**Study Overview:** This research is about understanding Major Depressive Disorder (MDD), a mental illness where people feel very sad and lose interest in things. The study looks at how something called **endotoxin** (a part of bacteria) and **cytokines** (proteins that help cells communicate) might affect the brain in people with MDD. Endotoxins can sometimes leak into the blood from the gut, causing inflammation, which might affect the brain and contribute to depression. The study will also look at genes to see if they play a role in this process. **Key Points:** - **Participation Requirements:** You must be 18 or older, give consent, and have been diagnosed with MDD. - **Length & Visits:** The study details about visits and duration are not specified. - **Exclusion Risks:** You cannot join if you have other mental disorders or certain physical diseases that might affect inflammation. This study is important because it may help us understand why some people with MDD don't get better with current treatments and could lead to new ways of helping them.