Last updated: October 30, 2026 · Dr Nidhi Joshi
In short: The immune system and its parts work as one defence network. Skin and mucous membranes block germs. White blood cells find and destroy them. Lymphoid organs (bone marrow, thymus, lymph nodes, spleen, tonsils) make, train and house those cells. Proteins such as antibodies, cytokines and complement mark invaders and coordinate the attack.
What is the immune system?
The immune system is the body’s defence network. It is not a single organ. It is a team of tissues, cells and proteins spread from your skin to your bone marrow. Its job is to tell “self” from “non-self”, and to remove bacteria, viruses, fungi, parasites and damaged cells while leaving healthy tissue alone.
To understand the immune system and its parts, it helps to sort them into four groups: barriers, organs, cells and proteins. Each group has a different job, and they constantly signal to each other.
Organs of the immune system and their functions
Primary lymphoid organs: where immune cells are made
Bone marrow is the soft tissue inside large bones. Its stem cells produce all blood cells, including every type of white blood cell. B cells also finish maturing here.
Thymus sits behind the breastbone. Young T cells travel here to mature and to learn not to attack the body’s own tissues. The thymus is most active in childhood and slowly shrinks with age.
Secondary lymphoid organs: where immune cells meet germs
Lymph nodes are small, bean-shaped filters along the lymph vessels. They trap germs from tissue fluid and bring them face to face with B and T cells. This is why nodes in your neck can swell during a sore throat.
Spleen, in the upper left abdomen, filters the blood. It removes old red blood cells and catches germs that reach the bloodstream.
Tonsils, adenoids and gut lymphoid tissue (such as Peyer’s patches in the small intestine) guard the entry points of the mouth, nose and gut.
Barriers: the first line of defence
Skin forms a tough physical wall. Mucous membranes line the airways, gut and urinary tract, and their sticky mucus traps germs. In the airways, tiny hair-like cilia sweep that mucus out. Tears, saliva and stomach acid add chemical protection.
Types of white blood cells and their functions
White blood cells (leukocytes) are the soldiers of the immune system. They all start in the bone marrow, but each type has its own job.
| White blood cell | Branch | Main job |
|---|---|---|
| Neutrophils | Innate | First responders; engulf and digest bacteria |
| Macrophages (from monocytes) | Innate | Engulf germs and debris; signal other cells |
| Dendritic cells | Innate → adaptive link | Carry pieces of germs (antigens) to lymph nodes to show T cells |
| Natural killer (NK) cells | Innate | Kill virus-infected and abnormal cells |
| Eosinophils, basophils, mast cells | Innate | Fight parasites; drive allergic inflammation |
| B cells | Adaptive | Become plasma cells that make antibodies |
| Helper T cells | Adaptive | Coordinate the response with cytokine signals |
| Cytotoxic (killer) T cells | Adaptive | Destroy infected cells directly |
Immune proteins: antibodies, cytokines and complement
Antibodies are Y-shaped proteins made by B cells. Each antibody fits one specific antigen. It can block a virus from entering cells or tag a germ for destruction.
Cytokines are small signalling proteins. They work a bit like local hormones, telling immune cells where to go and how strongly to react. Inflammation and fever are partly driven by cytokines.
Complement is a group of blood proteins that punch holes in bacteria and coat germs so phagocytes can swallow them more easily.
Innate vs adaptive immunity: what’s the difference?
| Innate immunity | Adaptive immunity | |
|---|---|---|
| Present from | Birth | Builds after exposure |
| Speed | Minutes to hours | Days on first exposure |
| Target | Broad patterns shared by many germs | One specific antigen |
| Memory | No lasting memory | Memory B and T cells |
| Main players | Barriers, neutrophils, macrophages, NK cells, complement | B cells, T cells, antibodies |
How the immune system fights an infection, step by step
- Breach. A germ gets past the skin, for example through a cut.
- Alarm. Macrophages and other tissue cells recognise it and release cytokines. Blood vessels widen, causing redness and swelling.
- First response. Neutrophils rush in and engulf the invaders. Complement helps tag and burst them.
- Messenger. Dendritic cells carry antigen to the nearest lymph node and present it to T cells.
- Targeted attack. Helper T cells activate matching B cells, which make antibodies. Killer T cells destroy infected cells.
- Memory. After the infection clears, memory B and T cells remain, so the next encounter is faster and stronger.
Types of immune system disorders
Problems arise when the immune system is too weak, too strong or aimed at the wrong target.
- Primary immunodeficiency is present from birth and is usually genetic. Severe combined immunodeficiency (SCID), once called “bubble boy disease”, is the best-known example.
- Acquired immunodeficiency develops later. HIV destroys helper T cells and, untreated, can lead to AIDS. Some medicines, such as chemotherapy, also weaken immunity.
- Allergies are overreactions to harmless substances such as pollen, dust mites or certain foods. Asthma, eczema and hay fever are linked to this kind of response.
- Autoimmune diseases occur when the immune system attacks the body’s own tissue. In type 1 diabetes, it destroys the insulin-making cells of the pancreas; in rheumatoid arthritis, it inflames the joints.
Vaccines: training the immune system
Vaccines use adaptive immunity’s memory on purpose. They show the immune system a harmless version or piece of a germ so it builds memory cells without the illness. See how the main vaccine types are grouped, how mRNA vaccines work step by step, and how some vaccine proteins are produced with recombinant DNA technology.
Key takeaways
- The immune system and its parts fall into four groups: barriers, lymphoid organs, white blood cells and immune proteins.
- Bone marrow and thymus make and train immune cells; lymph nodes, spleen and tonsils are where they meet germs.
- Innate immunity is fast and general; adaptive immunity is slower, specific and remembers.
- Disorders come from too little immunity, too much, or immunity aimed at the body itself.
Frequently asked questions
What are the main parts of the immune system?
The main parts are physical barriers (skin and mucous membranes), white blood cells, lymphoid organs (bone marrow, thymus, lymph nodes, spleen, tonsils and gut lymphoid tissue) and immune proteins such as antibodies, cytokines and complement.
What is the difference between primary and secondary lymphoid organs?
Primary lymphoid organs, the bone marrow and thymus, are where lymphocytes are made and mature. Secondary lymphoid organs, such as lymph nodes, the spleen and tonsils, are where mature lymphocytes meet germs and start an immune response.
Which white blood cells make antibodies?
B cells make antibodies. When a B cell is activated by its matching antigen, it turns into a plasma cell that releases large amounts of antibody into the blood.
What is the difference between innate and adaptive immunity?
Innate immunity is present from birth, acts within minutes to hours and responds the same way to many germs. Adaptive immunity takes days to build, targets one specific antigen and leaves memory cells that respond faster next time.
What are the main types of immune system disorders?
The four main groups are primary immunodeficiency (present from birth), acquired immunodeficiency (caused later, for example by HIV), allergies (an overreaction to harmless substances) and autoimmune diseases (the immune system attacks the body’s own tissues).
This article is for general education only and is not medical advice. Talk to a doctor about any health concern.
Sources
- MedlinePlus. Immune System and Disorders.
- Cleveland Clinic. Immune System: Function, Conditions and Disorders.
- NIAID. Overview of the Immune System.
- OpenStax Biology 2e. 42.1 Innate Immune Response and 42.2 Adaptive Immune Response.