Lesson Explainer: Components of the Human Immune System Biology

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

We come into contact with pathogens every day, from a small cut that may become infected to airborne viruses in public spaces. How do we not constantly end up sick? The answer is our hardworking and adaptable immune systems. In order to prevent infection and fight illness, the immune system must be able to detect potential danger and also respond to protect the body.

A pathogen is a biological agent that causes illness or disease. Pathogens include infectious organisms like bacteria, fungi, and parasites and nonliving entities such as viruses or prions. The organism being infected is referred to as the “host.”

Definition: Pathogen

A pathogen is a biological agent that causes illness or disease.

The immune system is a collection of organs that work together to perform the function of preventing illness. The immune system works by preventing entry of pathogens or destroying and removing pathogens that have entered the body.

Definition: Immune System

The immune system is a collection of organs, tissues, cells, and chemicals that work together to perform the function of preventing illness.

Since one organ or tissue can have several functions, many of the immune organs are also parts of different organ systems. For example, the skin is an important immune organ because it prevents pathogens from entering our bodies and serves in the so-called “first line of defense.” The blood is a special kind of connective tissue that plays an important role in transporting immune cells from place to place. Even our bones can be considered a part of the immune system because immune cells, called white blood cells, are formed inside the bone marrow. A diagram illustrating the organs of the immune system is shown in Figure 1.

Figure 1: A diagram showing the locations of the various immune system organs.

Let’s review some of these immune organs in more detail. We will discuss the lymph nodes, bone marrow, thymus, spleen, Peyer’s patches, and tonsils.

Lymph nodes are organs in what is commonly called the lymphatic system. The lymphatic system largely mirrors the circulatory system. However, unlike the circulatory system, the lymphatic system does not have a central pump like the heart and is an open system of vessels. The lymph vessels, illustrated in Figure 2, collect fluid from the tissues of the body that has been filtered out of the blood through the capillaries. They also act as transport vessels for immune cells.

Figure 2: This diagram shows how, as blood flows through the body tissue, fluids exit the capillaries to be reabsorbed by the lymphatic vessels and carried to the lymph nodes.

The lymph nodes are organs found at junctures throughout the lymphatic system. They can range in size from the point of a pinhead to the size of a bean. Lymph nodes are scattered throughout the body, but they are particularly common in the trunk area such as in the armpits, neck, chest, and groin.

The lymph nodes are where mature immune cells “live.” As the blood circulates, fluids containing nutrients and other important materials leave the blood vessels to enter the tissues. The lymph vessels collect this fluid, also called lymph, from the tissues of the body and transport it to the lymph nodes that return it to the bloodstream. The lymph nodes also filter the blood in order to detect the first signs of spreading infection.

Since they are home to many immune cells, the lymph nodes can quickly initiate a response to an invading pathogen. Sometimes one of the physical signs of an infection is swollen lymph nodes near where a pathogen has entered the body. This is because the lymph nodes may grow in size as immune cells divide in preparation for fighting a pathogen. A diagram illustrating the structure of a lymph node is shown in Figure 3.

Figure 3: A diagram showing the general structure of a lymph node. Lymph fluid and blood flow through the lymph node that allows them to detect infections and help coordinate an immune response.

Key Term: Lymph Node

A lymph node is a small organ found at junctures throughout the lymphatic system. Lymph nodes house mature and immature immune cells and filter the lymph fluid.

Example 1: Recalling the Cells Found inside a Lymph Node

The figure shows a cross section of a lymph node.

What cells can be found inside the structure?

  1. Lymphocytes, macrophages, and other white blood cells
  2. Only T lymphocytes in their final stages of maturation
  3. Stem cells that develop into all types of blood cells
  4. Aging red blood cells and their breakdown products


Lymph nodes are an important component of the immune system. Lymph nodes are organs in the lymphatic system, which consists of an open system of vessels called lymph vessels that collect lymph fluids from the tissues of the body and carry them to the lymph nodes.

Lymph nodes are small organs found at the juncture of several lymph vessels. They are especially numerous in the joints like the armpits and groin. The lymph passes through the lymph nodes where it is returned to the bloodstream. It is important that the rate of fluid leaving the bloodstream is equal to the rate of fluid entering it from the lymphatic system.

