At this point in the series, we’ve introduced aspects of the immune system
at the molecular, cellular, and organismal levels. So now it’s time to start learning
about how all these different components work together to recognize and eliminate pathogens.
Let’s start out by introducing the innate immune system, so that we can get an overview
of some of the things we will discuss in greater detail over the next few tutorials.
The innate immune system is an evolutionarily ancient defense
system, with elements conserved across plants, invertebrates, and even single-celled organisms.
It is called the innate immune system because it consists of immune mechanisms that are hard-wired
to quickly recognize and neutralize a broad range of threats, regardless of previous exposure. It
differs from the adaptive immune system, which relies on highly antigen-specific defenses
that take some time to launch, and which are strengthened by repeated exposure to the antigen.
So what are some of the most important features of the innate immune system
that we will be investigating? The first is barrier defense. We will learn about some of
the many strategies our bodies have evolved to keep pathogens out of various surfaces.
Barrier defense goes way beyond just structural barriers, however. There are also some incredible
chemical and biological defenses that occur at barrier surfaces, and many of these defenses
are coordinated by the cells lining the skin, airway, and gut. These cells are right at the
interface between what is outside versus inside the body, and they represent the
first line of defense that a pathogen has to overcome in order to infect a host.
If a pathogen is able to slip through physical and chemical barriers,
one of the first internal immune defenses it will meet is the complement system.
The complement system involves a series of cleavable proteins that circulate through the
bloodstream, and this is activated when proteins recognize microbial surfaces,
bind antigen-antibody complexes, or experience low-level spontaneous cleavage. Activation
of the complement system initiates a chain reaction of proteins cleaving other proteins,
and the byproducts of these reactions have a broad range of effects, including recruiting
phagocytes, coating pathogens to mark them for phagocytosis in a process called opsonization,
and even forming pores on microbial surfaces to directly kill pathogens. We’ll talk much more
about this process a bit later in the series. Once the initial signs of barrier breach
and infection begin to manifest, the affected area enters a state of inflammation. We will learn
how to recognize the four signs of inflammation, which include heat, redness, swelling, and pain,
as we will gain an understanding of what is transpiring on the cellular and molecular level
when this happens. Although inflammation gets a bad rap sometimes, we’ll learn how
important and effective this process is for quickly containing and neutralizing a threat.
We’ve introduced innate immune cells and pattern recognition receptors already,
but as we move forward we will devote more time to understanding these powerful mechanisms of immune
defense as well. Many immunology courses can be rather focused on the adaptive immune system,
and will thereby avoid going into detail regarding aspects of innate immunity,
as some consider it to be less sophisticated or less interesting than the adaptive immune system.
Again, this is perhaps due to the fact that innate immunity is highly conserved across
so many species, while adaptive immunity is found exclusively in vertebrates, thereby
representing evolutionary progress. But there is so much to learn about innate immunity as well,
as this system is truly complex, elegant, and incredibly important. So let’s get a
closer look at all these components of the innate immune system now.