Immune system explained

The immune system is a complex network of organs, cells and proteins that fights infection from microbes while protecting the body’s own cells.

It keeps a record of every microbe it has ever defeated, stored in types of white blood cells (B-lymphocytes and T-lymphocytes) known as memory cells. This means it can recognize and destroy a microbe quickly if it enters the body again, before it can multiply and make you sick.

Some infections, like the flu and the common cold, have to be fought many times, because many different viruses or strains of the same type of virus can cause these illnesses. Catching a cold or flu from one virus does not give you immunity against the others.

Parts of the immune system#

The main parts of the immune system are white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the bone marrow and the thymus.

White blood cells#

White blood cells are the key players in your immune system. They are made in your bone marrow and are part of the lymphatic system. They move through blood and tissue throughout your body looking for foreign invaders such as bacteria, viruses, parasites and fungi, and launch an immune attack when they find them. White blood cells include lymphocytes (such as B-cells, T-cells and natural killer cells) and many other types of immune cell.

Antibodies#

Antibodies help the body fight microbes or the toxins (poisons) they produce. They do this by recognizing substances called antigens on the surface of a microbe, or in the chemicals it produces, which mark the microbe or toxin as foreign. The antibodies then mark these antigens for destruction. Many cells, proteins and chemicals are involved in this attack.

Complement system#

The complement system is made up of proteins whose actions complement the work done by antibodies.

Lymphatic system#

The lymphatic system is a network of delicate tubes throughout the body. Its main roles are to:

• manage the fluid levels in the body
• react to bacteria
• deal with cancer cells
• deal with cell products that would otherwise cause disease or disorders
• absorb some of the fats in our diet from the intestine

It is made up of lymph vessels (tubes that carry lymph, the colorless fluid that bathes the body’s tissues and contains infection-fighting white blood cells), lymph nodes (also called lymph glands, which trap microbes) and white blood cells (lymphocytes).

Spleen#

The spleen is a blood-filtering organ that removes microbes and destroys old or damaged red blood cells. It also makes disease-fighting components of the immune system, including antibodies and lymphocytes.

Bone marrow#

Bone marrow is the spongy tissue found inside your bones. It produces the red blood cells the body needs to carry oxygen, the white blood cells used to fight infection, and the platelets needed to help blood clot.

Thymus#

The thymus filters and monitors your blood content and produces the white blood cells called T-lymphocytes.

The body’s other defenses#

As well as the immune system, the body has several other ways to defend itself against microbes, including:

• skin, a waterproof barrier that secretes oil with bacteria-killing properties
• the lungs, where mucous (phlegm) traps foreign particles and small hairs (cilia) wave the mucous upwards so it can be coughed out
• the digestive tract, where the mucous lining contains antibodies and stomach acid can kill most microbes
• body fluids such as skin oil, saliva and tears, which contain anti-bacterial enzymes
• the constant flushing of the urinary tract and the bowel

Fever is an immune response#

A rise in body temperature, or fever, is actually an immune system response. A rise in temperature can kill some microbes, and fever also triggers the body’s repair process.

Common disorders of the immune system#

It is common for people to have an over- or underactive immune system.

Overactivity#

Overactivity can take many forms, including allergic diseases, where the immune system makes an overly strong response to allergens. Allergic diseases are very common and include:

• allergies to foods, medications or stinging insects
• anaphylaxis (life-threatening allergy)
• hay fever (allergic rhinitis)
• sinus disease
• asthma
• hives (urticaria)
• dermatitis and eczema

Another form is autoimmune disease, where the immune system mounts a response against normal components of the body. Autoimmune diseases range from common to rare and include multiple sclerosis, autoimmune thyroid disease, type 1 diabetes, systemic lupus erythematosus, rheumatoid arthritis and systemic vasculitis.

Underactivity#

Underactivity of the immune system, also called immunodeficiency, means the immune system does not function correctly and makes people vulnerable to infections. It can be life-threatening in severe cases. Immunodeficiency can:

• be inherited, such as common variable immunodeficiency (CVID), x-linked severe combined immunodeficiency (SCID) and complement deficiencies
• arise as a result of medical treatment, for example medications such as corticosteroids or chemotherapy, including for people who have had an organ transplant
• be caused by another disease, such as HIV/AIDS or certain types of cancer

Immunoglobulin therapy#

Immunoglobulins (commonly known as antibodies) are used to treat people who are unable to make enough of their own, or whose antibodies do not work properly. This is known as immunoglobulin replacement therapy (IRT).

It is made from plasma, the liquid part of blood that contains important proteins such as antibodies. Traditionally, immunoglobulins were delivered through a drip into a vein, known as intravenous immunoglobulin (IVIg) therapy. They can now also be delivered into the fatty tissue under the skin, known as subcutaneous immunoglobulin (SCIg) therapy, which may offer benefits for some patients.

Many health services now offer SCIg therapy to eligible patients with specific immune conditions. If you are interested, discuss your particular requirements with your clinical immunology or allergy specialist.

Immunization#

Immunization works by copying the body’s natural immune response. A vaccine (a small amount of a specially treated virus, bacterium or toxin) is introduced into the body, and the body then makes antibodies to it. If a vaccinated person is later exposed to the actual virus, bacterium or toxin, they will not get sick, because their body recognizes it and knows how to attack it. Vaccines are available against many diseases, including measles and tetanus.

The immunizations you may need are decided by your health, age, lifestyle and occupation. Together these factors are sometimes referred to as HALO:

• Health: some conditions or factors may make you more vulnerable to vaccine-preventable diseases. For example, premature birth, asthma, diabetes, heart, lung, spleen or kidney conditions, Down syndrome and HIV may mean you benefit from additional or more frequent immunizations.
• Age: at different ages you need protection from different vaccine-preventable diseases. National immunization schedules set out recommended immunizations for babies, children, older people and other people at risk, including some populations who may be offered recommended vaccines at no cost.
• Lifestyle: lifestyle choices can affect your immunization needs. For example, traveling overseas to certain places, planning a family, sexual activity, smoking and playing contact sport that may expose you to someone else’s blood may mean you benefit from additional or more frequent immunizations.
• Occupation: if you work in aged care, childcare, healthcare, emergency services or sewerage repair and maintenance, discuss your immunization needs with your doctor. Some employers help with the cost of relevant vaccinations for their employees.

Key points#

• The main parts of the immune system include the thymus, white blood cells, antibodies, the complement system, the lymphatic system, the spleen and the bone marrow.
• White blood cells are the key players in your immune system.
• The lymphatic system has several roles, including managing fluid levels, reacting to bacteria and absorbing some of the fats in our diet from the intestine.
• It is common for people to have an over- or underactive immune system.
• Allergic diseases are very common.

Where to get help#