The lymph nodes are important immune organs. They are a site that comes into contact with large volumes of lymph fluid and blood on a regular basis. They are important in detecting the signs of infection and helping to coordinate an immune response.

All of the major immune organs, like the lymph nodes, are home to immune cells. The lymph nodes house many types of immune cells including B lymphocytes, T lymphocytes, macrophages, and other white blood cells.

Because of their function in monitoring the circulating fluids as well as their function of being the location of many types of immune cells, lymph nodes are able to help coordinate and carry out the immune response to a pathogen once it has been detected in the body.

For this reason, the cells that can be found in the lymph node are lymphocytes, macrophages, and other white blood cells.

Bone marrow is a type of tissue found inside our bones. There are two types of bone marrow, red and yellow. Red bone marrow is found inside flat bones like the ribs and pelvis as well as in the ends of long bones like the femur or thigh bone. Yellow bone marrow is mostly made of fat. Red bone marrow is where new red blood cells and white blood cells are made. The blood cells come from stem cells that differentiate into progenitor cells. Stem cells are special cells that can mature into many different types of cells and can reproduce indefinitely. Progenitor cells can differentiate into fewer types of cells than stem cells and can multiply a limited number of times.

Key Term: Red Bone Marrow

Red bone marrow is tissue found inside flat bones and in the ends of long bones that generates red and white blood cells.

The thymus is an organ found near the trachea just behind the sternum (or breastbone), between the lungs in the upper chest cavity. The thymus is where certain immune cells mature and differentiate. These cells are given the name T cells or T lymphocytes, which is short for “thymus cell lymphocytes.” The thymus also has endocrine functions, as it secretes thymic hormones such as thymosin directly into the bloodstream. These thymic hormones help to regulate the development of T cells and to stimulate the production of antibodies from B cells.

Key Term: Thymus

The thymus is an organ in the chest cavity. It has both endocrine and immune functions.

Example 2: Recalling the Maturation Site of T Lymphocytes

Where do precursor T lymphocytes go to complete their maturation?

  1. Thymus
  2. Liver
  3. Bone marrow
  4. Lymph nodes
  5. Spleen


All white blood cells originate in the red bone marrow found in flat bones like the hips and in the ends of long bones like the femur or thigh bone. This bone marrow is home to special cells called stem cells. The stem cells in the bone marrow can differentiate into two types of progenitor cells, and these progenitor cells differentiate into the various cells of the blood and of the immune system.

At a certain stage of their development, certain types of white blood cells will leave the bone marrow and travel to another location to complete their maturation process. Some cells remain in the bone marrow to mature, some travel to the thymus, and some mature in the spleen or the lymph nodes.

The thymus is an organ in the upper chest cavity, near the trachea and between the lungs. Lymphocytes that mature in the thymus are known as T lymphocytes, or T cells for short. T cells get their name because they are a type of lymphocyte that complete their maturation in the thymus.

Therefore, we know that precursor T lymphocytes go to the thymus to complete their maturation.

The spleen is a flat dark-red organ located on the left side of the body, near the stomach. It is about the size of the palm of your hand. The spleen functions similar to a large lymph node. It filters the blood, removing old and damaged red blood cells to be broken down. It also contains many white blood cells that can recognize antigens on the surface of foreign cells to initiate the adaptive immune response and eliminate pathogens. This allows the spleen to act as a communication hub for the immune system response.

Key Term: Spleen

The spleen is an organ in the abdominal cavity that functions in the lymphatic system and also removes old red blood cells from circulation and breaks them down.

The tonsils and the Peyer’s patches are both immune structures associated with the digestive system. The tonsils are two organs found on either side of the back of the throat. They are like sentinels positioned at the entrance to the respiratory and digestive systems. The tonsils sample the food we eat and the air we breathe in order to detect potential pathogens. They serve as an early detection system, alerting the rest of the immune system of any potential threats. The location of the tonsils in the throat is shown in Figure 4.

Figure 4: This diagram shows the location of the tonsils in the back of the throat. Tonsils are not very visible unless they become swollen due to infection.

Key Term: Tonsils

Tonsils are immune organs in the throat that detect pathogens in the air we breathe and the food we eat.

