Immune system and immunisation

MEDICAL SCIENCE

By GELINDE NAREKINE
DISEASE-causing microorganisms, commonly known as germs, are all around us, both in our environment and in our bodies.
That means, every day we are repeatedly exposed to these organisms – bacteria, parasites, fungi, protozoa, and viruses. Many of them are highly infectious and can cause dreadful diseases if they enter our body and multiply.
When people who are vulnerable to infections encounter a harmful microorganism, it can lead to disease that may have fatal end results, including lifelong physical disabilities and death.
The body protects itself through various defense mechanisms to physically prevent such infectious agents or pathogens from entering the body or to kill them if they do. When a pathogen does infect the body, our immune system is triggered and the pathogen is attacked and destroyed or overcome. Thus, our immune system is an extremely important defense mechanism that can identify an invading organism and destroy it.
Each pathogen has unique distinguishing components, known as antigens, which enable the immune system to differentiate between ‘self’, the body, and ‘non-self’, the foreign material. The first time the immune system sees a new antigen, it needs to prepare to destroy it. During this time, the pathogen can multiply and cause disease. However, if the same antigen is seen again, the immune system is poised to confine and destroy the organism rapidly. This is known as adaptive immunity.
Vaccines utilise this adaptive immunity and immune memory to expose the body to the antigen without causing disease, so that when the live pathogen infects the body, the response is rapid and the pathogen is prevented from causing disease.
Depending on the type of infectious organism, the response required to remove it varies. For example, viruses hide within the body’s own cells in different tissues, such as the throat, the liver and the nervous system, and bacteria can multiply rapidly within infected tissues.
Lines of defence
The body prevents infection through a number of non-specific and specific mechanisms working on their own or collectively, together. The body’s first lines of defense are external barriers that prevent germs from entering. The largest of all is the skin which acts as a strong, waterproof, physical barrier and very few organisms are able to penetrate undamaged skin. There are other physical barriers and a variety of chemical defences.
Non-specific defences include: (1) Skin – a strong physical barrier, like a waterproof wall, it prevents microbes entering our body; (2) Mucus – a sticky trap secreted by all the surfaces inside the body that are directly linked to the outside, also contains antibodies and enzymes that attack and destroy infectious agents; (3) Cilia – microscopic hairs in the airways that move to pass debris and mucus up away from the lungs; (4) Lysozyme – a chemical (enzyme) present in tears and mucus that damages bacteria; (5) Phagocytes – various cells that scavenge and engulf debris and invading organisms, which form part of the surveillance system to alert the immune system of attack; (6) Commensal bacteria – bacteria on skin and gut that do not cause disease under normal physiological condition but compete with potentially harmful bacteria for space and nutrients; (7) Acid – in stomach and urine, make it hard for any germs to survive; and (8) Fever – elevated body temperature make conditions unfavourable for pathogens to survive and multiply.
Immune response
An immune response is triggered when the immune system is alerted that something foreign has entered the body. Triggers include the release of chemicals by damaged cells and inflammation, and changes in blood supply to an area of damage which attract white blood cells.
White blood cells destroy the infection or convey chemical messages to other parts of the immune system. As blood and tissue fluids circulate around the body, various components of the immune system are continually surveying for potential sources of attack or abnormal cells. These chain of biochemical events collectively work toward preventing disease from occurring.
Antigens and antibodies
Antigens are usually proteins that make up the cell wall or membrane of the pathogen. The name ‘antigen’ is derived from these two words “antibody generators.” Any given organism contains several different antigens. Viruses can contain as few as three antigens to more than a hundred as for herpes and pox viruses. Whereas protozoa, parasites, fungi, and bacteria are larger, more complex organisms and therefore, contain hundreds to thousands of antigens.
An immune response initially involves the production of antibodies that can bind to a particular antigen and the activation of antigen-specific white blood cells. Antibodies, also called immunoglobulins, are protein molecules that bind specifically to a particular part of an antigen, the so called antigenic site or epitope.  They are found in the blood and tissue fluids, including mucus secretions, saliva, and breast milk.
There are five classes of antibodies (designated IgG, IgA, IgM, IgD and IgE), which have a range of functions. They act as ‘flags’ to direct the immune system to foreign material for destruction and form part of the innate or natural immune response. Normally, low levels of antibodies circulate in the body tissue fluids. However, when an immune response is activated, greater quantities are produced to specifically target the foreign material.
