How Does the Immune System Work?
How Does the Immune System Work?
The immune system is a network of barriers, organs, cells, and proteins that protects the body from infection. It blocks many germs before they enter, attacks those that get inside, and remembers some threats so it can respond faster in the future. It also removes damaged cells and watches for some cells that have become abnormal.
- What Is the Immune System and What Does It Protect the Body From?
- Where Is the Immune System Located in the Body?
- Which Cells and Proteins Make Up the Immune System?
- What Is the Body’s First Line of Defense Against Germs?
- What Is Innate Immunity and How Does It Work?
- What Is Adaptive Immunity and How Does It Work?
- How Do Innate and Adaptive Immunity Work Together?
- How Does the Immune System Remember a Previous Threat?
What Is the Immune System and What Does It Protect the Body From?
The immune system is not a single organ. It is a connected network that includes the skin, mucous membranes, bone marrow, thymus, lymph nodes, spleen, white blood cells, and many protective proteins.
Its main job is to recognize possible dangers and respond to them. These dangers may include:
- Viruses
- Bacteria
- Fungi
- Parasites
- Harmful substances made by germs
- Infected cells
- Damaged or abnormal cells
The immune system must do more than attack. It must also avoid harming the body’s own healthy cells. To do this, immune cells look for signs that help them tell normal cells from possible threats.
The immune response is not always the same. The body may block a germ at its surface, destroy it quickly with general defenses, or build a more targeted response against it. These different defenses work together rather than acting as separate systems. (NCBI)
The Immune System at a Glance
| Part of the defense | Main job | Examples |
|---|---|---|
| Physical and chemical barriers | Keep germs from entering | Skin, mucus, tears, and stomach acid |
| Innate immunity | Respond quickly to many kinds of danger | Phagocytes, natural killer cells, and complement |
| Adaptive immunity | Target a particular threat | B cells, T cells, and antibodies |
| Immune memory | Improve the response to a known threat | Memory B cells and memory T cells |
Where Is the Immune System Located in the Body?
The immune system is spread throughout the body. Immune cells travel through the blood and a separate network of vessels called the lymphatic system. They also live in tissues that are often exposed to germs, including the skin, lungs, airways, and digestive tract.
Several organs provide places where immune cells are made, trained, stored, or activated.
What Do the Bone Marrow and Thymus Do?
Bone marrow is the soft tissue inside many bones. It contains stem cells that produce all the major types of blood cells, including the white blood cells used by the immune system.
B cells develop and mature in the bone marrow. Early T cells also begin there, but they travel to the thymus to continue developing.
The thymus is a small organ behind the breastbone. It is where developing T cells are tested and trained. Cells that react properly may leave the thymus and join the immune system. Many cells that respond too strongly to the body’s own molecules are removed during this process.
What Do the Lymph Nodes and Spleen Do?
Lymph nodes are small, bean-shaped structures found along lymphatic vessels. They filter lymph, a clear fluid that drains from the body’s tissues.
Lymph nodes bring immune cells and material from nearby tissues together. If a germ enters through a cut, for example, parts of that germ may travel to a nearby lymph node. B cells and T cells that recognize it can then become active and multiply.
The spleen is an organ in the upper left part of the abdomen. It filters the blood rather than lymph. It helps immune cells detect germs and other foreign material carried in the bloodstream. The spleen also removes old or damaged red blood cells. (NCBI)
Which Cells and Proteins Make Up the Immune System?
The immune system contains many cell types. Each has a different role, but no immune cell works entirely alone. Cells communicate, share information, and call other cells to areas where they are needed.
Protective proteins in the blood and tissues also help immune cells find and remove threats.
What Do White Blood Cells Do?
White blood cells, also called leukocytes, are the main cells of the immune system. Different types perform different jobs:
- Neutrophils arrive quickly at many sites of infection and swallow germs.
- Monocytes travel in the blood and can develop into macrophages in tissues.
- Macrophages swallow germs, damaged cells, and cell remains.
- Dendritic cells collect material from germs and help activate T cells.
- B cells can develop into cells that make antibodies.
- T cells coordinate immune responses or destroy infected cells.
- Natural killer cells attack some infected or abnormal cells.
- Eosinophils help defend against certain parasites and are also involved in allergic reactions.
- Basophils and mast cells release substances that help start inflammation and allergic responses.
Some white blood cells circulate in the blood. Others remain in tissues, lymph nodes, the spleen, or other parts of the lymphatic system.
What Do Antibodies, Complement Proteins, and Cytokines Do?
Antibodies are proteins made by activated B cells. Each antibody recognizes a particular molecular shape, called an antigen. Antibodies may block a germ from entering cells or mark it so that other immune defenses can remove it.
Complement is a group of proteins that can activate one another in a chain reaction. Complement proteins can coat germs, attract immune cells, increase inflammation, and damage the outer membranes of some microbes.
Cytokines are small signaling proteins. Immune cells release them to communicate with nearby or distant cells. Cytokines can attract immune cells, increase or reduce inflammation, and guide the type of immune response that develops. (NIAID)
What Is the Body’s First Line of Defense Against Germs?
The body tries to stop germs before they enter its deeper tissues. The skin, mucous membranes, protective fluids, normal microbes, and several cleaning movements form this first line of defense.
These barriers are part of innate immunity, but they act before an internal immune response is needed.
How Do the Skin and Mucous Membranes Keep Germs Out?
The skin forms a strong physical wall. Its tightly joined cells make it difficult for most germs to pass through. Dead cells are continually shed from the surface, helping remove microbes attached to them.
