The Basic Principles of Vaccination

Vaccination simply means the introduction of a live or killed micro-organism or its antigens into a living healthy host to stimulate the immune response of the host to the disease-causing form of the organism.

It is the process of producing immunity to a disease by introducing a special preparation of appropriate antigenic material or a whole organism in the form of a vaccine into the host.

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. The agent stimulates the body’s immune system to recognize the agent as foreign, destroy it, and “remember” it so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters. (WHO, 2017)

Before we go on to the basic principles of vaccination, let us take a look at the historical perspective.

The Historical Perspective

The name vaccine is coined from the first vaccine produced against cowpox virus (vaccinia) by Edward Jenner. This vaccine conferred protection against cowpox and other related viruses like smallpox virus after it caused a mild infection.

Relevant Terms in Vaccination and Immunity

• Antigen: Any molecule on the surface of a microbe or within it that makes it recognizable by immune cells.
• Antibody: This is a molecule that binds to an antigen during immune reactions.
• Attenuation: This is the scientific process of rendering a pathogenic (disease-causing) organism too weak to cause disease.
• Antigen Presenting Cells (APCs): These are cells in the body that are capable of processing antigens on the surface of microorganisms for recognition by the immune cells. These include macrophages, dendritic cells, etc. Click here to Read about Cells of the Immune System.
• Microbe: These are micro-organisms, small living organisms that are capable of causing human diseases or destruction of certain substances.
• Major Histocompatibility Complex(MHC): These are surface proteins essential for immune cells to recognize foreign antigens in vertebrates.
• Naïve cells: These are cells that have not had initial contact with an antigen. That is, they have not been sensitized.
• Sensitization: The process of exposing the body’s immune cells to antigen and stimulating them to produce substances that will destroy the invading particle or organism.

When a host is exposed to a new microorganism for the first time, it becomes primed or sensitized and it attempts to fight against the invading agent. This it does by certain humoral (antibody-mediated) and cellular mechanisms. This occurs in every case of infections but the progress or not of the infection into a full-blown disease is dependent on the ability of the host immune system to completely eliminate or destroy the offending agent. Thus, the full disease and its classical clinical features are seen if the host immunity was incapable of totally destroying the agent.

It is worth noting that Immunization is a very important concept in vaccination. Immunization is often confused for vaccination but they do not exactly mean the same thing.
Immunization refers to the immunological changes that occur following vaccination.

Steps of Immunization

1. Antigen Presentation and Immune Recognition
2. Stimulation of the Immune System
3. Protection against Infection and Disease

When a new particle is encountered within the body of the host, immune mechanisms are triggered to ascertain if the particle is indigenous or foreign. This is possible because different cells have characteristic surface antigens that differentiate them from other cells. No two organisms have exactly similar antigens, even among humans. We have unique antigens referred collectively to as the Human Leucocyte Antigen (HLA). 

Certain molecules on the surface of microorganisms like bacteria and viruses are specific for them and their presence will almost always indicate their presence. For example, N-formyl methionine is especially seen in bacteria and a few other organisms but not produced by human cells.

After this encounter, the antigenic material is then carried and processed by certain cells known as Antigen Presenting Cells(APCs) [see terms above]. These APCs break down the polypeptide sequence of the antigen into smaller fractions of oligopeptides, dipeptides and simple amino acids. Then the APCs present these simple forms(epitopes) of the antigen on their surface couple with certain surface molecules referred to as the Major Histocompatibility Complex(MHC) to naïve T-cells.
The MHCs are of two types depending on the types of antigens they are associated with, and the subset of naive T cells they present them to.
The subset of T cells includes the CD4, CD8 and NK cells. However, in this process of antigen presentation, only CD4 and CD8 T cells are involved.

MHC I occurs in all nucleated cells, platelets and other blood cells and they are involved in presenting epitopes to CD8 cells(Cytotoxic T Lymphocytes -CTLs). They are important in recognizing and destroying intracellular pathogens such as bacteria(Mycobacteria tuberculosis), viruses(HIV), Rickettsias and Chlamydia.
MHC II normally occurs on professional antigen-presenting cells like macrophages and dendritic cells. They are related to extracellular organisms and particles, presenting these antigenic epitopes to CD4 cells(Helper T Lymphocytes). The helper cells have three other subsets namely; T Helper-1, T Helper-2 and T Helper-17 cells.
Just after activation of the naive T Helper cells, they differentiate into one of the three subsets depending on the nature of the invading organism whose antigen was presented.

The immune system stimulation can be broadly viewed in two ways. First involving the humoral immunity or production of antibodies. Here, the helper T cells play a major role in activating the B lymphocytes, or in the activation of soluble complement proteins. These reactions aim to produce cytokines or chemokines that will recruit other host’s effector cells to destroy and phagocytose or eat up the offending agent.

The other way in which immunity can be achieved is through the stimulation of cytotoxic T cells as we mentioned earlier. This is otherwise known as the cell-mediated immunity because the cytotoxic T cells are capable of destroying the intracellular pathogen without necessarily involving chemicals or humoral means. One notable mechanism by which this is achieved is through the Perforin-Granzyme Pathway
The NK cells are activated in response to interferons or macrophage-derived cytokines. They serve to contain viral infections while the adaptive immune response generates antigen-specific cytotoxic T cells that can clear the infection. NK cells work to control viral infections by secreting IFNγ and TNFα.

At the end of the processes involved in the stimulation of the immune system, not only does the host acquire immediate protection from the organism but it also watches out for any subsequent exposure by producing memory cells that act to remember the identity of the foreign particle and alert the immune system when there is a recurrence.

This is what I have described above in Steps in Immunization.

However, in vaccination, the whole process is triggered artificially by deliberately inoculating antigenic epitopes of the organism you want to protect the individual against. By removing the virulence of the organism, it can no longer cause disease but because the antigenic epitopes are preserved in vaccination, it is still capable of inducing immune stimulation or immunization.
It should be noted that vaccines are not available for every pathogenic organism yet because of the innate ability of some organism to evade immunity several times or due to the difficulty in extracting the active epitope and at the same time pathogenically inactivating the organism.

The subtle difference in the way vaccines stimulates the body’s immune system is seen in the state of the organism or antigen used. In a natural exposure, the live pathogenic organism or its toxins are responsible for causing disease in a susceptible individual. The disease presents because the organism was able to surmount the host responses. 

In vaccination, a weakened form of the organism or the toxins is inoculated. These are composed of harmless antigens that are incapable of causing disease but is sufficient to stimulate the body’s immune system.

Vaccination Can Be Done In Childhood and Adulthood

Bearing in mind what we have discussed above, we can deduce that vaccines are safe and effective for both children and adults. However, for maximum protection from childhood vaccine-preventable diseases, policies have been made by the World Health Organisation and National Health Ministries to encourage vaccination in children.
Some of the vaccine-preventable diseases include:

• Diphtheria
• Haemophilus influenzae type b (Hib)
• Hepatitis A
• Hepatitis B
• Human Papillomavirus (HPV)
• Influenza (Flu)
• Measles
• Meningococcal Infections
• Mumps
• Pertussis (Whooping Cough)
• Pneumococcal Infections
• Polio
• Rotavirus
• Rubella (German Measles)
• Tetanus
• Varicella (Chicken Pox)

Conclusion