When someone is vaccinated, the immune system is triggered to increase the levels of circulating antibodies against a certain antigen. Antibodies are produced by a type of white blood cell called B cells. Each B cell can only produce antibodies against one specific epitope. When activated, a B cell will multiply to produce more clones that are able to secrete that particular antibody. This goes to form what is known as cell-mediated or adaptive immunity.
Primary response
Upon exposure to a pathogen, the body will attempt to isolate and destroy that particular infectious agent. Chemicals released by inflammation increase blood flow and attract white blood cells to the area of infection. Specialist cells, known as phagocytes, engulf the target and dismantle it. These phagocytes then travel to the nearest lymph nodes where they ‘present’ the antigens to other cells of the immune system to induce a larger, more specific response. This response leads to the production of antigen-specific antibodies.
Circulating antibodies then find the organism and bind to its surface antigens. In this way it is labelled as ‘target’. This specific response is also called the adaptive or cell-mediated immune response, since the immune system adapts to suit the type of invader, which now becomes the immune system’s focus of attack. When the body is first exposed to an antigen, several days pass before this adaptive response becomes active. Upon first exposure to a pathogen, immune activity increases, then levels off and falls. Since the first, or primary, immune response is slow it cannot prevent disease, although it may help in recovery.
Once antigen-specific T and B cells white blood cells, also called lymphocytes, are activated, their numbers expand. Following an infection some memory cells remain resulting in memory for the specific antigens. This immune memory takes a few months to fully develop.
Secondary response
During subsequent exposures to the same pathogen, the immune system is able to respond rapidly because of the presence of immune memory cells, and thus, the activities of the immune system reaches higher levels. The secondary immune responses can usually prevent disease, because the pathogen is detected, attacked and destroyed before symptoms appear. In general, adults respond more rapidly to infection than children. They are able to prevent disease or reduce the severity of the disease by mounting a rapid and strong immune response to antigens they have previously experienced. In contrast, children have not experienced as many infectious antigens and therefore, are more likely to get sick easily.
Memory of the infection is reinforced and long lived antibodies remain in circulation. Some infections, such as chickenpox, induce a life-long memory of infection. Other infections, such as influenza, vary from season to season to such an extent that even an adult’s immune system is unable to adapt to preventing reoccurrence of infection.
Vaccination
Vaccination utilises this secondary response by exposing the body to the antigens of a particular pathogen and activates the immune system without causing disease. The initial response to a vaccine is similar to that of the primary response upon first exposure to a pathogen, slow and limited. Subsequent doses of the vaccine act to boost this response resulting in the production of long-lived antibodies and memory cells, as it would naturally following subsequent infections.
Vaccines vary in how they stimulate the immune system. Some provide a broader response than others. Vaccines influence the immune response through the nature of the antigens they contain, including number and characteristics of the antigens, or through the route of administration, such as orally, intramuscular or subcutaneous injection. The use of adjuvants in vaccines can help to determine the type, duration and intensity of the primary response and the characteristics of resulting antigen-specific memory. For most vaccines, more than one dose may be required to provide sustained, long-lasting protection – to be fully immunized against a particular pathogen and therefore, occurrence of disease is prevented.
Through vaccination, immunisation is achieved. This enables the body to rapidly respond to attack, with the immune system being enhanced in its response to a particular organism.
Types of immunisation
The two types of immunisation are active and passive immunisation. In active immunisation, when challenged by invading pathogen, the body generates its own response to protect against infection through specialised cells and antibodies, as stimulated by vaccines. Full protection takes time to develop but is long lasting. In passive immunisation, ready-made antibodies are passed directly to the person being immunised.
This allows for immediate protection, but immunity generated through this means may only last a few weeks or months. A classic example is that antibodies being passed from mothers to infants across the placenta and in breast milk, to protect the infants for a short time after birth. It is interesting to mention that antibodies are also purified from blood in laboratories. Such antibodies can be directly injected to provide rapid but short-lived protection or treatment for certain diseases, such as rabies, diphtheria and tetanus.
Source of information:
1. Pollard, AJ and Bijker, EM 2021, A guide to vaccinology: from basic principles to new developments, Nature Reviews of Immunology, 21, viewed 23 October 2021, https://www.nature.com/nri
2. The University of Auckland Immunisation Advisory Centre 2020,The immune system and immunization, viewed 02 November 2021, http://www.immune.nz.org/immunisation
3. World Health Organisation 2020, How do vaccines work?, viewed 02 November 2021, http://www.who.int

• Gelinde Narekine is a technical officer in the discipline of Medical Laboratory Science, School of Medicine and Health Sciences, University of Papua New Guinea.