitivity to inflammation-related depression. Low-dose LPS has been used by multiple groups to safely induce transient inflammatory responses in humans. The pilot study is expected to take approximately one year to complete.