The immune system is a complex system of structures and processes that has evolved to protect us from disease. Molecular and cellular components make up the immune system. The function of these components is divided up into nonspecific mechanisms, those which are innate to an organism, and responsive responses, which are adaptive to specific pathogens. Fundamental or classical immunology involves studying the components that make up the innate and adaptive immune system.
Innate immunity is the first line of defence and is non-specific. That is, the responses are the same for all potential pathogens, no matter how different they may be. Innate immunity includes physical barriers (e.g. skin, saliva etc) and cells (e.g. macrophages, neutrophils, basophils, mast cells etc). These components ‘are ready to go’ and protect an organism for the first few days of infection. In some cases, this is enough to clear the pathogen, but in other instances the first defence becomes overwhelmed and a second line of defence kicks in.
Adaptive immunity is the second line of defence which involves building up memory of encountered infections so can mount an enhanced response specific to the pathogen or foreign substance. Adaptive immunity involves antibodies, which generally target foreign pathogens roaming free in the bloodstream. Also involved are T cells, which are directed especially towards pathogens that have colonised cells and can directly kill infected cells or help control the antibody response.
The word “immunity” originates from the Latin word immunis meaning exempt. Generally speaking, immunity consists of all of the body’s defense systems that resist infections and toxins. The immune system therefore has a crucial role in fending off intruders.
The immune system can be thought of as a military defense system resisting uninvited guests. To put it dramatically, the troops patrolling the body mercilessly attack intruders, blasting bacteria, poisoning parasites, devouring viruses and forcing cancer cells to commit suicide. However, sometimes the soldiers get hit by friendly fire.
Autoimmune diseases such as arthritis and psoriasis involve a state of imbalance where the immune system erroneously attacks the body’s own protein, sugar or cell structures. Food and pollen allergies, on the other hand, are examples. Although functioning as a cohesive system, immunity is made up of various parts. A rough distinction can be made between innate (nonspecific) and adaptive (specific) immunity.
You may have noticed that sometimes your reaction to a pathogen may be
1) asymptomatic (you may still spread it to others although you have no symptoms),
2) mild (it goes away in a few days), or
3) “full-on” pathogenic (noticeable and takes 1–2 weeks to resolve at minimum).
This is due to individual differences in the immune system response and the fact that you may or may not have encountered the pathogen previously. The immune system is fast to react when it notices a familiar pathogen and can get rid of it in just a few days. If a pathogen is previously unknown, it takes a few weeks for the adaptive immune system to recognize, mark, and eliminate the intruder.
Pathogens may also mutate, which makes it harder for the immune system to recognize a previously known pathogen. In some cases, a virus can also hide from the immune system and remain dormant until later activation (such as HIV, herpes, and papillomavirus).
Effective drugs can be developed to kill potential intruders (such as antibiotics against bacteria), inhibit their replication (such as antivirals against viruses), block their way into the system (such as receptor antagonists), and vaccines (where the immune system is taught with weakened viral agents with the aim to help it recognize the pathogen in the future).
It has been known for centuries that fevers and infections, especially in childhood, may actually be healthy, leading to better health outcomes as adults. A study published in 2019 found that children who contract influenza early in their lives receive strong immunity against potential mutated subtypes of the influenza virus.
However, in some cases, you are out of luck, especially as an adult. For example, measles is known to wipe out the immune system memory leaving one vulnerable to other diseases, and the influenza virus may permanently damage the nervous system and increase the risk for Parkinson’s disease. Infections, therefore, can be a double-edged sword: they can make us either stronger or weaker depending on the situation.
Immune dysfunction and clinical immunology
The immune system is a highly regulated and balanced system and when the balance is disturbed, disease can result. Research in this area involves studying disease that is caused by immune system dysfunction. Much of this work has significance in the development of new therapies and treatments that can manage or cure the condition by altering the way the immune system is working or, in the case of vaccines, priming the immune system and boosting the immune reaction to specific pathogens.
Immunodeficiency disorders involve problems with the immune system that impair its ability to mount an appropriate defence. As a result, these are almost always associated with severe infections that persist, recur and/or lead to complications, making these disorders severely debilitating and even fatal. There are two types of immunodeficiency disorders: primary immunodeficiencies are typically present from birth, are generally hereditary and are relatively rare. Such an example is common variable immunodeficiency (CVID). Secondary immunodeficiencies generally develop later in life and may result following an infection, as is the case with AIDS following HIV infection.
For more information, please see our briefing on immunodeficiency.
