Hello everyone! Welcome back. From the last chapter you already know that immune system is very important and it is with us almost every aspects in medicine. So, today I will give you detail introduction of the Host defense and immunity. As the definition of immune, probably you already heard that it is from a Latin word, means freedom from the exemption of an individual from service or duty. So, basically in immune system, we know the immunity is a state of having sufficient biological defense to avoid kinds of infection, disease, or other unwanted biological invasions. And our immune system is quite complex. At least it can be divided into immune organs, and then after immune organs we have all the immune cells and molecules function in this system. The key feature of our immune system is discriminate “self” and “non-self”, like foreign antigens and others, to recognize and response to tolerate this kind of self-antigen, and then recognize to response, rise an immune response to non-selves. For the antigens, depends on whether it is self or non-self. This can be detected by a lot of immune cells, for example, the T lymphocytes and the B lymphocytes. Here, for the T cell, it’s mainly controls the Cellular immunity. And, for the B cell, the major function is using antibody to rise a Humoral immunity. In summary, we have known that a lot of functions of immune system already been studied. For example, it can be an immune defense, and also a key thing immune system needs to be is immune homeostasis, and the last thing is immune surveillance. For the immune defense, it’s basically to neutralize the toxin or to get rid of this kind of invasion of pathogens. But when there is kind of things getting wrong, then you will see a lots of diseases happens, for example, it can be a Hypersensitivity reactivity. And if you have Immunodeficiency, then all the pathogens become very very dangerous. And for the immune homeostasis, also is very key, because those kind of Immunoregulation is important for keep our immune system to tolerate to self-components. If there is something gets wrong, you will heard lots of people suffer from Autoimmune disease. That’s why the immune system attacked ourselves. And for the Immune surveillance, the main function is to help us away from Tumor. For example, if they have some mutation, the immune surveillance can detect those kinds of mutations, get rid of this tumors before the tumor grow up. And also immune surveillance is used when you control those kind of viral infection. If a viral infection became persistent, for example, for a long time you cannot get rid of, then that’s will be some serious problem. After that, you will get some sense about specificity because we always use the words “weak” and “strong” to describe our immune system. Someone says that “oh, my immune system is very strong.” or someone said “I got sick very often because my immune system is weak.” But, for now you will get some sense, only weak and strong cannot be enough to describe the proper function of immune system. Besides that, we should have another key feature of our immune system, that is specificity. That means whether your immune system can recognize those kind of foreign dangerous antigen very specifically or recognize those kind of invasion very specifically. For example, in this figure you can see, if you got weak response or strong response, you still can have some Tumor infection or infection problem. Then if you got too strong, you will have Autoimmune disease. But, if you add another “specificity”, then you will see, okay, for this time, if you have a very good specificity to keep your immune system weak that means self-tolerance. That’s the situation we want to go. And if you have a high specificity and the immune system is very strong that means you can recognize those kind of dangerous stuff to get rid of these invasions very efficiently. So, adding the specificity, you will have all the features of immune system. I will introduce this specificity’s importance later also. So, for our immune system, there are mainly two parts: one is called innate immunity and the other is called adaptive immunity. Innate immunity is kind of immunity that can recognize a pattern in total. They called “not that specific”, but it still has specificity, because they should know that it is a bacteria, so that’s kind level of specificity. I mean it’s a non-specific is because they lacking a specific antigen response. They don’t have antigen specific response. They only recognize pattern and those innate immune system is kind of naive immune system. They don’t have memory in this system, but they are very quick. I will introduce later. And another thing in the immune system called adaptive immunity, that’s usually refer very very specific immune system. This also called acquired immunity, so, that means it can specific against a certain antigen. And it is also important to know that those adaptive immunity has a memory. That means if we recognize some antigen this time, and then we can remember those kind of antigen. When those antigen comes again, we will recognize it much faster. So, here is some pictures about these three layers of immune system. The first layer is just barrier function. We use our skin and our body barriers to keep those kind of invasions away. And if those bacteria already inside of us, they will first mount innate immune response to see whether we can get rid of them. If not, we have to develop a relative slow response to mount an efficient adaptive Immune system to get rid of the specific antigens. This figure also summary what innate and adaptive immune response do. For example, a bacteria that invading us. Firstly, we use barrier function to get rid of those kind of things. If those microbe already inside ourself, and then within hours, usually within 12 hours, our innate immune system already became function to try to fight with those bacteria. But, if that could not get rid of those bacteria efficiently, it’s usually need from 1 to about 5 days roughly ， roughly ，one week to mount a strong adaptive immune response, usually involved in this adaptive immune response is B cell and T cell and then to get rid of those infection later. Here is also a summary about that, when you get infection of some pathogen. And then, if your innate immunity work very well, then you get rid of disease. But if not, you go to the adaptive part, and then ok, you can recover, get rid of the disease. But it’s important is that in the meantime, this adaptive immune system became memory with those antigens. And then, when you get re-infection later, then it’s can mount a specific immunological memory response, very quickly, and to make you recover very quick. And this complex figure also detailed every every moleculars in this process. For example, the innate immunity, they usually recognize different microbes’ pattern. For example, some common features in bacteria or in virus that can be recognized in the innate immune part. And then, we recognize “Oh, that is virus”, “that is a bacteria”, then mounting an efficient immune response. And for adaptive immunity, we mainly use antibody like this, and also using the T cell receptor to recognize specific antigen. Here is a summary of the features of the innate and adaptive immune system. So, basically innate immunity let’s remember again, is basically a naive response, don’t need antigen, they don’t have specific antigen response, but they do know what kind of pathogen, they need to find of this. And then adaptive immune is basically an acquired immune system. And they need antigen to stimulate, specific antigen to stimulate this system. Usually innate immune response is very quick, can be done within hours. But adaptive immune system need at least several days to develop. And the specificity of innate immunity is determined by some pattern recognition receptor like PRR. And adaptive immune, because they need to recognize specific antigen, then they need T cell receptor and B cell receptor (antibody) to function. We don’t have memory in the innate immunity. When next time, those bacteria come again, we still don’t remember them. So, we just response again like new things. But for the Adaptive immunity, we can remember specific antigen to mount much stronger, much quicker responses. And then different components involved, for the innate parts, usually phagocyte and NK cell. And in the adaptive side, usually players are T cells and B cells. Okay, throughout this chapter, you will know some basic components in the immune system. And the next chapter I will go with you to the history of immune system.
