The immune system
The body’s immune system is made up of a group of special cells and proteins that work together to protect the body against infection and diseases.
There are many cells that make up the immune system, these include:
- Lymphocytes: B-cells and T-cells
These cells are all types of white blood cells.1
The main proteins of the immune system are cytokines, antibodies and complement proteins.1
Although all parts of the immune system work together, immune responses are grouped into two types; innate immune responses and adaptive (acquired) immune responses.1,2
- The innate immune response is the first line defence against a pathogen. It includes barriers such as skin and mucosa and is not specific to a particular pathogen. Innate immune responses involve cells such as neutrophils, monocytes and complement proteins. Even small infants have excellent innate immune responses.
- The adaptive (or acquired) immune response is specific to a particular pathogen. The T-cells and B-cells of the adaptive immune response have a long memory and remember pathogens they have attacked, so they can quickly attack them again in the future if needed. The cells can also adapt to attack a new pathogen. Although the adaptive immune system works at birth, it is not experienced enough to fully protect infants.
How the immune system protects against infection
When the body is exposed to a pathogen, such as through the nostrils or a cut on the skin, white blood cells, and other cells of the innate immune system, quickly travel to the site through the blood, or in special vessels called lymphatics, to begin fighting an infection.1 These cells also call the adaptive immune response into action. The adaptive immune response is carried out by two types of white blood cells, B cells and T cells. 2
B-cells produce special proteins called immunoglobulins (also known as antibodies) to fight a specific infection. Each antibody is designed to attach to a specific pathogen - much like a lock and key, which will either block the pathogen from infecting healthy cells, or mark it for destruction by other cells called phagocytes.2 The immune system is able to make a different antibody to protect it against each different type of infection.1
Antibodies are a vital part of the body’s defense against foreign invaders like bacteria, or viruses. Antibodies are also important because they protect the body from getting certain types of infection more than once.
There are five main classes of antibodies:1
Made in large amounts by the body, IgG protects against infections in the blood and tissues. IgG molecules are the only antibodies that can cross the placenta and pass some immunity from the mother to child.
Found in tears, bile, saliva and mucus, IgA protects against infections of your eyes, mouth, throat, lungs and intestines.
These antibodies are the first antibodies produced to protect against infection.
Responsible for allergic reactions. Only small amounts of these antibodies are found in the body.
The role of these antibodies is not well understood.
T cells are responsible for long-lasting protection against infections. T-cells directly attack cells infected with viruses as well as regulate the immune system.1 There are four types of T cells, each with a special purpose.
- Helper T cells detect infection and tell B cells to produce antibodies
- Cytotoxic, or killer, T cells destroy infected cells.
- Regulatory T cells tell the immune system when the battle with the germs is over and to stop fighting.
- Memory T cells remember how to defeat an infection and can respond rapidly if the same infection reoccurs1.
Phagocytes are a type of white blood cells that surround and then ingest (“eat”) a pathogen, such as bacteria and fungi. Phagocytes also contain a chemical which can then digest (breakdown) the pathogen. The main types of phagocytes are neutrophils, and monocytes/macrophages. Macrophages act like scavengers. They look for foreign invaders and then send alert signals for other macrophages to come and destroy them.1
The complement system is a group of proteins in the bloodstream that work in an organised fashion to defend against foreign invaders. These proteins work with antibodies and phagocytes to rid the body of infection.1
Although everyone experiences infections from time to time, people with an immunodeficiency tend to experience infections more frequently and the infections may be severe.1
Signs and symptoms of a primary immunodeficiency3
- An unusually large number of infections needing treatment, including:
- Frequent ear infections
- Serious sinus infections
- Chronic sinus or lung disease
- Infections caused by unusual types of bacteria or viruses
- Infections in unusual places
- Recurrent deep skin or organ abscesses
- Infections that do not respond to treatment as expected, including:
- Infections that are not resolved by antibiotic treatment
- Infections that come back soon after stopping antibiotics
- Persistent, unexplained thrush in the mouth after one year of age
- Severe infections needing intravenous antibiotics
- Middle ear infections with pus persistently oozing out
- Recurrent serious sinus infections
- Persistent production of infected phlegm (sputum)
- Failure of an infant to gain weight or grow normally or weight loss in adults
- Family history of primary immune deficiency
- Any other unusual symptoms related to infections, such as persistent diarrhoea
People with a healthy immune system may also experience some of the above.
Aside from recurrent infections, deficiency and defects in the immune system may sometimes also result in:
- Autoimmune disorders
- Inflammation disorders, such as inflammatory bowel disease or arthritis
- Cancer, such as lymphoma
- Allergic disease, such as eczema, food allergy, hay fever, and asthma4
- Immune Deficiency Foundation. Patient & Family Handbook for Primary Immunodeficiency Diseases. 5th Edition. Available at: https://primary immune.org/patient-family-handbook/
- Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002. Available at: https://www.ncbi.nlm.nih.gov/books/NBK21070/; https://www.ncbi.nlm.nih.gov/books/NBK21052/#A5336
- ASCIA. Primary immunodeficiencies (PID) Clinical Update 2017. Available at: https://www.allergy.org.au/ health-professionals/papers/pid
- Stiehm, R. Laboratory Evaluation of the Immune System. UpToDate, 23 Jan 2017. Available at: https://www.uptodate.com/contents/laboratory-evaluation-of-the-immune-system