Introduction to Immunology: History, Cells, and Immune System

As we have discussed at great length in my  series on microbiology and infectious diseases,   we are constantly surrounded by infectious  and potentially pathogenic microbes, meaning   microorganisms which cause disease. They’re in  the air, on our skin, in our food, pretty much  

everywhere. So why don’t we get sick all the time?  Beyond this, have you ever wondered what causes   allergies? And what’s the deal with vaccines,  how do those work? All of these questions, and   many more, can be answered by understanding how  the immune system works. The immune system is an  

astonishingly complex set of tissues, cells, and  specialized molecules which protect us from any   foreign particles and microbes that may enter our  bodies. And this series will focus on immunology,   which is the study of the immune system. Now we  have already discussed the very basics of the  

immune system over in the anatomy and physiology  series. A quick review of that tutorial, as   well as a few others from that playlist, will be  quite useful for context as we study immunology,   since we will frequently be discussing various  features of the human body, and this knowledge  

will be assumed. Basic information regarding cell  biology and biochemistry will also be presumed as   a prerequisite, so be sure to visit my playlists  on those topics if you’re feeling rusty. Rest   assured, immunology is exceedingly complicated,  even with this prerequisite understanding,  

so without it, almost nothing discussed in this  series will make any sense at all. But if you’re   up to speed on the fundamentals, then get ready  to go much deeper with the fascinating subject   of immunology than you ever have before,  as this will not be a surface level study,  

we are going to really dig into the minute  details over several dozen tutorials.  Now before diving into the components of the  immune system itself, any study of immunology   will benefit from an overview of its history for  appropriate context. To do this properly, let’s  

go back in time a couple thousand years. Before  we knew what the immune system was, and before   we knew that pathogenic microbes exist, people  were still able to recognize the fundamental   principles of immunity. In his description of  a terrible plague that swept Athens in 430 BCE,  

the Greek historian Thucydides wrote, “Yet it  was with those who had recovered from the disease   that the sick and the dying found most compassion.  These knew what it was from experience, and had   now no fear for themselves; for the same man was  never attacked twice – never at least fatally”. 

It wouldn’t be until centuries later that  scientists like Robert Koch and Louis Pasteur   demonstrated that diseases such as smallpox and  plague are caused by infectious microorganisms.   However, Thucydides’ observation that  people who had previously been infected  

were protected from subsequent infection is an  excellent example of what we call adaptive or   acquired immunity. This principle is the basis  of how vaccines work, which we will discuss in   considerable depth later in the series. Although Edward Jenner has largely been  

credited with the development of the first  vaccine, which was against smallpox in 1796,   the principles of immunization, or intentional  exposure to a pathogen to avoid future infection,   had already been in practice across the world  for centuries. In the 9th century CE, the  

Persian physician Al Rhazes noticed that smallpox  survivors were no longer susceptible to subsequent   bouts of the disease. In China around 1000 CE,  people learned that inhaling the powdered crusts   of smallpox scabs would protect them against  future cases of smallpox. Then in the early 1700s,  

an African slave in Boston named Onesimus  described how he had been inoculated against   smallpox, which was a common practice in southern  Libya. Around the same time in Constantinople,   the British poet Lady Mary Wortley Montagu  learned about Turkish women who performed  

operations to inoculate people against smallpox.  After having her son inoculated in Turkey,   she brought this knowledge back to England. It was  only after all of this activity that Edward Jenner   introduced the concept of vaccination by using  a weaker, less dangerous version of the virus  

that causes smallpox in order to immunize people,  instead of using smallpox itself. This process was   safer and easier to regulate, leading to the  widespread adoption of the smallpox vaccine.   Much of this information is discussed from a  strictly historical perspective in the history  

of drugs series, so be sure to head over there  to learn more about the development of this   and other medical innovations. But moving forward from here,   even with Jenner’s astounding breakthrough,  immunology didn’t really take off as a discipline  

until scientists began using experiments to  uncover the major components of the immune system.   The Russian zoologist Elie Metchnikoff discovered  phagocytes, which are cells that consume things   like other cells. In a landmark experiment in  1883, Metchnikoff pushed a rose thorn into a  

transparent starfish larva. After a few hours, he  noticed that cells had rushed in and surrounded   the thorn, and he correctly hypothesized  that these cells could respond similarly   to an infectious threat. His theory became known  as the “cellular theory of immunity,” which states  

that immune protection from foreign invaders is  primarily carried out by cells like phagocytes.  A few years later, in 1890, Emil von Behring  and Shibasaburo Kitasato discovered another   element of the immune system. They realized that  after an animal had been infected with diphtheria  

or tetanus, their serum contained a powerful  antitoxin, that, if transferred to a new animal,   could protect it from future infection. This  phenomenon is called “passive immunity.”   Although they didn’t know this at the time,  the “antitoxin” they discovered was antibodies,  

which are specialized proteins made by the immune  system that bind to specific targets, keeping   them from being able to enter cells, while also  flagging them for destruction by immune cells.   Their discovery led to a rival theory,  the “humoral theory of immunity,”  

which said that the protective elements of the  immune system could be found in bodily fluids,   sometimes referred to as humors, such as serum. Whether immunity was carried out by cells or by   humors was the subject of intense debate for  many years. As technology developed, we began  

to understand that the immune system is made up of  both cellular components and humoral components,   or as we now call them, soluble components, and  that these work together to provide immunity.  During this series, we will study the tissues,  cells, and molecules that make up the immune  

system. We will work to understand what the  major components of the immune system are,   and precisely how they work together  to provide defense against pathogens.  In doing this, we will talk about how barrier  surfaces like the skin, stomach lining,  

and even tears are the first line of defense  against microbes, and how each type of tissue has   distinct ways of preventing microbes from entering  the body. We will learn about the structure of the   lymphatic system, which is made up of miles of  specialized vessels that run alongside blood  

vessels. The lymphatic system filters fluid from  tissues and circulates it through organs like   lymph nodes and the spleen, where white  blood cells can sense if there’s an infection   in a different part of the body. We will perform  an in depth analysis of the different kinds of  

immune cells, which have highly specialized roles  for detecting and destroying different kinds of   threats. We will learn about how white blood cells  develop, and how they are able to distinguish   between healthy cells and any kind of foreign  threat. We will see that these cells don’t just  

recognize microbes, they can even identify  and destroy cells in our own bodies when they   become sick. And we will learn how these cells  provide a kind of “memory” for the immune system.  During this series, we will also talk about  signaling molecules like cytokines and chemokines,  

which tell white blood cells where to go and what  to do during an infection. We will investigate the   evolutionarily ancient system of complement,  which is made up of circulating proteins   that trigger a molecular chain reaction which  can kill bacteria. We will examine how antibodies  

develop, how they are able to target pathogens so  specifically, and their important applications in   the world of biotechnology as research tools  and therapies against many kinds of diseases.  And finally, throughout this series, we will  discuss some aspects of clinical immunology.  

We want to understand what happens when  the immune system doesn’t function properly   and actually harms us, such as is the case with  allergies, autoimmune disease, and Celiac disease.   As you can see, we have a lot of ground  to cover, so without further delay,  

let’s start learning about the immune system.