Peyer’s patches are clusters of immune cells, called lymphoid follicles, located in the lower portion of the small intestine. The small intestine is populated by a large, diverse community of bacteria that aid in the digestion of food. The Peyer’s patches monitor these bacteria to ensure they remain balanced and under control. The Peyer’s patches also detect harmful bacteria that may have entered the small intestine.

Key Term: Peyer’s Patches

Peyer’s patches are clusters of immune cells located in the lower small intestine.

All the organs of the immune system work together to keep the body safe from pathogens by detecting, or noticing, when something is wrong and working together to coordinate an appropriate response. These organs are each quite different, but they share the feature of housing the immune cells that do most of the work in fighting pathogens.

The immune cells are called white blood cells. They get their name as a matter of contrast with the red blood cells that have a distinctive color that comes from hemoglobin.

There are several different types of white blood cells that each play an important role in the immune system. These cells include lymphocytes, such as T cells, B cells, and natural killer (NK) cells, as well as neutrophils, basophils, eosinophils, and macrophages.

T lymphocytes, also simply called T cells, are lymphocytes that mature in the thymus. There, they differentiate into three different cell types. These cell types are helper T cells, cytotoxic T cells, and suppressor T cells.

The different types of T cells are often distinguished by the different cell surface receptors present in their cell membranes. These receptors are called “clusters of differentiation,” or CD for short. The different types of lymphocytes and some of their cell surface receptors are shown in Figure 5.

Figure 5: This diagram shows the different types of lymphocytes. Different lymphocytes possess different cell surface receptors and perform different functions.

Th lymphocytes, also called helper T cells, are lymphocytes that activate other types of T cells. They also stimulate B cells to make antibodies against a particular pathogen. Helper T cells are distinguished by their expression of the CD4 cell surface receptor.

Tc lymphocytes, also called cytotoxic T cells or killer T cells, are lymphocytes that kill infected or abnormal cells. These T cells are responsible for destroying cells that are infected with a virus. They also can detect and kill cells that are dividing too rapidly and may develop into cancer. Cytotoxic T cells are also the lymphocytes that will attack a transplanted organ, potentially causing rejection. Cytotoxic T cells are distinguished by their expression of the CD8 cell surface receptor.

Ts lymphocytes, also called suppressor T cells or regulatory T cells, are lymphocytes that suppress or regulate the function of the other types of immune cells. They keep the immune system from attacking the body, which can lead to autoimmune disorders. They also reduce the activity of immune cells after a pathogen has been eliminated. Suppressor T cells are derived from helper T cells and also express the CD4 cell surface receptor along with several others.

B lymphocytes, also called B cells, mature in the bone marrow, as opposed to T cells that mature in the thymus. B cells make antibodies, which are globular proteins adapted to attach to and help destroy pathogens.

NK cells are also known as natural killer cells. These are lymphocytes that have the ability to recognize stressed or abnormal cells and destroy them. They are responsible for identifying and destroying infected cells in the early stages of a viral infection. They also attack tumors and kill cancerous cells in the body.

White blood cells and red blood cells are all made in the red bone marrow. A blood stem cell can differentiate into either a myeloid or lymphoid progenitor cell. The lymphoid progenitor cells mature into the different kinds of lymphocytes. A diagram illustrating the differentiation of various immune cells is shown in Figure 6.

Figure 6: A diagram showing the differentiation of blood cells, including white blood cells, from a common stem cell progenitor.

Definition: Lymphocytes

Lymphocytes are immune cells that originate from lymphoid progenitor cells that generate immune responses against pathogens in the body. There are three main types of lymphocytes: B cells, T cells, and NK cells.

The myeloid progenitor cell differentiates into a type of cell that can further mature into a neutrophil, eosinophil, basophil, or monocyte. The monocyte can then further differentiate into several types of cells, one of which is a macrophage. Red blood cells are also derived from myeloid progenitor cells.

Macrophages, neutrophils, eosinophils, and basophils are all different types of cells that have the ability to either engulf foreign pathogens in the body or attack parasites that are too large to engulf by degranulation in the case of basophils. We call this process of “eating” pathogens phagocytosis. “Phago-” comes from the Greek word for “eat” and “-cyto” refers to “cell.”

