Though the evolution of medicine and medical resources has led some to postulate that infections will no longer threaten humanity, epidemics and pandemics continue to strike nowadays. From the H1N1 pandemic of 2009 to this year’s measles pandemic, no one is safe. No one can escape unless we’re immunized. To become immunized one must either get sick going through all the ensuing risks that come with infection, or get vaccinated. So what is vaccination? What is immunization? And how do they help our immunity prevent infections?
No talk of vaccination can take place without a mention of a British physician named Edward Jenner, of bovine animals that are cows and their relation to the origin story of the world’s first vaccine. In 1798, it came into Jenner’s notice that milkmaids that were in contact with cowpox, got mildly sick but never thereafter contracted the smallpox disease. One must bear in mind that smallpox was quite the killer in those days and that mild disease seemed rather an attractive alternative compared to death. Not long after this observation, Jenner decided to inoculate people with cowpox so that he’d immunize them from smallpox. Therefore, by Pasteur’s denomination and in Jenner’s honor, the word “vaccination” came into existence from the Latin for cow, vacca.1
Immunization Vs Vaccination?
Vaccination is to purposefully expose an individual to a non-pathogenic form of a microbe in order to immunize him (i.e. confer him a long-lived state of immunity against this disease-causing pathogen).2,3
The terms immunization and vaccination are often erroneously used as synonyms. Here is why:
There are two types of immunization. Passive immunization which is the transfer of immune effectors in order to procure the individual with a temporary immunity against a given pathogen. This process may be done naturally, in case of maternal transfer to the fetus of antibodies through the placenta or through breastfeeding.2 It is used therapeutically in case of severe infections or envenomation where the specific adaptive immunity will take too long to activate its effectors. Therefore, transferring these effectors (especially antibodies) from an already immunized person or animal will help save the affected person’s life. This is the case of viper or scorpion venom exposure, rabies, diphtheria and tetanus in unimmunized patients.2
Active immunization is when the adaptive immune system is activated against a pathogen, successfully neutralizes it and creates an immunological memory that will permit a rapid and more effective response if further infections occur. In this case scenario, we are talking about a natural infection: pathogen encounters a susceptible host, infects it; the host reacts through his innate and adaptive immunity, creates an immunological memory and never will get sick another time. This doesn’t mean that the pathogen has ceased to exist. It is still there. It may try to re-infect the host, but the immune system remembers it, therefore it stops it before any signs of sickness occur.2
In vaccination’s case, as per our prior explanation: by favoring an encounter with a non-pathogenic form of a microbe with the host’s immune system, we will reduce all infection-related risks while maintaining the benefit of stimulating the adaptive immune system to produce specific effectors that are capable of effectively neutralizing the pathogen and inducing an immunological memory via memory cells that will ensure a faster and stronger immune response in case of a true encounter with the enemy in question.2,3
Consequently, we would have trained our immune system to remember, and it will ALWAYS remember.
For those of you, who are not familiar with the immunology subject, here are some fundamental notions that you need to take along in order to get further in our article.
Our immune system has two main defensive lines:
The innate immune response that assures a rapid, non-specific elimination of pathogens. The adaptive immunity, on the other hand, is slower to initiate, but insures a specific response that is capable of generating an immunological memory.2-6
If we look at the human body from a microbial point of view, we will face first a huge surface of the dermal epithelium that would seem impenetrable. Not only that, it also contains proteins that have the capacity to inhibit bacterial proliferation (mainly E. coli).4
If we try a different port of entry and end up choosing the respiratory system, we will find a mucus-producing epithelium with ciliated cells that will kick us out. If we try penetrating from the GI system, we’ll be welcomed by a highly acidic environment in the stomach, an alkaline one in the small intestines and a whole microbiota in the colon. If we’re still feeling persistent we might try the eyes, but this area too is protected by the lachrymal secretions that contain lysozymes capable of lysing different pathogens.
