The Immune System: Innate Defenses and Adaptive Defenses
Summary
TLDRProfessor Dave's video script delves into the human immune system's dual defense mechanisms: innate and adaptive. The innate system, our first line of defense, includes physical barriers like skin and mucous membranes, along with internal defenses such as antimicrobial proteins and phagocytes. The adaptive system, more sophisticated, involves antibodies and lymphocytes that target specific pathogens. The script explains how these systems collaborate to maintain our health, detailing processes like inflammation, phagocytosis, and the generation of antibodies for a tailored immune response.
Takeaways
- 🛡️ The immune system is our body's defense against pathogens like bacteria and viruses.
- 🔁 It consists of two main parts: the innate defense system and the adaptive defense system.
- 🧬 The innate defense system provides immediate, non-specific responses to pathogens, including physical barriers like skin and mucous membranes.
- 🔬 Internal innate defenses include antimicrobial proteins, phagocytes, and inflammatory responses to infection.
- 🌡️ Inflammation is a key part of the innate immune response, involving the release of chemicals like histamine and cytokines.
- 👨🔬 Phagocytes, such as macrophages and natural killer cells, engulf and destroy pathogens and infected cells.
- 🧬 The adaptive defense system is more sophisticated, involving specific responses to different pathogens through antibodies.
- 💉 Antibodies are Y-shaped proteins that can mark pathogens for destruction by phagocytes.
- 🌐 Antigens are foreign substances that trigger an adaptive immune response, and their determinants are what antibodies bind to.
- 📈 There are two types of adaptive immune responses: humoral (involving antibodies) and cellular (involving T cells).
- 💊 Vaccines can provide active humoral immunity by prompting an immune response to an inactivated pathogen.
Q & A
What are the two main parts of the human immune system?
-The human immune system can be split into two parts: the innate defense system and the adaptive defense system.
How do surface barriers like skin and mucosae protect the body from pathogens?
-Surface barriers such as skin and mucosae protect the body by acting as physical barriers that block pathogens from entering. The skin is tough to penetrate when unbroken, and mucosae are often acidic, have lysozymes, and can trap microorganisms in mucus, inhibiting their entry.
What is the role of phagocytes in the innate immune response?
-Phagocytes play a crucial role in the innate immune response by engulfing and digesting pathogens or debris through a process called phagocytosis, preventing their spread within the body.
How does the inflammatory response contribute to the immune system?
-The inflammatory response contributes to the immune system by increasing blood flow to the affected area, causing vasodilation and vascular permeability, which allows for the removal of foreign substances and aids in the delivery of clotting proteins for tissue repair.
What are antibodies and how do they function in the adaptive immune system?
-Antibodies are Y-shaped proteins produced by lymphocytes that circulate in the blood and lymph, marking pathogens for destruction by phagocytes. They recognize and bind to specific antigens, facilitating the immune system's targeted response.
What is the difference between the humoral and cellular immune responses?
-The humoral immune response involves B cells producing antibodies to neutralize pathogens, while the cellular immune response involves T cells that can directly kill infected or abnormal cells, such as those infected by viruses or cancer cells.
How does the adaptive immune system develop a memory for specific antigens?
-The adaptive immune system develops a memory for specific antigens through the proliferation of lymphocytes that recognize those antigens. Once a lymphocyte is committed to a particular antigen, it produces thousands of surface receptors for that antigen, allowing for a rapid and specific response upon future encounters.
What is the primary immune response and how does it differ from the secondary immune response?
-The primary immune response is the initial production of antibodies by B cells upon first encountering an antigen, which takes a few days. The secondary immune response is a faster and more effective response that occurs if the same antigen is encountered again, due to the presence of pre-existing antibodies and memory cells.
How do vaccines contribute to the humoral immunity?
-Vaccines contribute to the humoral immunity by introducing an inactive form of a pathogen, which allows the primary immune response to occur without causing the disease. This primes the immune system so that if the real pathogen is encountered, the immune system is already prepared to respond quickly and effectively.
What are the two major types of T cells and what is their role in the immune system?
-The two major types of T cells are CD4 and CD8. CD4 T cells, also known as helper T cells, activate B cells, T cells, and macrophages, while CD8 T cells, also known as cytotoxic T cells, destroy body cells that have been infected by viruses or bacteria, as well as cancer cells.
