**Study on Depression, Immune System, and Brain Changes** This study looks at how the immune system might affect depression, a mental illness that makes people feel very sad. It also checks how this ties to changes in the brain. The researchers will focus on something called *endotoxin*, which is a substance made by bacteria. They think that endotoxin might cause brain inflammation and damage, leading to depression symptoms. The study will measure levels of endotoxin in the blood and see if it affects the brain and mood. They also want to see if genes (parts of DNA that give instructions to our body) have any role in this process. **Key Points:** - The study involves people over 18 with major depressive disorder (MDD). - Participants shouldn't have other mental disorders or recent suicide risks. - The study involves looking at blood samples and genetic information. If you are considering joining, know that you will need to sign a consent form, and ensure you fit the criteria mentioned. This study aims to better understand how the immune system and genes might affect depression, possibly helping to find new treatments in the future.