Natural oils, a slightly acidic surface, and substances made by skin cells also limit the growth of many germs. Harmless microbes that normally live on the skin compete with other microbes for space and nutrients.
Mucous membranes line passages that open to the outside, including the nose, mouth, lungs, digestive tract, and urinary and reproductive tracts. These membranes produce mucus, which traps germs and particles.
Tiny hair-like structures called cilia move mucus out of parts of the airways. Coughing and sneezing also help remove trapped material.
How Do Tears, Saliva, and Stomach Acid Help?
Tears wash particles from the eyes, while saliva helps clean the mouth. Both contain substances that can damage certain bacteria.
Stomach acid creates a harsh environment that destroys many germs swallowed with food or mucus. Digestive enzymes and the movement of material through the intestines provide further protection.
These defenses are not perfect. Some germs can survive them, and others may enter through wounds or damaged tissues. When this happens, immune cells inside the body begin a more active response. (NCBI)
What Is Innate Immunity and How Does It Work?
Innate immunity is the body’s rapid, general response to injury and infection. It is already present before the body meets a particular germ.
Innate immune cells recognize common patterns found on groups of germs or released by damaged cells. This allows them to act quickly, often within minutes or hours.
The innate response may cause inflammation. Blood vessels widen, more fluid enters the tissue, and immune cells move toward the affected area. This can produce redness, warmth, swelling, and pain.
How Do Phagocytes Destroy Germs?
A phagocyte is a cell that can surround and swallow germs, damaged cells, and other material. The word includes cells such as neutrophils, macrophages, and dendritic cells.
The process occurs in several steps:
- The phagocyte detects and moves toward the threat.
- Its cell membrane surrounds the material.
- The material becomes enclosed in a small compartment inside the cell.
- That compartment joins with others containing destructive enzymes.
- The germ or material is broken down.
Macrophages and dendritic cells may then display small pieces of the germ on their surfaces. This helps connect the innate response with adaptive immunity.
What Do Natural Killer Cells Do?
Natural killer cells, or NK cells, inspect the body’s cells for signs that something is wrong. They can destroy some cells infected by viruses and some cells that have become abnormal.
Unlike B cells and T cells, natural killer cells do not need to recognize one exact antigen before acting. They respond to patterns of signals on a cell’s surface.
Natural killer cells release proteins that damage the target cell and cause it to shut itself down. They also release cytokines that affect other immune cells.
What Is Adaptive Immunity and How Does It Work?
Adaptive immunity is a more targeted form of defense. It develops when B cells and T cells recognize a particular antigen.
The first adaptive response is slower than the innate response because the matching immune cells must be found, activated, and multiplied. This often takes several days.
Once active, adaptive immunity can attack a threat with great precision. It can also form long-lasting memory cells.
What Do B Cells Do?
Each B cell carries receptors that recognize a particular antigen. When a suitable B cell is activated, it multiplies and produces a group of matching cells.
Some become plasma cells, which release large amounts of antibodies. These antibodies travel through blood and body fluids.
Antibodies can:
- Block viruses or toxins from attaching to cells
- Bind to germs and prevent them from spreading
- Clump foreign material together
- Mark germs for phagocytes
- Help activate complement proteins
Other activated B cells become memory B cells that remain after the immediate threat has passed.
What Do T Cells Do?
T cells recognize pieces of antigens displayed by other cells. Different T cells have different jobs.
Helper T cells release signals that guide the immune response. They help activate B cells, macrophages, and other T cells.
Cytotoxic T cells, sometimes called killer T cells, can destroy body cells infected by viruses or certain other organisms. They may also attack some abnormal cells.
Regulatory T cells help limit immune responses and reduce the chance of unnecessary damage to healthy tissues.
Some activated T cells become memory T cells. These cells help the body respond more quickly if the same threat returns.
How Do Innate and Adaptive Immunity Work Together?
Innate and adaptive immunity are often described separately, but they are closely connected.
Innate immunity acts first. It may destroy the threat before adaptive immunity is needed. If the threat continues, innate immune cells help start and shape the adaptive response.
Dendritic cells and macrophages can swallow germs and display their antigens. They then travel to lymph nodes or interact with nearby T cells. Cytokines released during the innate response help tell T cells what kind of response may be needed.
Adaptive immunity also strengthens innate defenses. Antibodies coat germs, making them easier for phagocytes to swallow. They can also activate complement. Helper T cells can increase the germ-killing activity of macrophages.
Together, the two systems provide a response that is both rapid and highly targeted. (NIAID)
How Does the Immune System Remember a Previous Threat?
After an adaptive immune response, most of the activated B cells and T cells are no longer needed and disappear. However, some remain as memory cells.
Memory B cells and memory T cells can survive for years, and sometimes much longer. Some long-lived plasma cells also continue releasing antibodies after an infection or vaccination.
This lasting protection is called immunological memory. The strength and length of that memory vary with the germ, the type of immune response, and the person.
Why Is the Immune Response Faster the Second Time?
During the first encounter with an antigen, the immune system must find the small number of B cells and T cells able to recognize it. Those cells must then multiply and develop into working immune cells.
Afterward, the body has a larger group of memory cells that already recognize the antigen. If it appears again, these cells can become active more quickly.
Memory B cells can rapidly produce new antibody-making cells. Memory T cells can provide help or attack infected cells sooner. Existing antibodies may also begin blocking the threat immediately.
The second response is therefore often faster and stronger than the first. It may prevent illness completely or make the illness less severe. Vaccines use this ability by exposing the immune system to a safe form or part of a threat so that memory can develop without the full disease. (NCBI)
Written by Chris Morais, MSc, MPhil, PhD — Making complex biology simple
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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