Neutrophils, basophils, and eosinophils are immune cells known as granulocytes that usually have a short life span of just a few days. Granulocytes have a distinctive multilobed nucleus. While eosinophil and basophil nuclei typically have two lobes, neutrophil nuclei have between two and five lobes. Granulocytes also contain structures in their cytoplasm called granules that assist in the breakdown of pathogens.

Neutrophils, basophils, and eosinophils are similar cells, named for the difference in the pH of dyes that they have an affinity for and are therefore stained most effectively by. For example, basophils have an affinity for dyes that are basic or alkaline, eosinophils have an affinity for dyes that are acidic (eosin), and neutrophils have an affinity for dyes that are neutral. The different granules within them makes each of these cell types efficient at engulfing and destroying different types of pathogens.

Definition: Neutrophil

A neutrophil is an immune cell called a granulocyte. Neutrophils are phagocytes cells that engulf and destroy pathogens.

Macrophages are also cells that engulf pathogens to destroy them. They do not possess granules, so they are not considered granulocytes like neutrophils. Macrophages and monocytes also typically have a single large nucleus compared to the lobed nucleus of a granulocyte. Macrophages can either be resident, remaining in a certain body tissue, or mobile, traveling around the body to detect information about potential infection and alert other immune cells.

Macrophages engulf pathogens by phagocytosis, break them apart, then display parts of the pathogen on their cell surfaces to allow the pathogen to be easily identified by other immune cells. A diagram illustrating the process of phagocytosis in macrophages is shown in Figure 7.

Figure 7: A diagram showing the phagocytosis of a pathogen in a macrophage. The pathogen is broken down by enzymes. The antigens will later be displayed on the cell surface.

Definition: Macrophage

A macrophage is a phagocytic immune cell that can display an antigen on its cell surface proteins.

In a typical sample of human white blood cells, approximately 40%60% will be neutrophils, 20%40% will be lymphocytes, 2%8% will be monocytes, 1%4% will be eosinophils, and 0.5%1% will be basophils. A further 0%3% will consist of young neutrophils. Within the lymphocyte population itself, 70%85% of these will be T cells, 5%10% B cells, and 5%20% NK cells.

All of these immune cells originate from stem cells in the bone marrow.

When the immune system is severely compromised or destroyed due to illness or chemotherapy, patients can receive a bone marrow transplant from a suitable donor that will provide them with healthy stem cells that can differentiate into all the various types of cells needed to repopulate their entire immune system!

Example 3: Recalling the Organ Where White Blood Cells Originate

Where do all white blood cells originate?

  1. Thymus
  2. Spleen
  3. Lymph nodes
  4. Liver
  5. Bone marrow


White blood cells are immune cells. All of our blood cells, white blood cells, red blood cells, and platelets, arise from a type of stem cell that is found in the red bone marrow. Red bone marrow is a type of tissue found inside flat bones like the ribs and pelvis and in the ends of long bones like the femur or thigh bone.

Red bone marrow is home to stem cells that can differentiate into different kinds of cells and are able to divide infinitely. These stem cells can either make more stem cells or differentiate into either the myeloid or the lymphoid progenitor. Progenitor cells are like stem cells because they can differentiate into many different kinds of cells. However, they are different from stem cells because they can only divide a certain number of times.

The myeloid progenitor differentiates into cells that eventually become our red blood cells, thrombocytes or platelets, and white blood cells including neutrophils and macrophages.

The lymphoid progenitor differentiates into cells that eventually become lymphocytes including T lymphocytes, B lymphocytes, and natural killer cells.

Using this knowledge, we can conclude that all white blood cells originate in the bone marrow.

The immune system is often divided into two categories, shown in Figure 8. These are innate immunity and adaptive, or acquired, immunity. Innate immunity is the part of the immune system that fights all infections the same way. It is immunity you are born with. Adaptive immunity adjusts depending on the specific pathogen or threat. It is immunity you acquire over time.

Figure 8: A flow chart showing the two major immune divisions, innate, and adaptive.

The entire immune system can be split into two components: cell-mediated immunity, which relies on the function of cells, and humoral immunity, which relies on the function of chemical or molecular components in the blood.

Cell-mediated and humoral immunity are closely linked to each other. The chemicals that are the main factors in humoral immunity are produced by cells, often immune cells. These chemicals include two main categories in the adaptive immune system: antibodies and cytokines.

