If we are going to learn about the immune system, the best way to start is by introducing the major
cell types that make up the immune system and their basic functions. Each of these types will
be examined in more depth in their own separate tutorials later in the series, but it’s a good
idea to do a quick survey up top, so that we can get a sense of all the components, and then expand
on each one later. Now, unlike other systems in your body, such as the digestive system or the
circulatory system, which are made up of distinct organs that are fixed in place, the immune system
is made up of billions of individual white blood cells that are mobile throughout your body.
These circulate through your bloodstream and move around in your tissues, patrolling
for signs of infection or tissue damage. When they do detect a threat, they communicate with
other immune cells so as to quickly mobilize a counterattack. As much as we may be tempted to
bestow these cells with sentience, as though they are literal bodyguards, in actuality they
are simply responding to specific chemical stimuli. But we are talking about a dynamic,
complex system, involving billions of different cells constantly moving and interacting with
one another, so let’s first boil down the roles of immune cells to four main functions.
Function one, recognize. An immune cell needs to be able to recognize when there is a threat.
This involves being able to distinguish between self and non-self, meaning that
it must differentiate between a cell of the human body and an infectious bacterium or virus.
It also needs to recognize when self cells become cancerous and need to be destroyed,
or when there has been damage to self tissue that needs to be healed, such as when you get a cut.
An immune cell can also receive chemical messages from other cells that can influence
its activity. These messages are often in the form of proteins that are secreted,
such as cytokines, which influence how an immune cell responds to a particular threat,
or chemokines, which help guide immune cells to the site of damage or infection.
Funcion two, react. Once they recognize a problem, immune cells react by deploying
effector functions, which are specific to the type of threat at hand. These
effector functions can include mechanisms which kill pathogens or infected cells,
stimulation of wound healing, clearance of cellular debris,
and coordination of the immune response through cytokine and chemokine signals.
Function three, regulate. Although immune cells are very good at killing infectious organisms,
they can also cause damage to self tissue, so immune cells need to be able to regulate
themselves and each other to prevent an immune reaction from getting out of control.
Before they are able to carry out their effector functions, immune cells need to be “activated.”
Immune cell activation can happen by receiving multiple signals indicating the presence of
a pathogen, or by receiving a specific set of signals called “co-stimulatory signals”
from other immune cells. The requirement for activation and co-stimulation works like a
system of checks and balances, ensuring that immune cells don’t accidentally become activated
and cause unnecessary damage to host tissue. We’ll talk about this process in considerable
depth later in the series. Additionally, many types of immune cells send cytokine signals
to tell other cells to ramp down the immune response and begin repairing damaged tissue.
And finally, function four, remember. Immune cells called T and B lymphocytes
have the ability to remember specific threats. Emerging research suggests that other immune cells
may also be able to remember previous infections. This concept of immune memory
is the basis behind vaccinations, and it will be discussed extensively later in the series.
Now, the manner in which immune cells carry out these four functions of recognizing, reacting,
regulating and remembering is largely dependent on where in the body the cells are located,
and later we will learn about precisely how immune cells are spatially distributed
throughout lymphoid tissue, as well as the ways in which the lymphatic system
helps control these properties. However, there are also lots of unique cell types with distinct
identities and effector functions that help determine what a cell does
during an immune response, so let’s move forward and talk about these next.