Med-Surg Immune System, part 2: Acquired Immunity - Humoral and Cellular
by Cathy Parkes September 25, 2020 Updated: December 29, 2021 7 min read
In this article, we give a brief overview of how acquired immunity works, which is really important knowledge you need to have to understand Med-Surg and Critical Care topics in the immune system. These topics are covered in our Medical-Surgical Flashcards (Immune system), and Cathy’s video follows along with the cards.
Cathy noticed that most textbooks make this really confusing, so we broke it down to be as simple as possible.
Humoral immunity involves B cells creating antibodies and is a process that occurs mostly in the blood. Humoral immunity is so named because it takes place in the “humors,” which is what scientists thousands of years ago called body fluids. The more modern term is antibody-mediated immunity, but humoral has stuck around because it’s shorter.
B cell activation
An antigen binds to an antibody on a B cell. The antibodies act a little like a claw grabber arm, as you can see in the illustration.
The B cell then engulfs (swallows) the antigen, which is called endocytosis. The prefix endo- means inner, absorbing, or containing; and cytosis means cells.
So the B cell absorbs/engulfs the antigen, breaks it down into pieces, and presents those pieces on its own surface, like you would display a flag on the front of your house. To do this, the B cell uses major histocompatibility complex (MHC) II proteins.
The MHC II proteins act as the flagpole in that analogy, or like little golf tees, allowing the B cell to show off these pieces of the antigen. The B cell needs to alert other types of cells that this is what it’s doing, so that it can receive help. This antigen-presenting B-cell is now a complex.
B cell proliferation
A helper T cell, which is green in our illustration, notices this antigen-presenting B cell complex that’s asking for help, and they bind to the complex. The T cell releases cytokines, which are chemical messengers that prompt the B cell to start rapidly dividing into two other types of cells: plasma cells and memory cells.
The B cell rapidly divides into plasma cells, which produce and release antibodies that work against the specific antigen it has been working with. The plasma cells are working now to help the body heal.
The B cell also divides into memory cells. These memory cells don’t forget. They are going to recognize that antigen if it ever appears again, and wage war against it. The memory cells will work in the future to help the body heal.
The plasma cells release antibodies, and these antibodies are on the hunt for more of the same antigen. Once they find that antigen, they bind to it, and a reaction takes place which triggers neutralization and destruction of those antigens.
The other type of acquired immunity is called cellular immunity, or cell-mediated immunity. Though the common terms you will encounter are humoral and cellular immunity, it can be helpful to remember them as antibody-mediated vs. cell-mediated, so you know who’s running the show. Cellular immunity involves mainly T cells.
Initiation & activation
This cycle begins with a “naive” (virgin) T cell, one that has never encountered an antigen. When that naive T cell comes in contact with the aforementioned MHC antigen complex, it becomes activated. The T cell is ready to do its job.
Once the T cell is activated, it releases cytokines (chemical messengers) that prompt the proliferation and activation of other T cells, including: helper/inducer T cells, regulatory T cells, and cytotoxic T cells.
Helper/inducer T cells
Helper/inducer T cells are CD4+ cells, which you will learn a lot more about when we cover HIV/AIDS. CD4+ cells recognize self (part of your body) vs. non-self (not part of your body) cells. In response to non-self (foreign) cells, these CD4+ cells release cytokines that function as a call-to-arms to other white blood cells to destroy any non-self cells. When you think of CD4+ cells releasing cytokines, imagine a cell yelling through a megaphone, “I NEED BACKUP HERE!”
Regulatory T cells
Regulatory T cells prevent hypersensitivity or overreaction of the immune system by releasing cytokines that suppress the immune response. Imagine these cells over the loudspeaker repeating, “Please remain calm and move in an orderly fashion.”
Cytotoxic T cells
Cytotoxic T cells are CD8+ cells. These cells are aggressive, dyed-in-the-wool killers, but for good reason. Cyto refers to cell, and toxic means harmful, so these cells seek out and destroy dangerous cells, like infected, cancerous, and damaged cells.
Our Medical Terminology flashcards can help you break down and understand medical terminology that you come across in your studies!
Active vs. Passive & Natural vs. Artificial Immunity
Immunity can be either natural or artificial, and this reflects how the antigen got to your body. Natural immunity refers to antibodies created in response to a disease-causing agent. Artificial immunity refers to antibodies created in response to a deliberate exposure (like with a vaccine).