Antibodies are globular proteins made by B cells that attach to a specific antigen. An antigen is any molecule that can attach to an antibody. Antigens are usually proteins or carbohydrates found on the surface of pathogens. Antibodies are also known as immunoglobulins. “Immuno-” refers to the immune system, and “globulin” to the globular structure of the protein.

Antibodies have the ability to attach to a specific antigen. This helps the immune system to identify pathogens or infected cells so that they can be destroyed. Antibodies are also one of the ways that the immune system remembers an infectious agent, which allows the body to be prepared to fight it more efficiently in the future.

Definition: Antibody (Immunoglobulin)

An antibody is a globular protein produced by B lymphocytes that is adapted to bind with a specific antigen.

Cytokines are a group of signaling chemicals that operate within the immune system. They facilitate communication between the different immune cells and are an important part of stimulating and coordinating the immune response.

There are several different types of cytokines including chemokines, interferons, interleukins, and lymphokines. They are made by a wide variety of cells including T cells, B cells, and macrophages. Cytokines serve to regulate the balance between cell-mediated and humoral immunity. They also have a regulatory effect on the maturation of certain immune cells. Cytokines have many different functions in fighting disease.

Definition: Cytokine

A cytokine is a chemical messenger that functions in communication between cells of the immune system.

For example, interleukins are secreted as a result of a stimulus such as pathogenic infection and moderate communication between cells, particularly cells of the immune system. Interleukins literally means “between leukocytes,” which is another name for white blood cells as “leuko-” means “white” and “-cyte” means “cell.” Though they are now known to have effects on cells beyond the immune system, some of the key functions of interleukins are to activate T cells and B cells, stimulating their growth, differentiation, and movement. They therefore play an important role in the immune response.

Chemokines are small proteins that are mostly responsible for initiating movement of immune cells among their other functions. For example, chemokines can recruit phagocytic cells to the site of injury or infection in the body.

Interferons are glycoproteins that are also released in response to pathogens or other cytokines and are capable of inducing targeted cell growth and inflammatory responses. For example, interferons can target healthy cells that are close to an infected cell, stimulating them to produce enzymes that inhibit replication of pathogenic viruses.

Another group of molecules that play an important role in the human immune system are called complements. Complements are different proteins, specifically enzymes, that circulate in the blood and tissue fluids and can be activated by antibodies bound to antigens on pathogens. One of the many effects that complement proteins might cause as a result of this activation is breaking down the cell membrane of the attached pathogen through a process called lysis. This attracts phagocytes and makes it easier for them to recognize and engulf the remains of the pathogen.

Example 4: Describing a Cytokine

What is a cytokine?

  1. A globular protein that binds specifically to antigens
  2. An enzyme that destroys microbes in the blood and other body fluids
  3. A chemical messenger that mediates communication between immune cells
  4. A phagocyte that is active during an inflammatory response
  5. A structure in the lymph node where lymphocytes mature


The human immune system can be divided into two categories, innate immunity, which is immunity we are born with, and adaptive or acquired immunity, which is immunity we develop over time.

The innate immune system includes the physical barriers that keep pathogens from entering the body; it also includes some cells like neutrophils that engulf and destroy pathogens through phagocytosis.

The adaptive immune system is often further divided into humoral immunity that relies on chemical components in the blood and cell-mediated immunity that relies on the actions of lymphocytes and some other cells.

Humoral immunity involves many different chemical components. These include antibodies, or globular proteins that bind to specific antigens, and cytokines, which are chemical messengers that facilitate communication between immune cells.

This means that a cytokine is a chemical messenger that mediates communication between immune cells.

The immune system is a vast and complex network of organs, cells, and chemicals that work together to maintain homeostasis by fighting infection and other potential dangers to the body. The various divisions, innate, adaptive, humoral, and cell mediated, all work together seamlessly and constantly to prevent illness and disease.

Let’s summarize what we have learned in this explainer.

Key Points

  • The organs of the human immune system are located throughout the body.
  • The immune system consists of many types of cells with different functions.
  • Cytokines are chemical messengers that mediate communication between immune cells.
  • The human immune system can be divided into innate immunity and adaptive, or acquired, immunity.

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