Autoimmune diseases occur when the immune system attacks the body it is meant to protect. People suffering from autoimmune diseases have a defect that makes them unable to distinguish ‘self’ from ‘non-self’ or ‘foreign’ molecules. The principles of immunology have provided a wide variety of laboratory tests for the detection of autoimmune diseases. Autoimmune diseases may be described as ‘primary’ autoimmune diseases, like type-1 diabetes, which may be manifested from birth or during early life; or as ‘secondary’ autoimmune diseases, which manifest later in life due to various factors. Rheumatoid arthritis and multiple sclerosis are thought to belong to this type of autoimmunity. Also, autoimmune diseases can be localised, such as Crohn’s Disease affecting the GI tract, or systemic, such as systemic lupus erythematosus (SLE).
For more information, please see our briefing on autoimmune diseases.
Allergies are hypersensitivity disorders that occur when the body’s immune system reacts against harmless foreign substances, resulting in damage to the body’s own tissues. Almost any substance can cause allergies (an allergen), but most commonly, allergies arise after eating certain types of food, such as peanuts, or from inhaling airborne substances, such as pollen, or dust. In allergic reactions, the body believes allergens are dangerous and immediately produces substances to attack them. This causes cells of the immune system to release potent chemicals like histamine, which causes inflammation and many of the symptoms associated with allergies. Immunology strives to understand what happens to the body during an allergic response and the factors responsible for causing them. This should lead to better methods of diagnosing, preventing and controlling allergic diseases.For more information, please see our breifing on allergies.
Asthma is a debilitating and sometimes fatal disease of the airways. It generally occurs when the immune system responds to inhaled particles from the air, and can lead to thickening of the airways in patients over time. It is a major cause of illness and is particularly prevalent in children. In some cases it has an allergic component, however in a number of cases the origin is more complex and poorly understood.
Cancer is a disease of abnormal and uncontrolled cell growth and proliferation and is defined by a set of hallmarks, one of which is the capacity for cancer cells to avoid immune destruction. With the knowledge that evasion of the immune system can contribute to cancer, researchers have turned to manipulating the immune system to defeat cancer (immunotherapy). Cancer immunotherapy seeks to stimulate the immune system’s innate powers to fight cancerous tissue and has shown extraordinary promise as a new weapon in our arsenal against the disease. Other applications of immunological knowledge against cancer include the use of monoclonal antibodies (proteins that seek and directly bind to a specific target protein called an antigen. An example is Herceptin, which is a monoclonal antibody used to treat breast and stomach cancer).For more information, please see our briefing on cancer immunotherapy.
Transplants involve transferring cells, tissues or organs from a donor to a recipient. The most formidable barrier to transplants is the immune system’s recognition of the transplanted organs as foreign. Understanding the mechanisms and clinical features of rejection is important in determining a diagnosis, advising treatment and is critical for developing new strategies and drugs to manage transplants and limit the risk of rejection.For more information, please see our briefing on transplant immunology.
Vaccines are agents that teach the body to recognise and defend itself against infections from harmful pathogens, such as bacteria, viruses and parasites. Vaccines provide a sneak ‘preview’ of a specific pathogen, which stimulates the body’s immune system to prepare itself in the event that infection occurs. Vaccines contain a harmless element of the infectious agent that stimulate the immune system to mount a response, beginning with the production of antibodies. Cells responsive to the vaccine proliferate both in order to manufacture antibodies specific to the provoking agent and also to form ‘memory cells’. Upon encountering the infectious agent a second time, these memory cells are quickly able to deal with the threat by producing sufficient quantities of antibody. Pathogens inside the body are eventually destroyed, thereby thwarting further infection. Vaccines are also of course highly controversial and a strong argument is that they may also be harmful.
Hereditary immunity deficiencies or disorders are relatively rare. Impaired immunity is usually caused by environmental factors and general health factors.
Immunosuppressive factors include:
NOTE: the extended use of long-acting cortisone treatments for various inflammatory conditions and diseases such as asthma and arthritis may impair the immune response and increase the risk of bacterial and fungal infections.
Exercise is a double-edged sword. On one hand, exercise strengthens immunity – on the other hand, overtraining may also impair it. Symptoms of overtraining include impaired performance, chronic fatigue, frequent illness, impaired sleep quality, and mood swings. Overtraining may lead to a state of imbalance in which humoral immunity becomes excessively active at the expense of cell-mediated immunity.
Typical symptoms of overtraining individuals include a drop of 5–10 units in the maximum heart rate, the decreased level of noradrenaline in the urine and a decrease in the natural killer cell, white blood cell and immunoglobulin levels in the blood. In particular, the latter of the previous listing are detrimental changes to the immune system.