DEFINITION OF IMMUNOLOGY

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Immunologyis simply defined as the study of how the immune system of a living organism functions in either a disease condition or a healthy state. It is the study of how an organism responds to foreign bodies (antigens) that it comes in contact with in the environment. The field of immunology studies how the cells of the immune system differentiate foreign bodies or antigens from the body’s own self molecules that are not pathogens or antigens. The immune system is generally a complex system of network of cells, tissues and organs and cell products (e.g., antibodies and cytokines) which work cooperatively to protect the body from invading pathogenic microorganisms and the disease/infection that they cause. Susceptible individuals can be immunized through the process of immunization or vaccination. Immunization / vaccination is a process of injecting a susceptible human or animal host with an antiserum or immune system booster (e.g., vaccines) in order to give the body immunity or protection from an infection or infectious disease agent. These infectious disease agents include pathogenic viruses, bacteria, fungi and protozoa that causes plethora of diseases or infections in man and animals. The phrase immunology also encompasses all the biological, chemical, and physical properties of an organism’s immune system that help it to fight against infectious diseases. In other words, immunology deals basically with the study of our body’s protection from foreign agents or antigens (inclusive of pathogenic microorganisms) that invade the host’s body to cause infection or disease. The environment in which we live is inundated with diverse foreign bodies or macromolecules and pathogens which are generally termed antigens.

An antigen is a molecule or substance that is foreign to the body (i.e., it is unknown by the host immune system). They also include pathogenic microorganisms as aforesaid that cause diseases in humans and animals. Molecules which are unfamiliar to the host’s body and which the immune system sees as potential harm are known as non-self molecules while those molecules, cells, tissues and organs that are valid parts of the body are called self molecules. To be functional and active, an immune system must be able to distinguish self molecules (which are harmless) from non-self molecules (which are usually harmful to the host). These non-self molecules are infectious disease agents or particles that invade the human body and cause disease. They include pathogenic viruses, fungi, bacteria, parasites/worms or protozoan and every other molecule that should not naturally be found in the human body, and which has the capacity to cause or induce an infectious disease process in the host. Non-self molecules may also include grafts or transplants of cells or tissues, even though they are not pathogenic in nature. The field of immunology has had tremendous success in clinical/human medicine in the area of containing the excesses of infectious diseases, and this can be envisaged in the ‘eradication’ of some diseases (e.g., smallpox, measles, polio etc) which used to be a burden to humanity. Some diseases such as polio have been eradicated in many parts of the world; and in places where polio and other infectious diseases may still be present, the disease(s) have been controlled to the barest minimum and are almost nearing the ‘eradication status’. This is the concept of immunology; and why the study of the immune system is important.

As infectious diseases and their causative agents continue to emerge, re-emerge and mutate from time to time as seen in the recent coronavirus disease 2019 (COVID-19) caused by a new strain of coronavirus (severe acute respiratory syndrome coronavirus type-2, SARS-CoV-2) that ravaged the entire world in 2020, the field of immunology and the study of immune system will continue to remain relevant in the field of medicine in order to keep pace with the changing and mutating nature of pathogens. This is the only way that potent vaccines and antimicrobials can be discovered and developed to assuage the excesses and pains caused by microbial pathogens in human or animal populations. As seen with the COVID-19 pandemic which was discovered in Wuhan China in December 2019, the global economy was almost brought to a halt – as economies of different nations and continents was put on a lockdown that caused several emotional, psychological, social, mental and financialconstraints and pace on the human race. Infectious diseases do not respect the borders of any nation no matter how safe, protected and secure such a border or nation might be. The recent outbreak, infectiousness and spread of the COVID─19 is one typical example that infectious diseases and their causative agents that truly, pathogens does not know the borders of any country, they strike and act whenever the conditions become favourable for them to thrive, spread and cause havoc in the human population. COVID-19 pandemic has indeed tested the ability of the world’s healthcare systems to mount a coordinated and successful response against the disease. Most importantly, the COVID-19 pandemic has really awakened our consciousness to some novel but ‘often-neglected practices’ such as hand washing – that is important for the containment of infectious disease outbreak and spread. Interesting, the practice of ‘social distancing’ is another novel and important preventive measures that the COVID-19 pandemic has drawn our attention to. People across the globe practiced social distancing at all times in order to stop the contraction and spread of the virus.

Today, there is plethora of vaccines and vaccination / immunization program around the globe that help to prevent the acquisition and transmission of infectious diseases or their agents within a defined human or animal population. Vaccine research and development for other infectious diseases of man is also currently ongoing for the protection of the human race against any futuristic outbreak. Through proper immunization/vaccination of the susceptible human or animal hosts, the spread of infectious diseases or pathogens can be adequately contained and possibly managed or eradicated. Prior to birth, the foetus in the womb of an expectant mother is usually free from microbes or foreign bodies because of the germ-free environment provided by the uterus. But immediately after birth, the newborn becomes bathed with different types of infectious and non-infectious microbes including microorganisms that constantly attack the body from childhood to adulthood. As earlier said, mankind live in a potentially harsh environment that is packed with different kinds of microorganisms (both pathogenic and non-pathogenic microbes) that can cause diseases and infections of different types, but the human body has evolved mechanisms that allows it to evade the excesses of these microbes. This mechanism that helps the host’s body to be free from foreign agents including pathogenic microorganisms is known as the immune system.

