 So, we have talked about response of immune cells to injury. The example we were talking about is a thorn prick. And incidentally, that thorn also carried some pathogens, some bacteria with it. So, we said that first response to this injury is release of histamine by mass cells to dilate the blood vessels, allowing other cells to come in and take action. So, before we go any further and see what happens next, first let's talk about the cells that make up immune system. So, here's a slide which shows you the major immune cells. Immune cells, all these cells originate from cells precursor cells in the bone. These cells reside in the bone marrow. These are truly potent hematopoietic cells. These cells divide and they can form two types of progeny, myeloid progenitor cells and the lymphoid progenitor cells. The lymphoid progenitor cells give rise to lymphocytes, there are three different types of lymphocytes, B cells which make antibodies, T cells which kill virus infected cells and most importantly they regulate the activities of other immune cells. The third category is natural killer cells. These cells lies virus infected cell or cancer cells. This is these cells come from the lymphoid progenitor cell. Myeloid progenitor cell gives rise to two groups of cells. One group does not have any nucleus and they compose of basically RBCs or the red blood cells which give our blood the red color because they have iron in it. We know that iron is required to carry oxygen to different parts of the body. Platelet cells, these are actually fragments of a cell, they initiate blood clotting. The other group is the white other white blood cells are include basophil cells and the mass cells, basophil cells and the mass cells they both contain histamine and they release histamine when it is required of them. Acenophil cells kill antibody coated parasites, neutrophils and macrophages, they engulf and digest microorganisms or pathogens. Macrophages come from basically monocytes, monocytes develop into macrophages. There's another class of cells which we will talk about in more detail later on are the dendritic cells. These dendritic cells are also phagocytic in nature. They ingest bacteria or the pathogen, degraded into little fragments and present the parts of the pathogen they display it on their surface and present it to other immune cells. So let's see, we have talked about dendritic cells, let's see what exactly they do. So the dendritic cells are also distributed in various parts of the body. They when they see pathogens, they engulf them, they engulf them and they form the phagosome or the the the vacuole or the vesicle which contains the pathogen. They fuse that vesicle with the lysosome causing degradation of that contents of the phagosome and then they display the parts of that pathogen on their surface. You can see right there, right here, these are the parts of this bacteria which are being displayed on the surface of this dendritic cell. After eating up a pathogen engulfing a pathogen, this dendritic cell moves to a special part of the body called the lymph node. We have several of these nodes all around all over our body. So this dendritic cell now is going to go to the lymph node and it is going to present little pieces of the pathogen or whatever bacteria engulf to the T cells. So here the torch is being passed on from the innate immune system to adaptive immune system. I will describe what I have just said. Innate immune system I mentioned earlier also are nonspecific responses. For example, dendritic cells have special receptors on them. These receptors recognize certain parts of the pathogens and they are not very specific. These receptors, it's a group of receptors that can bind different parts of pathogen or bacteria. They have evolved over a course of evolution and these are one of the more primitive type of receptors. So they cause this cell, our dendritic cell to eat this bacteria and when these dendritic cells move to the lymph node, they can present those pieces to the T cells. Dendritic cells are part of innate immune system because they are not specific. They will eat any micro, whether it's bacteria or virus or doesn't matter what type of bacteria it is, it will eat it. The adaptive immune system, which includes the T cells and B cells, we will talk more about them, they are specific. The T cells, we have many different types of T cells in our body. Only the T cells that have the ability to recognize this particular pathogen, whatever bacteria it is, let's say it is a salmonella bacteria, only T cells that can recognize pieces of salmonella will get activated. The first thing they will do is, one of the first things they do is they go back to the site of infection and they tell other, they help other pegocytic cells kill and destroy the salmonella in our case. So I have talked about two things. I have talked about adaptive immune system and I have talked about lymph. So first of all, let me tell you more about lymph. Here's a diagram of lymphatic system. So we have another type of vessels in our body which is distributed. They carry lymph, which is a fluid, it is a clear fluid. Lymph is derived from blood and it accumulates in the intracellular spaces. So from these spaces, lymph is collected through a network of ducts and lymph is then, this lymphatic fluid is then returned to the blood at special part in a special blood vessel, which is the thoracic duct. While this lymph is traveling in the lymphatic system, it encounters these structures which I referred to as lymph nodes earlier. These lymph nodes, we'll see a larger photograph of this. These lymph nodes have a population of specialized cells, T cells and the B cells, which are performing an inspection. Just like when passengers come out of the airport, there's a custom stall. The luggage of these passengers is inspected. Lymph nodes have cells in which we have custom inspectors. You can think of these B cells and T cells as the custom inspectors that are looking, are inspecting the contents of the lymph and trying to see if there are any foreign particles in it. So these lymph nodes, let's see a larger picture of them. Here's a lymph node. So the lymph fluid comes into the lymph node. The lymph fluid, lymphatic fluid comes in and here it gets filtered. There are specialized areas of the lymph node where specific cells live. For example, in lymphoid follicle area of the lymph node, the B cells reside and the T cells are primarily present in the cortex. And when the lymphatic fluid comes in, these cells do their job, which is to inspect the contents of the lymph. So it is filtered and inspected. OK, so this is the structure of lymph node. Next we will see, when I said adaptive immune system, we will look at what adaptive immune system is and what are the responsibilities of adaptive immune system. And more importantly, what are the components of adaptive immune system?