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What Are the Immune System's Lines of Defense?

By Sandi Johnson
Updated May 17, 2024
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The immune system's lines of defense are comprised of three major components: the innate, the adaptive, and the acquired immune systems. Each system is created separately and works both as an independent defense system and a symbiotic system working in conjunction with the others. Systems are layered in such a way that if threats slip past one line of defense, the next in line is triggered to respond. Of the three components making up the immune system's lines of defense, each one serves in either a defensive or offensive capacity against pathogens like viruses, bacteria, and other foreign bodies.

Every human being is born with an innate immune system. As an integral part of the immune system, innate immunity is also known as a non-specific immunity, owing to its more passive, generalized role in the immune system. Physical barriers, such as the skin and mucous membranes, are the primary component of the innate immune system. These are the body's first lines of defense, preventing pathogens from getting inside the body in the first place. Innate immunity is considered a defensive member of the immune system team, using tools such as inflammation to signal other immuno-related systems to mount a response.

Next in line to defend the body against pathogens is the adaptive immune system. Should bacteria, viruses, or other foreign bodies get past the body's physical barriers, certain cells known as natural killer or T cells attack pathogens directly. When the body signals the presence of toxins, bacteria, damaged body cells, viruses, or other foreign molecules, killer cells and their associated helper cells seek out and destroy the threat before it has a chance to create illness, disease, or abnormal growth. Adaptive immunity is considered an offensive component in the immune system's lines of defense, taking its name from the ability to appropriately adapt in response to a variety of threats.

Acquired immunity encompasses numerous individual factors. Immunizations and antibodies created after previous exposure to a pathogen fall under acquired immunity, creating another of the immune system's lines of defense. Considered both an offensive and a defensive component of the human immune system, certain types of acquired immunity are temporary or passive. For example, newborn babies borrow immunity from their mothers, first through the placenta during pregnancy and after birth through colostrum in the mother's breast milk. Later, as the child matures and experiences exposure to certain pathogens, the adaptive immune system assists the acquired immune system by creating memory cells or antibodies. This allows the acquired immune system to establish permanent defenses against certain types of threats.

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Discussion Comments
By anon329039 — On Apr 07, 2013

Some viruses, like the one that causes the flu, will adapt to survive by changing an epitope on their surface. The epitope is the location in which an antibody from your memory B cells attach to the virus and destroy it. So your immune system is not getting worse. It still works just as well; it's just that the virus itself is adapting.

By anon328931 — On Apr 06, 2013

In response to burcinic, the good news is that you immune system is not "not working" as well as we think it is, its just that the viruses are adapting.

Each time you get the flu, the innate immune system steps in for the fight and gives its battle plans to the adaptive immune system. So next time when you get that particular version of the flu, the adaptive immune has antigens that will recognize the identifying spot on that flu virus (called an epitope) and eliminate it before you even feel sick.

But here is the kicker (and this is where your confusion comes in): The adaptive immune system has memory cells with a very specific antigen. It will only attach to the right epitope. So each year, the flu virus learns that in order to survive, it must change its epitope.

When it does this, the flu virus will still cause problems for you and make you sick because the adaptive immune system cannot fight it.

The good news is that the old strand will never hurt you again because your immune system is prepared. And also, if you get a new version of the virus, the innate is always ready for a fight.

By serenesurface — On Jul 07, 2012

@burcinc, @ysmina-- It also helps to get vaccinated.

There is a lot that happens before the immune system gets to the third line of defense. There are many cells and proteins that fight dangerous invaders in the nonspecific line of defense. When it gets to the third line of defense, antibodies are created which helps T and B cells target the enemy. These cells also remember this antigen the next time they see it and kill it even faster.

That's how vaccination helps. It introduces the pathogen to the immune system so that it knows what to do much faster come next time around. The third line of defense is strengthened and there is less chance of that pathogen causing an infection or disease.

By ysmina — On Jul 06, 2012

@burcinc-- I'm not an expert but I think that in an ideal environment, our immune system works perfectly.

Do you remember a cartoon series in the late nineties? It was a series where children would become tiny and enter the body to show how the body works. I specifically remember an episode which showed how our immune system fights germs to prevent and treat sickness. I have been amazed with this system ever since!

But this doesn't mean that our immune system doesn't weaken. I know for example, that stress is a huge factor that suppresses our immune system and prevents us from fighting infections as we normally would. When we're stressed, we release too much of a hormone called cortisol. This hormone gets in the way of other hormones that allow our immune system to remain strong and do its job correctly.

Under such conditions, pathogens can get through our first and second line of defense and develop into disease more easily.

By burcinc — On Jul 06, 2012

It seems like the immune system is well organized and has a great plan in terms of preventing illness. But still, we get sick so much!

There has rarely been a winter where I haven't caught a cold or the flu. My dad has diabetes, which I've heard is because of an infection as well. And almost everyone I know has had an infectious ailment of one kind or another in the past couple of years. Does this mean that our immune system is not working properly?

Because in order for me to be sick, the pathogen not only has to get through non-specific immunity but it has to get past the T cells too. I wonder if our defense system is not working as well as we think it is.

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