 Hey everybody, Dr. O here. So we just got done talking about your primary lymphoid organs, where your lymphocytes mature. Now we're going to talk about the first of your secondary lymphoid organs, which is where they actually concentrate like a staging area where they can mount their immune responses. So what's called naive lymphocytes are going to leave your primary lymphoid organs, your bone marrow and your thymus, and they're going to concentrate in these secondary lymphoid organs, lymph nodes and your spleen being the key ones. And that is where they're going to learn what their target is and then go ahead and mount their immune response. So the lymph node here, as you can see at the bottom on the right hand side, there are multiple afferent lymphatic vessels that are going to carry lymph into a lymph node. And you also will notice that there are less efferent lymphatic vessels. Reason for that is you want this lymphatic flow to be slow. So think about like a traffic jam. You want it to be slow as it travels through a lymph node. And as this lymphatic fluid swirls through this lymph node, about 99% of what's in this lymph that shouldn't be there is being filtered out. And the other key thing, so think of it kind of like a roadblock and a checkpoint where they're monitoring and looking. They're looking for things that shouldn't be there, potential pathogens. And as they filter out and remove these things, your lymph node also has every type of immune cells. You have B cells, T cells, antigen presenting cells, macrophages, you name it. So the lymph has to walk through these checkpoints full of all these immune cells, all these soldiers that are looking at them to make sure that they should or shouldn't be there. And if they shouldn't be there, you're going to see an immune response. And if that happens, that's why you get swollen lymph nodes when you're sick. Because the lymph node that found that pathogen is now targeting it and cloning millions and millions and millions of cells to go off and fight that infection in your blood or wherever it is. So that's why you see the structure of a lymph node. It's basically just the sac holding on to all these immune cells that are constantly on high alert for you. And that's also why you see less efferent lymphatic vessels than afferent lymphatic vessels because you want the flow to be really slow through here. So all these immune cells have a crack at seeing the fluid and seeing if there's something that they should respond to. Okay. So that's a lymph node. I hope this helps. Have a wonderful day. Be blessed.