The Immune System: Innate Defenses and Adaptive Defenses

Summary

Professor 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

00:00

🛡️ 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.

05:06

🧬 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.

10:07

🔬 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.

Keywords

💡Immune System

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful pathogens such as bacteria and viruses. In the video, it is described as having two main components: the innate and adaptive defense systems, both of which are crucial for protecting the body from infections and diseases. The immune system's role is central to the video's theme of understanding how our bodies protect us from microscopic threats.

💡Innate Defense System

The innate defense system is the body's first line of defense against pathogens. It includes physical barriers like the skin and mucous membranes, as well as internal defenses such as antimicrobial proteins, phagocytes, and natural killer cells. The video explains that this system is always 'ready to go,' providing immediate yet non-specific protection against a wide range of invaders. It is highlighted as a critical component of the immune system's ability to withstand daily exposure to countless pathogens.

💡Adaptive Defense System

The adaptive defense system is a more sophisticated and specific response to pathogens. It involves the production of antibodies by lymphocytes, which can recognize and target specific invaders. The video describes how this system 'learns' from encounters with foreign substances, known as antigens, and retains a 'memory' of them for future immune responses. This concept is key to understanding the body's ability to mount more effective defenses against previously encountered pathogens.

💡Antibodies

Antibodies, also known as immunoglobulins, are Y-shaped proteins produced by lymphocytes that circulate in the blood and lymph. They play a central role in the adaptive immune response by binding to specific antigens, marking them for destruction by phagocytes. The video emphasizes the importance of antibodies in the humoral immune response, where they are produced in large quantities following an initial exposure to an antigen, providing a quicker and more effective defense upon subsequent exposures.

💡Antigens

Antigens are foreign substances that can trigger an immune response. They are typically large molecules or surface proteins found on pathogens that are recognized as 'non-self' by the immune system. The video explains that antigens provoke the production of antibodies, which are specific to each antigen. Understanding antigens is essential for grasping how the immune system differentiates between self and non-self and mounts targeted responses.

💡Phagocytes

Phagocytes are a type of white blood cell that can engulf and destroy pathogens, dead cells, and debris through a process called phagocytosis. The video describes macrophages as the most significant type of phagocytes, which play a critical role in the innate immune response by clearing invaders and contributing to the inflammatory response. Phagocytes are an essential part of the body's defense mechanism, as they help prevent the spread of infection.

💡Inflammatory Response

The inflammatory response is a protective mechanism of the innate immune system that occurs in response to injury or infection. It involves the release of chemicals like histamine and cytokines, leading to vasodilation and increased vascular permeability, which results in redness and swelling. The video explains that this response not only helps to remove foreign substances but also delivers proteins necessary for clotting and tissue repair. Inflammation, while sometimes uncomfortable, is a vital part of the body's healing process.

💡Lymphocytes

Lymphocytes are a type of white blood cell that plays a central role in the adaptive immune response. They include B cells, which produce antibodies, and T cells, which are involved in cell-mediated immunity. The video describes how lymphocytes originate from hematopoietic stem cells in the bone marrow and become immunocompetent, meaning they can recognize specific antigens. Lymphocytes are crucial for the body's ability to remember and respond more effectively to previously encountered pathogens.

💡Humoral Immune Response

The humoral immune response is a component of the adaptive immune system that involves the production of antibodies by B cells. The video explains that this response occurs when B cells encounter their specific antigen, leading to the proliferation and differentiation into plasma cells that produce large amounts of antibodies. This response is termed 'humoral' because it takes place in the body fluids such as blood and lymph. The humoral response is critical for neutralizing pathogens and toxins in the body's extracellular fluids.

💡Cellular Immune Response

The cellular immune response is another aspect of the adaptive immune system, primarily mediated by T cells. As described in the video, activated T cells can directly kill infected or cancerous cells. The cellular response is essential for dealing with intracellular pathogens, such as viruses, which live inside the body's cells. The video emphasizes the diversity and complexity of T cells, which include CD4+ T cells that help activate other immune cells and CD8+ T cells that directly destroy infected cells.

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.