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Lesson Video: Components of the Human Immune System Biology

In this video, we will learn how to describe the structure and function of the organs, cells, and chemicals of the immune system.

14:14

Video Transcript

In this video, we will learn about the organs of the immune system. We will discover where they are located and their different functions. We will describe the roles of the different cells of the immune system and learn about the chemicals they produce. We will also recall that immunity in humans can be divided into two categories: innate or nonspecific immunity and adaptive or specific immunity.

We must constantly defend ourselves against unwelcome intruders. Potentially deadly pathogens like bacteria and viruses can be found in the air we breathe, the food we eat, or gain entry through cuts in our skin. Abnormal body cells, which in some cases may develop into cancer, must also be dealt with. The cells and organs of our immune system stop us from getting constantly sick by detecting potential threats and responding to them in order to protect our bodies.

The immune system is a collection of organs, tissues, cells, and chemicals with the collaborative role of preventing illness. We can divide the immune system into three lines of defense. The first line of defense is external and made up of the epithelial tissues that cover our bodies and line our organs, along with the secretions they produce. The skin forms part of this first line of defense. The second line of defense is internal, involving white blood cells called phagocytes, which attack invading pathogens that have managed to breach the first line of defense. The third line of defense is the specific immune response. This tackles pathogens simultaneously with the second line of defense. However, particular microorganisms are responded to in a particular way.

We described the first two lines of defense as nonspecific, since any foreign invaders are responded to in the same way. The third line of defense is specific and includes the production of proteins called antibodies, specifically produced to target a particular pathogen or toxin.

Now let’s look at the main organs of the immune system, which is shown in this diagram. Let’s find out more about each organ in turn, starting with the lymph nodes. The lymph nodes are organs in the lymphatic system. The lymphatic system is a network of vessels closely linked to the circulatory system, shown in our diagram in green. The vessels carry lymph, a fluid formed from tissue fluid, which surrounds cells and tissues. Tissue fluid is made from substances which leave the blood through the capillary wall, for example, water, oxygen, and nutrients.

Cells take in useful substances from the tissue fluid and release metabolic waste into it. Any excess tissue fluid is drained into the lymphatic system, where it becomes lymph by name. Unlike the blood, lymph is not pumped. Instead, it moves passively as the lymphatic vessels are compressed during movement by the contraction of the muscles surrounding them. Lymph nodes are the filtering and storage areas of the system. They’re organs found at junctures throughout the lymphatic system, ranging in size from the point of a pinhead to the size of a bean. They’re commonly found in areas such as the armpits, neck, groin, and chest. Lymph nodes are filled with white blood cells specialized for defense. When a pathogen is detected, the white blood cells in the lymph nodes multiply, preparing to fight the pathogen. This explains why swollen lymph nodes can be a sign of an infection.

Now let’s move on to our next organ, the bone marrow. Bone marrow is a type of tissue that fills the central cavity of our bones. There are two types of bone marrow: red bone marrow and yellow bone marrow. The red bone marrow contains hematopoietic stem cells, which are unspecialized cells that can differentiate to form different types of specialized blood cell. The red bone marrow is where new red blood cells and white blood cells are formed. It produces around 200 billion new red blood cells every day. Yellow bone marrow is involved in the storage of fats.

Lymphocytes, a type of white blood cell, originate from stem cells in the bone marrow. One subgroup of lymphocytes, called B lymphocytes, remain in the bone marrow to mature. The other, T lymphocytes, migrate to the thymus gland to complete their maturation. The T stands for thymus. So what is the thymus gland? The thymus is an organ found just behind the sternum between the lungs in the upper chest cavity. As well as being the location for the maturation of T lymphocytes, the thymus gland produces hormones in its role as an endocrine gland.

Now let’s learn about the spleen, the largest lymph organ. The spleen is located in the upper-left side of the abdomen. It functions like a large lymph node. It is a major site for blood filtering, removing old and damaged red blood cells so they can be broken down. It also acts as a store for some types of white blood cells, which can recognize antigens on the surface of pathogens and destroy them. So it has an important role in the immune response.

Now let’s learn about the tonsils. Tonsils are groups of lymphatic tissues found in the mouth at the back of the throat and close to the entrance of the nasal cavity. They help to detect pathogens we might inhale or ingest and can alert the immune system if they detect a potential threat.

Let’s move on to the appendix. The appendix is a small tube of tissue that projects from the lower-right side of the large intestine. Previously, the appendix was thought to be a largely redundant organ. However, we now know the appendix has immune functions, mostly during childhood and early adulthood. It contains lymphoid tissue, which develops during childhood and gradually diminishes with age. The appendix assists with the maturation of white blood cells, the production of immune proteins called antibodies, along with other aspects of the immune response.

Peyer’s patches are located in the small intestine. They’re clusters of immune cells called lymphoid follicles. They help to protect us against bacteria we may have ingested in our food. They also have a role in monitoring and regulating the populations of resident bacteria in the small intestine, which help with the digestion of food.

In summary, the organs of the immune system work together to protect us from pathogens. The bone marrow and the thymus are the primary sites for the development of white blood cells. The remaining organs are primarily involved in detecting and trapping foreign particles and pathogens.