How do antigen-presenting cells (APCs) participate in the activation of T cells?
-Antigen-presenting cells participate in the activation of T cells by presenting antigens on their surface. This interaction with T cell antigen receptors, along with co-stimulation from other molecules on the APC surface, leads to T cell activation, proliferation, and differentiation.
Outlines
🛡️ Innate and Adaptive Defense Systems
The immune system is our body's defense mechanism against pathogens. It is divided into two main parts: the innate defense system, which provides immediate but non-specific responses, and the adaptive defense system, which offers a tailored response to specific threats. The innate system includes physical barriers like skin and mucous membranes, as well as internal defenses such as antimicrobial proteins and phagocytes. Phagocytes, including macrophages and natural killer cells, engulf and destroy pathogens. The adaptive system involves antibodies produced by lymphocytes, which can specifically recognize and mark pathogens for destruction. This system also includes the humoral and cellular immune responses, with the former focusing on antibody production and the latter involving T cells that can directly kill infected cells.
🧬 Adaptive Immunity and Antigens
Adaptive immunity is the immune system's ability to 'remember' and respond more effectively to previously encountered pathogens. This is facilitated by antibodies, which are Y-shaped proteins produced by lymphocytes. Antigens are foreign substances that trigger an adaptive immune response, and they are recognized by specific antigenic determinants. Lymphocytes, originating from hematopoietic stem cells in the bone marrow, can become B cells with membrane-bound antibodies or T cells that can directly kill infected cells. The adaptive immune response involves the humoral response, where B cells produce antibodies, and the cellular response, where T cells target infected cells. Vaccines can stimulate the production of antibodies without causing disease, providing immunity against future infections.
🔬 Structure and Function of Antibodies
Antibodies, also known as immunoglobulins, are large proteins composed of four polypeptide chains. They have a Y-shaped structure with two identical halves, each with a variable (V) region that binds to specific antigens and a constant (C) region. The V regions are responsible for antigen recognition, while the C regions determine the antibody's function. There are five classes of antibodies, each with distinct roles and locations within the body. Antibodies tag antigens for destruction by other immune cells. The cellular immune response, involving T cells, is also discussed, with CD4 and CD8 T cells playing different roles in activating other immune cells or killing infected cells, respectively.
Mindmap
Keywords
💡Immune System
💡Innate Defense System
💡Adaptive Defense System
💡Antibodies
💡Antigens
💡Phagocytes
💡Inflammatory Response
💡Lymphocytes
💡Humoral Immune Response
💡Cellular Immune Response
Highlights
The immune system protects us from countless pathogens that could interfere with our cellular activity.
The immune system is divided into innate and adaptive defense systems.
Innate defense begins with external barriers like skin and mucous membranes.
Internal innate defenses include antimicrobial proteins, phagocytes, and other entities.
The adaptive defense system is sophisticated and involves antibodies for specific responses.
Innate and adaptive systems communicate and work together for overall immunity.
Surface barriers like keratinized epithelial cells prevent pathogen entry.
Mucosal barriers are acidic and contain lysozymes to inhibit bacterial growth.
Inflammatory response is a hallmark of internal innate defenses.
Phagocytes perform phagocytosis to engulf and destroy pathogens.
Macrophages and natural killer cells are key players in the immune response.
Inflammation involves the release of chemicals like histamine and cytokines.
Phagocytes rush to the site of injury during inflammation to eliminate pathogens.
Antibodies are Y-shaped proteins that mark pathogens for destruction.
Antigens are foreign substances that provoke an adaptive immune response.
Lymphocytes recognize specific antigens and develop immunity against them.
Humoral immunity involves B cells producing antibodies in response to antigens.
Cellular immunity is mediated by T cells that can kill infected or cancerous cells.
T cells differentiate into CD4 and CD8 types, each with distinct roles in immunity.
Vaccines provide active humoral immunity by simulating the primary immune response.
Antibodies have a Y shape with variable regions that bind to specific antigens.
There are five classes of antibodies, each with unique roles and locations in the body.
Transcripts
It’s Professor Dave, I wanna tell you about the immune system.
Every single day, in everything we do, we are coming into contact with countless pathogens.
These are bacteria and viruses that could potentially do great harm to us macroscopic
beings by interfering with our cellular activity.
