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.