Immunity can be either active or passive, and this refers to whether or not your immune system made antibodies. Active immunity is when your own body creates antibodies. Passive immunity is when your body ends up with antibodies that it didn’t have to create.
|Types of Immunity||Active Immunity||Passive Immunity|
Natural active immunity: Your body produces antibodies (active) in exposure to a live pathogen (natural).
For example, you get chicken pox, and after your initial infection, you build immunity against the disease.
Natural passive immunity: You receive antibodies (passive) that were originally created in response to a disease-causing agent (natural).
This occurs when a mother passes immunity to a child through breast milk or the placenta.
Artificial active immunity: Your body produces antibodies (active) in exposure to a vaccine (artificial).
For example, you get a polio vaccine as a child and you build immunity against polio.
Artificial passive immunity: You receive antibodies (passive) that were created in exposure to a vaccine (artificial).
For example, if you are bit by an animal, you will get a rabies shot containing rabies antibodies so you don’t become infected with rabies.
Cathy’s teaching on the immune system is intended to help prepare you for Medical-Surgical nursing exams. The Medical-Surgical Nursing video series is intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI and NCLEX.
In this video we will be talking about acquired immunity. Acquired immunity includes both humoral immunity, as well as cell-mediated immunity. And based on the textbooks I've read, a lot of these texts just make a huge, complicated mess out of this topic. So I'm going to try to break it down Barney style and try to make it as easy as possible. So I will be bringing up an illustration from our Medical-Surgical Edition 2.0 deck. If you are following along with cards, this illustration is on card number four.
Okay. Let's go through the steps of humoral immunity using this illustration.
So first of all, we have an antigen that will bind to an antibody on a B cell. The B cell will engulf the antigen and present it on the surface of the cell by MHC II proteins.
Then this helper T cell here in green will bind to the complex and release cytokines, which are basically like chemical messengers. This will induce this B cell to rapidly divide into plasma cells, as well as memory cells.
These plasma cells will produce and release antibodies that will work against this specific antigen.
The memory cells will become sensitized. But they will not function until the next exposure to the antigen. At that time, when they are re-exposed, they will mount a large and rapid response to that antigen. So they're like, "Yeah. I'm waiting for you. You come around again and you're going to be in trouble." So they're ready to go next time the body encounters that same antigen.
So then we have these antibodies here, which will bind to the antigen. And with that binding, a reaction takes place. And that triggers neutralization and destruction of those antigens.
Now that we've gone through the steps of humoral immunity, let's talk about cellular immunity.
So when that naive T cell comes in contact with the MHC antigen complex, it becomes activated.
And it releases cytokines or those chemical messengers. And those messengers in turn allow for the proliferation and activation of a number of T cells. This includes other helper or inducer T cells, as well as regulatory T cells and cytotoxic T cells.
Let's go through each of those very briefly.
With helper or inducer T cells, these are CD4+ cells. And we'll definitely be talking more about CD4+ cells when we talk about HIV and AIDS. These cells recognize self versus non-self cells. And in response to non-self cells, these CD4+ cells will release cytokines that are basically a call to arms to other white blood cells to come and destroy this non-self cell.
Then we have regulatory T cells. These prevent hypersensitivity or overreaction of the immune system by releasing cytokines that suppress the immune response.
And then finally, we have cytotoxic T cells, which are CD8+ cells. These cells destroy infected, cancerous, or damaged cells.
Okay, now that you have a better understanding of humoral and cellular immunity, let's talk about the differences between active and passive immunity as well as natural and artificial immunity.
With active immunity, your body is actively producing antibodies. So with active natural immunity, your body produces antibodies in response to exposure to a live pathogen.
If you have active artificial immunity, this is where your body produces antibodies in response to exposure to a vaccine.
Then we have passive immunity. So with passive immunity, your body is not making these antibodies. These antibodies are given to you.
So we have passive natural immunity. This is where mom passes antibodies to her baby through the placenta or through the breast milk. So the baby didn't have to make these antibodies. They were a gift from mom.
Then we have passive artificial immunity. This is where a patient is administered immunoglobulins. So they are given antibodies to help them fight an infection. Because their body didn't create these antibodies, there will be no memory cells available if they are exposed to that infection again in the future. However, the antibodies they get through these immunoglobulins will help them fight the current infection.
So hopefully, that's a helpful explanation to help you differentiate these different types of immunity. In my next video, we will be going over immune system malfunction as well as infection. Thanks so much for watching!
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