How do we withstand these microscopic threats?
Fortunately, every human possesses an immune system that is well-equipped to protect us,
so let’s take a look at how it works.
The immune system can be split up into two parts.
There is the innate defense system, and the adaptive defense system.
The innate is the part that is always ready to go.
It begins with external membranes, like the skin and a variety of mucous membranes.
This separates what’s inside from what’s outside, but of course, there are lots of
ways for pathogens to get past this barrier.
That’s why we have internal defenses like antimicrobial proteins, phagocytes, and other
entities that are able to inhibit the spread of the invaders throughout the body.
Then there is the adaptive defense system.
This is much more sophisticated, as it involves a response that is specific to the type of
invader, made possible by things called antibodies, which we will get to a little later.
These two systems communicate and work together to keep us healthy and safe every day.
Let’s start by taking a closer look at the innate defenses, starting with the surface
barriers, the skin and mucosae.
These are very effective at blocking pathogens from entering the body.
Epithelial cells on the surface are highly keratinized, so as long as it is unbroken,
it’s tough to get through.
That’s why we can easily get infections when we have cuts on the skin, because pathogens
can suddenly get in that way.
Wherever we have natural openings and body cavities, these are lined with mucosae that
have important features.
They tend to be acidic, which inhibits bacterial growth.
Many of them have lysozymes, which destroy bacteria.
If mucus lines a particular passageway, microorganisms tend to get stuck there.
We can even find defensins, which are antimicrobial peptides.
But of course, no matter how effective these are, some pathogens will get through.
That’s where the internal innate defenses come into play.
The hallmark of this system is the inflammatory response.
Before we go through that, let’s mention phagocytes, which can perform phagocytosis.
This is when a cell engulfs some pathogen or other debris, and it sits inside in a vesicle.
This vesicle will merge with a lysosome, which has acid hydrolase enzymes that can digest
whatever is nearby, leaving it in tiny pieces.
These pieces then leave the cell by exocytosis, unable to do any harm.
The biggest and best phagocytes are macrophages, which are derived from white blood cells.
There are also natural killer cells which circulate in blood and lymph that can kill
cancer cells and virus-infected cells early on, simply by detecting certain abnormalities
in the cell, and inducing apoptosis in the cell, which is programmed cell death.
Phagocytes are also part of the inflammatory response.
This occurs when body tissues are injured in some way, by physical trauma, heat, or infection.
This begins with the release of inflammatory chemicals like histamine into the extracellular fluid.
In addition, macrophages as well as cells of certain boundary tissues have receptors
that enable them to recognize pathogens, sometimes with great specificity, and this kind of event
will trigger a release of cytokines, which are another type of inflammatory chemical.
What these chemicals do, is they cause local arterioles to dilate, and nearby capillaries
to leak slightly, otherwise known as vasodilation and vascular permeability.
The excess of blood in the area causes the redness and swelling that we can visibly see
when a part of the body is inflamed.
Although this generates pain because of the pressure on nearby nerve endings, it is a
favorable strategy, because the rush of fluid sweeps any foreign material into lymphatic
vessels, so that it can be broken down in the lymph nodes, and the fluid also delivers
proteins that are important for clotting to aid in repair.
Once inflammation has initiated, phagocytes then rush the scene, first neutrophils, and
then macrophages soon after.
This begins when phagocytes enter the bloodstream from the red bone marrow, so they can get
to the injury.
Then in margination, they cling to capillary walls at the site of injury, recognizing molecular
signals on inflamed cells.
In diapedesis, they squeeze out of the capillary.
Chemotaxis will then occur, where phagocytes migrate up the gradient of certain molecules
that act as a homing device for the site of injury, ready to eat up any intruders.
So that covers the basics regarding the innate defenses.
So what about the adaptive defenses?
This is the part of the immune system that can learn about any foreign substance it comes
into contact with, which we call antigens, and develop the ability to protect the body
from that specific antigen any time in the future.
But how can your immune system have such an incredible memory?
And what does this even mean?
To understand this, we have to learn about antibodies.
These are large Y-shaped proteins that are produced by lymphocytes, and they circulate
in the blood and lymph, looking for pathogens, which they are able to mark such that phagocytes
can recognize them for destruction.
So what are these antigens?