Now let’s move on to talk about some of the important cells of the immune system. We’ve already seen how white blood cells are the key players in the defense against pathogens. There are many different types of white blood cells. They each have different roles and are often distinguished by the type of receptor found on their surface. So let’s start by looking at the T lymphocytes in more detail.

T lymphocytes mature in the thymus, where they differentiate into three different cell types: T cytotoxic cells, helper T cells, and suppressor T cells. T cytotoxic cells or killer T cells are distinguished by their expression of the CD8 cell surface receptor. T cytotoxic cells kill infected or abnormal cells, for example, cells infected with a virus or those which are dividing too rapidly, as this could lead to cancer. Helper T cells express the CD4 cell surface receptor. These cells activate other types of T cells and stimulate B lymphocytes to produce antibodies. Suppressor T cells also express the CD4 cell surface receptor. These cells help to suppress or regulate the function of other immune cells. They prevent the immune system from attacking self-cells, therefore helping to prevent autoimmune disease. They also help to switch off immune cells after a pathogen has been eliminated.

Unlike the T lymphocytes, B lymphocytes remain in the bone marrow to complete their maturation. B lymphocytes produce antibodies, which attach to and destroy pathogens. NK cells, or natural killer cells, are a type of lymphocyte that destroy abnormal cells, those infected with a virus, and cancerous cells. Red blood cells and white blood cells are all produced from stem cells in the red bone marrow. These hematopoietic stem cells first develop into either a common myeloid progenitor or a common lymphoid progenitor, shown here on the two different branches. The common lymphoid progenitor goes on to differentiate to form some of those white blood cells we’ve already discussed, natural killer cells, T lymphocytes, and B lymphocytes, which can specialize further to become antibody-secreting plasma cells. The common myeloid progenitor develops into red blood cells along with a number of different white blood cells, some of which are shown here on the diagram.

Eosinophils, basophils, neutrophils, and macrophages are all different types of white blood cell that have the ability to either engulf foreign pathogens in the body or attack parasites. This engulfing of pathogens is called phagocytosis. The white blood cell can surround, engulf, and digest the pathogen, therefore destroying it.

We can divide immunity into two categories: cell-mediated immunity, which relies on the function of cells, and humoral immunity, which relies on the function of chemicals. We’ve already seen how antibodies, globular proteins made by B lymphocytes, attach to pathogens and destroy them.

Cytokines are signaling chemicals, which enable immune cells to communicate. There are many different types of cytokines made by many different cells with many different functions. Interleukins are one example. Inter- means between, and leukins refers to leukocytes, another term for white blood cells. Some of the roles of interleukins involve activating B cells and T cells, stimulating growth, differentiation, and movement.

The human immune system is often divided into two categories. These are innate immunity and adaptive or acquired immunity. Innate immunity is immunity that you were born with. It is nonspecific, which means it fights all infections in the same way. It responds to infections quickly, and it prevents entry of pathogens or acts as a barrier. For example, the skin, mucous membranes, and phagocytic cells are all part of the innate immune system. In contrast, the adaptive immune system develops over time. It is specific, so each pathogen is dealt with in a particular way, and the response is slower. Cytotoxic cells and antibodies both form part of the adaptive immune system.

Let’s see how much we’ve learnt about the components of the immune system by having a go at a practice question.

Where do all white blood cells originate?

The question is asking us where in the body white blood cells come from. White blood cells have a variety of roles in the immune system, including engulfing and digesting harmful pathogens and producing antibodies. You may be familiar with the fact that all white blood cells and red blood cells develop from stem cells. Hematopoietic stem cells can differentiate to form all types of blood cells. Hematopoietic stem cells are found in the red bone morrow. These differentiate to form the many different types of white blood cells, including different types of lymphocytes and a range of phagocytic cells. The B lymphocytes remain in the bone marrow to mature, while the T lymphocytes migrate from the bone marrow to the thymus gland to complete their maturation. All other white blood cells, for example, the phagocytic cells, macrophages, and neutrophils, also develop from hematopoietic stem cells in the red bone marrow.

So the answer to the question “Where do all white blood cells originate?” is bone morrow.

Let’s review the key points covered during this lesson video. The immune system organs are the bone marrow and thymus, which are the primary sites of the development of white blood cells, and the spleen, lymph nodes, Peyer’s patches, appendix, and tonsils, with the main role of detecting and trapping foreign particles and pathogens. The main cells of the immune system include T lymphocytes, which differentiate to form T cytotoxic cells, which kill infected or abnormal cells; helper T cells, which activate other immune cells; and suppressor T cells, which regulate the immune response.

B lymphocytes produce antibodies, which attach to and destroy specific pathogens. Other immune cells include phagocytes, such as neutrophils and macrophages. These destroy pathogens by engulfing them and digesting them. Along with antibodies, cytokines are another example of a chemical produced by cells of the immune system. Cytokines, for example, interleukin, act as signaling chemicals, allowing the cells of the immune system to communicate. Immunity in humans can be divided into innate immunity, which is nonspecific, and adaptive immunity, which is specific.

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