If by some chance, we find a gap through all these barriers, we will face an army of phagocytes, which would recognize and phagocyte (eat) us directly or by using opsonins. Opsonizations (from the Greek word that signifies “to make tasty”) is to favor phagocytosis through an opsonin-pathogen complex. Many molecules can act as opsonins, among which are complement molecules, collectins such as surfactant proteins, antibodies (IgA and IgG) bound to their specific antigens.4
The direct recognition of pathogens by phagocytic cells is done via specific receptors that identify certain molecular patterns that are common to all microbes. These receptors are known as Pattern-Recognition-Receptors or PRRs. Their corresponding ligands on microbes are known as Pathogen-Associated-Molecular-Patterns or PAMPs. There’s a wide array of these PRRs, each classified as a family and each is specific to a particular sequence in a pathogen. Some identify LipoPolySaccharides or LPSs (i.e. endotoxins) of Gram-Negative bacteria’s wall. Others locate certain viral or bacterial DNA in the cytoplasm of infected cells. Once this recognition has occurred, a cascade of signals will ensue and eventually cause the transcription and synthesis of opsonins, cytokines and other antimicrobial molecules such as interferon alpha and beta with their potent antiviral activity.2-4
A special mention in this fatal encounter of ours goes to phagocytes, the complement system and Natural Killer cells. They play major roles in both the innate and adaptive immune system. Indeed, phagocytes are responsible for processing epitopes and presenting them to effectors of the adaptive system. They act as Antigen-Presenting-Cells or APCs. The dendritic cell is often qualified as the professional APC. Furthermore, the collaboration of these phagocytes with adaptive immunity is apparent through the process of opsonization: one of the major opsonins are immune complexes.3,4,6 As for the complement system, its main pathways are the classic pathway that is activated by immune complexes (especially complexes made of immunoglobulins of certain IgG types neutralizing their specific epitopes on certain pathogen); the alternate and Lectin pathways that are activated directly by the circulating pathogens. In the three cases, all pathways will ultimately cause a lysis of their target by forming on its wall numerous pores or Membrane Attack Complexes (MACs). This will induce osmotic shock and cell death.3,4,6,8
NK T cells (Natural Killer) come from the T lineage and have a common role in both adaptive and innate immunity. They act through similar mechanisms to those observed in cytotoxic LTs. Besides, they can bind to immune complexes via FcRs (Receptors that bind constant fraction of the immunoglobulin) in order to lyse the neutralized pathogen. This phenomenon is called ADCC or Antibody Dependent Cell-mediated Cytotoxicity.3,5,6
As we’ve seen, the innate immune response is a rapid and strong one. It has some specificity into it throughout the use of PRRs. Recently, it has been proved that reinfection could induce a sort of “trained immunity” or an “innate immune memory”. It works in coordination with adaptive immunity and covers the battleground during the time necessary for its conspirator to get ready.9,10
Classically, the adaptive immune response is centered around Lymphocytes T. After their interactions with APCs especially dendritic cells or DCs, they will be activated. They’ll proliferate and differentiate. This requires an interaction between TCR (T-cell receptor) and antigen-holding MHC. Consequently, this ignites a signal that needs costimulation and the presence of certain specific cytokines. The whole process will help orientate the type of the subsequent response.5
LT CD4+ will recognize antigens presented on MHC type II. This type of Major Histocompatibility Complex is found on APCs. Once LT CD4+ is activated, it will differentiate into a T helper (LTH). It is responsible for regulating all immune cells including LBs, macrophages and others LTs. There exist at least 5 subtypes of LTH: LTH1, LTH17 which orchestrate cell-mediated immunity; LTFH and LTH2 that coordinate the antibody-mediated immunity or humoral immune response; and last but not least, Tregs which are the regulators (i.e. inhibitors) of activated T cells.5
LT CD8+, on the other hand, interact with antigens presented on MHC type I available on all the body’s nucleated cells. It is activated to cytotoxic lymphocyte T (CLT) that is capable of killing malignant and infected cells.5
Activated dendritic cells can interact with LBs through their BCR, thus activating them into plasma cells. They will be subjected to, through series of signals and cytokine stimulations, hypersomatic mutation and class switch recombination that will permit it to synthesize IgGs specific to the epitope that was initially presented. Furthermore, LBs are capable of switching to IgAs accountable for mucosal surfaces’ immunity and IgEs responsible for immunity against parasites and incriminated in allergy pathogenesis.6
LBs and CD8+ LTs have the capacity of recognizing antigens without T CD4+ help. This kind of immunity is said to be T independent and does not procure an immunological memory. In fact, the generation of LB, LT CD8+ and LT CD4+ memory cells is conditioned by a T dependent immune response.5 In case of T independent humoral response, it is important to know that no hypersomatic mutation or class switch recombination occurs; therefore, the antibodies produced subsequently will be mainly IgMs and will have low affinity towards the antigen. Nonetheless, this type of immune response is thought to be enhanced by interactions with other cell types such as macrophages, LTs and possibly neutrophils.6
As complex and strong as our immune system is, sometimes an infection can overwhelm it. Some pathogens have even found ways to escape our immunity’s different defenses.3-6 This has promoted the use of vaccination, a way to induce immunization while escaping the tedious and dangerous infection.