The word antigen is derived from the phrase “antibody generating”, so the word refers
to any foreign substance that will be recognized as being not of the self, and will thus provoke
an adaptive immune response.
These can be proteins, polysaccharides, lipids, any large molecule that doesn’t belong,
as well as many pathogens, as these will bear foreign surface proteins that can also be
recognized.
The part of the foreign substance that interacts with the immune system is called the antigenic
determinant.
An antibody or lymphocyte will bind in a way that resembles enzyme-substrate interactions,
and different lymphocytes will recognize different determinants.
These include B lymphocytes or T lymphocytes, depending on what type of immunity they oversee,
and there are also antigen-presenting cells.
Lymphocytes of either variety originate inside red bone marrow, from hematopoietic stem cells.
They then become immunocompetent, meaning they gain the ability to recognize a particular
antigen, and once committed to a particular antigen, thousands of surface receptors are
produced that are devoted to that recognition.
On B cells these receptors are actually membrane-bound antibodies.
These lymphocytes must recognize certain proteins, but also learn self-tolerance, meaning they
must not attack the body itself.
Cells that fail to do this are forced to undergo apoptosis, and in fact only about two percent
of T cells will make it, but the ones that do will rapidly divide to make many copies
of itself, all with the same antigen recognition.
Now let’s talk about two types of adaptive immune response.
There is the humoral immune response, and the cellular immune response.
The humoral immune response occurs when a new B cell encounters its antigen, which causes
endocytosis, followed by proliferation and differentiation into plasma cells.
These will then mass produce the antibody that recognized the antigen, and these will
circulate in the blood and lymph, looking for that same thing again.
This is referred to as the primary immune response, and it takes a few days to make
all those antibodies, which is one drawback to this defense strategy, since it protects
against future invasions, but it can’t act so quickly upon the initial invasion.
However, if that antigen does come back, the secondary immune response begins, and this
will be swift and effective, with plenty of antibodies to tag the antigen for destruction.
This kind of humoral immunity can be attained naturally, through infection, or artificially,
with vaccines, which allow for the primary immune response to occur with an inactive
form of a pathogen, so that if the real thing ever comes by, the immune system is already
ready for it.
More on vaccines at another time.
In either case, we are describing active humoral immunity.
Passive humoral immunity is different because the body doesn’t go through the work of
recognizing an antigen and generating antibodies, instead these antibodies can be introduced
directly into the body, either through a mother’s milk, or through injection of gamma globulin.
Let’s zoom in on an antibody for a closer look.
As we said, these are large proteins, and they consist of four polypeptide chains connected
by disulfide bridges.
The two halves of the Y shape are identical.
There are two heavy chains, and two light chains, and a hinge region where the kink occurs.
Each chain has a C region, which is always almost the same, and a V region, which changes
shape depending on which antigen it will recognize, and this region is at the tip of the Y arms,
which we call the antigen-binding site.
There are five classes of antibody, listed here, where Ig stands for immunoglobulin,
another name for antibody, followed by M, A, D, G, or E. These have different roles
and locations.
Some of these are monomers, some are dimers, some are even pentamers, depending on how
many antibodies come together.
But in any case, antibodies tag their specific antigen when they find it, so that it can
be destroyed later.
We will get more specific about this process when we look at particular infectious diseases.
For now let’s continue on and switch over to the cellular immune response.
Here we will look at T cells.
These operate a bit differently, as activated T cells have the ability to kill cells of
the body that have been infected by viruses or bacteria, as well as cancer cells.
This cells are more diverse and complex than B cells, but they come in two major types,
CD4, and CD8, which refer to glycoproteins that act as surface receptors, though they
differ from antigen receptors, rather they interact with other cells.
CD4 cells activate B cells, T cells, and macrophages, while CD8 cells destroy foreign cells, or
body cells with foreign agents.
T cells undergo activation and differentiation when T cell antigen receptors interact with
antigen-presenting cells.
Then, co-stimulation must occur from other molecules on the surface of the antigen-presenting cell.
This leads to proliferation and differentiation.
The resulting T cells can be of a wide variety, and we will examine these types at a later time.
For now, we should simply understand the differences between innate defenses and adaptive defenses,
as well as humoral immunity and cellular immunity.
With that covered, let’s move forward and finish up with a few more systems of the human body.
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