The immune system of a human host helps to establish a germ-free state(known as immunity) against invading pathogenic microorganisms. Immunity which is derived from the Latin word “immunis” meaning “exempt or freedom from a burden” is a state of the host’s body that is characterized by the body’s ability to remove or counteract any trace of non-self molecules (antigens) that enters the body. An immune state can also be achieved in a human host through immunization – during which the host’s body becomes more responsive and prepared to work against foreign molecules or pathogens as they come in contact with the body. If truth be told, the science of immunology is centered and built around immunity – since this branch of biomedical science deals with how the host body is being protected (either naturally or artificially) from infectious disease agents. The immune system is indeed a significant security of the human host since the immunity it provides helps a great deal to help keep the body safe from the myriad of pathogens that surrounds the environment. This system (i.e., the host’s immune system) which learns and recognizes pathogens or antigens prior to and after the first attack has evolved to mass produce cells and molecules with which it uses to eliminate a variety of pathogenic microorganisms. Of particular interest is the ability of a host immune system to produce antibodies and other immunological responses that are unique and specific for each of the different pathogenic microorganisms that invade the human body. In most cases, these antibodies are produced first, after a natural infection (i.e., when the host is exposed to a pathogen or infectious disease) and secondly, when the individual is immunized or vaccinated against an infectious disease or infectious disease agent. Antibodies and immunological responses produced in advance helps to put the host in a better state to effectively fight off any infectious disease or pathogen attack against it.

Antibodies are protein molecules produced by the immune system on exposure to an antigen, and that can combine specifically with foreign molecules (i.e., antigens or pathogens). The immune system performs two basic functions in the body and these are: the function of recognition and discrimination and the function of response. Following the invasion of the body by pathogenic microorganisms, the immune system recognizes these foreign agents (non-self molecules) and discriminates or differentiates them from one another, especially from the self molecule of the host. The immune system also goes further to differentiate the non-self molecules from the body’s own molecules (i.e., self molecules which are harmless to the body expect in cases of autoimmunity). After doing this, the immune system recruits and engages a variety of immune system cells and molecules (known as effector cells) to specifically respond and eliminates the invading antigen or pathogen. This initial attack of the host body by antigens leaves behind memory cells – which help the immune system to recognize similar pathogen(s) in a second attack.

And in the second attack by the same or similar antigen or pathogen in the future, the host immune system directs a more robust and fast immune reaction or response that gets rid of the antigen and thus prevents the development and further spread of an infection or disease in the host. Thus, when the body’s immune system develops immunity to a specific antigen or pathogen, it will continue to remain free of the infection caused by that particular pathogen for (almost) a lifetime. A properly functional immune system is an asset to the body because such functional immune system will constantly police the body for the presence of antigens and other foreign bodies (inclusive of microbial pathogens) in order to discriminate the invading antigens or pathogens from the host’s self tissues and cells (Figure 1). Once identified, the body’s immune system mounts an immunological attack with its associated cells and organs to ward-off and contain the nefarious activities of the invading disease-causing agent. The spleen, white blood cells (WBCs), antibodies, the thymus and the lymph nodes are some of the organs and cells of the immune system that help to protect the body from disease and disease-causing microorganisms.

Figure 1. Schematic representation showing the various location and organs of the immune system. Photo courtesy: https://www.healthdirect.gov.au/immune-system

To ward-off antigens and possibly keep pathogenic microorganisms at bay, it is critical that the human host maintains a good nutrition and ensures that diets are taken in their balanced forms inclusive of all the necessary nutritional requirements required for normal growth and body development. Proper nutrition as well as proper and sufficient exercise and rest is vital for the conservation of a functional and robust immune system ready to challenge and fight off infectious diseases and their causative agents. This is vital because some infectious diseases of man (inclusive of immunodeficiency diseases) have been linked to malnutrition. Children and the elderly with immature immune systems are often at risk, and undernourishment makes these individuals to be more susceptible to infections caused by pathogens. Maternal undernourishment could also affect the immune system of the neonates as well as their birth weight. Expectant mothers are always advised to maintain good nutrition for both their health and that of the unborn child. And even in a diseased state, the loss of nutrients from the body in some disease conditions (e.g., diarrhea and vomiting) weakens the individual’s immune system; and proper replacement of lost fluids and intake of good diet is advisable in such scenarios. Malnourished individuals (especially children and the elderly) have higher morbidity and mortality rates to infectious diseases compared to people who eat well; and this also affects the course of infections in the former since their immune system have been impaired from protecting the body against invading pathogenic microorganisms and other disease causing particles or antigens.    

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

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