 Hey everybody, Dr. O here. In this video we're going to talk about really just the thymus, but I want to introduce the primary lymphoid organs here in this video. There are two primary lymphoid organs, your thymus and your bone marrow, so I won't show you the bone marrow here, but that's going to be where lymphocytes mature, lymphocytes being one class of your white blood cells. The key difference here is the T cells and B cells mature in different locations. B cells are called B cells because they mature in the bone marrow, so I'll cover B cells in a separate video. But B cells are going to become the plasma cells that make antibodies. Antibodies are responsible for taking care of extracellular pathogens or pathogens outside of your cells as well as toxins that are in your body fluids. So that's going to be what's known as antibody mediated or humoral immunity. Humorals the term you're going to hear more often. So that's your B cells. They come from the bone marrow. We'll cover them later. But here we're going to talk about the thymus gland, which is where your T cells mature. These cells are responsible for the targeting and destruction of intracellular pathogens. Think cells infected with viruses, cells infected with intracellular bacteria, small bacteria that can live inside our cells. That's going to be what's called cell mediated or cellular immunity. So here we see the thymus gland is resting on top of the heart. Its absolute size is biggest about puberty, but relatively speaking it's largest when you're born because it's you grow, but faster than your thymus gland does. But the thymus gland is very important location for the maturation of your T cells. So here you see in the inside of the thymus gland we have this outer cortex and intermedulla. The cortex is where the thymocytes, which is the name for your immature lymphocytes, immature T cells, are going to develop into a mature T cell. And then it's going to leave through the medulla and you'll see that it comes into contact with dendritic cells, macrophages, these are going to be what are called antigen presenting cells that help teach these T cells what they should be targeting. You just remember the function of the thymus is the maturation of your T cells. And there are hormones involved here, they're called thymus and hormones. One last key thing about the thymus gland though is that it is what's called thymic involution. So your thymus is going to atrophy as you age. At puberty it probably weighed about, or if you're near there now, weighs about 1.4 ounces. And then it's going to slowly break down about 3% a year until you reach the age of 35 to 45, then it's going to continue to break down but more slowly. So the older you are, the less your thymus weighs. So mine probably weighs 0.4 ounces right now when I'm 50 it'll be near 0.3 ounces and it's going to continue to decline. So this is one of the reasons we think there's a limit on how long humans can live because at the rate of decline that's normal by about the age of 120 your thymus gland will be gone, which means you can't produce any new mature T cells which means you can't fight off any new infections. So this could be the reason that the genetic limit for a human being appears to be in that 120 to 130 year age. Alright, so key things here, thymus is where your T cells are going to mature and the thymus does atrophy as we get older in a process called thymic involution, which means it's going to work less and less than the older you are, the more susceptible you are to disease for that reason. Okay, I hope this helps. Have a wonderful day. Be blessed.