A mosquito lands on your arm, injects its chemicals into your skin,
and begins to feed. You wouldn’t even know it was there,
if not for the red lump that appears, accompanied by a telltale itch. It’s a nuisance, but that bump
is an important signal that you’re protected
by your immune system, your body’s major safeguard
against infection, illness, and disease. This system is a vast network of cells,
tissues, and organs that coordinate your body’s defenses
against any threats to your health. Without it, you’d be exposed to billions
of bacteria, viruses, and toxins that could make something as minor
as a paper cut or a seasonal cold fatal. The immune system relies on millions
of defensive white blood cells, also known as leukocytes, that originate in our bone marrow. These cells migrate into the bloodstream
and the lymphatic system, a network of vessels which helps
clear bodily toxins and waste. Our bodies are teeming with leukocytes: there are between 4,000
and 11,000 in every microliter of blood. As they move around, leukocytes
work like security personnel, constantly screening the blood,
tissues, and organs for suspicious signs. This system mainly relies
on cues called antigens. These molecular traces on the surface
of pathogens and other foreign substances betray the presence of invaders. As soon as the leukocytes detect them, it takes only minutes for the body’s
protective immune response to kick in. Threats to our bodies are hugely variable, so the immune response
has to be equally adaptable. That means relying on many
different types of leukocytes to tackle threats in different ways. Despite this diversity, we classify
leukocytes in two main cellular groups, which coordinate a two-pronged attack. First, phagocytes trigger
the immune response by sending macrophages
and dendritic cells into the blood. As these circulate, they destroy
any foreign cells they encounter, simply by consuming them. That allows phagocytes to identify the antigen on the invaders
they just ingested and transmit this information
to the second major cell group orchestrating the defense, the lymphocytes. A group of lymphocyte cells called T-cells
go in search of infected body cells and swiftly kill them off. Meanwhile, B-cells and helper T-cells use the information gathered
from the unique antigens to start producing special proteins
called antibodies. This is the pièce de résistance: Each antigen has a unique, matching
antibody that can latch onto it like a lock and key, and destroy the invading cells. B-cells can produce millions of these, which then cycle through the body
and attack the invaders until the worst of the
threat is neutralized. While all of this is going on,
familiar symptoms, like high temperatures and swelling, are actually processes designed
to aid the immune response. A warmer body makes it harder for bacteria
and viruses to reproduce and spread because they’re temperature-sensitive. And when body cells are damaged, they release chemicals that make fluid
leak into the surrounding tissues, causing swelling. That also attracts phagocytes, which consume the invaders
and the damaged cells. Usually, an immune response
will eradicate a threat within a few days. It won’t always stop you from getting ill,
but that’s not its purpose. Its actual job is to stop a threat from escalating to dangerous
levels inside your body. And through constant
surveillance over time, the immune system
provides another benefit: it helps us develop long-term immunity. When B- and T-cells identify antigens, they can use that information
to recognize invaders in the future. So, when a threat revisits, the cells
can swiftly deploy the right antibodies to tackle it before it affects
any more cells. That’s how you can develop immunity
to certain diseases, like chickenpox. It doesn’t always work so well. Some people have autoimmune diseases, which trick the immune system into attacking the body’s own
perfectly healthy cells. No one knows exactly what causes them, but these disorders sabotage
the immune system to varying degrees, and underlie problems like arthritis, Type I diabetes, and multiple sclerosis. For most individuals, however, a healthy immune system will successfully
fight off an estimated 300 colds and innumerable other potential infections
over the course of a lifetime. Without it, those threats would escalate
into something far more dangerous. So the next time you catch a cold
or scratch a mosquito bite, think of the immune system. We owe it our lives.
