 Hey everybody, Dr. O, in this video we're going to talk about the second major secondary lymphoid organ, and that is your spleen. So first let's talk about structurally where it is. As you can see in the image there in the center, the spleen is right under the diaphragm. It's actually attached to the lateral border of the stomach by a little ligament, the gastro splenic ligament, so you can see where it sits. One thing that's kind of noticeable about the spleen is it has a very weak capsule around it, which makes it a fragile organ. You hear about people getting their spleen bursting and football injuries, car accidents, those kind of things. So it's a pretty weak organ compared to some of the other ones. So basically we're going to break it down into two parts, the red pulp and the white pulp. It's often called the filter of the blood because this is where the red pulp is going to have a ton of blood flowing through it, and it's going to have all sorts of macrophages, which are the cells that grab and recycle damaged and old red blood cells. So a huge chunk of the recycling of red blood cells is going to occur here in the red pulp of the spleen. The other key thing about the red pulp is there are immune cells here, but they're primarily the cells that are involved in innate immunity or non-specific immunity. So while they're filtering their blood, they are also looking for blood-borne pathogens, pathogens that are going to be right in your bloodstream. So there is some immunity to the red pulp portion of the spleen as well, but the white pulp is going to be full of all your T cells and B cells, the cells involved in the adaptive immune response, the specific immune response. So what they're going to be doing, think about the white pulp of your spleen as a huge lymph node. It has the same lymphoid follicles as a lymph node does, and it's going to be filtering things, looking at things, then mounting immune responses from there. So the red pulp and the white pulp of the spleen, they're really both involved in immunity, but primarily when you think of the red pulp, think of the filtering of blood, the white pulp, think of it like a huge lymph node where it's going to be filtering, removing, and then mounting immune responses to whatever it finds from the blood. The nice thing about your lymphatic system is since lymphatic fluid flows a lot more slowly than blood, it gets a real good look at what's going on in there. So that's one advantage of having these 500 to 600 lymph nodes and having the white pulp of the spleen. Okay, that's an introduction to the spleen. I hope it helps. Have a wonderful day. Be blessed.