Theoretically, introducing a form of a pathogen that doesn’t cause disease will successfully stimulate both the cellular and humoral immune response, generate an immune memory and will have minimal to no side effects.
There are several types of vaccines, mainly: live attenuated vaccines, killed vaccines, and subunit vaccines of which there are many varieties that we will mention briefly.
Let’s begin with the live attenuated vaccines. These vaccines consist of live but non-pathogenic microbes. Microbes can be non-pathogenic naturally as is the case of the vaccinia virus in humans. This virus causes cowpox. It is a relative of the Variolae virus that causes smallpox and was used in immunizing individuals against this disease.1,2
Attenuation can also be obtained by cultivating a pathogen in abnormal conditions and creating a strain that would only cause disease in these harsh conditions. It is the case of the Bacillus of Calmette and Guérain (BCG), an attenuated strain of Mycobacterium bovis that is used to immunize newborns against tuberculosis.11 The benefit in an attenuated live vaccine is that the microbe still has its ability to replicate and by doing so continues to challenge the immunity, therefore inciting a robust immune response and a solid memory with little or no use of boosters. However, since the pathogen is only attenuated, there remains the risk of reverting back to its disease-causing form especially in immunocompromised hosts. Of these vaccines, we cite: oral polio vaccine (OPV), the measles mumps rubella vaccine or MMR and the above-mentioned BCG.1-3
Next are the killed or inactivated vaccines, which are pathogens that are rendered inactive through treatment with chemicals such as formalin or phenol or by using heat waves. By doing so the pathogen loses its ability to replicate thus reducing (without completely eliminating) reactivation risk while upholding immunogenicity. Because there is no replication, the use of boosters is sometimes required. It is the case of the injectable polio vaccine, the rabies vaccine, and the seasonal flu vaccine.1-3
The reason for seasonally vaccinating against the flu is because there are two main human influenza viruses: type A and type B. Each have two main antigens: Hemagglutinin H and Neuraminidase N. Point mutation in their genes insures a great variability of these antigens in the same host, and when neutralizing antibodies diminish enough of the dominant strains, a new one may emerge as the main one. This occurs throughout the year and is called antigenic drift. With the emergence of new strains unrecognized by the host’s immunity, infection occurs and an epidemic spreads because not enough individuals are immunized against it. This may also happen when an exchange of antigens between two different strains occurs in the same host. This phenomenon is called antigenic shift resulting in a novel strain that cause a pandemic. This is why global surveillance systems are watching out for circulating strains and updating the vaccine’s formulations semi-annually to prevent further outbreaks. However, the insufficient vaccination throughout the world continues to be a great setback.15
Subunit vaccines came to exist in order to diminish risks associated with using the attenuated or inactivated vaccines. They consist mainly of isolating and administering macromolecules or parts of the pathogen that are immunogenic. Of these vaccines we can identify inactivated exotoxins or toxoids as is the case of tetanus, diphtheria and acellular pertussis vaccine (DTap). They can also consist of capsular polysaccharides or surface glycoproteins such as the pneumococcal and meningococcal vaccines and the hepatitis B vaccine.2
A major issue encountered with polysaccharides-based vaccines is their lack of immunogenicity. Indeed, they cause especially in infants (because they have a low number of DCs) T-independent immune responses. This is mainly short-circuited by the numerous boosters required for these vaccines. They are quite shortly spaced in the early life (2, 4 and 12 months of life) because of this peculiarity in babies. Further will be discussed below regarding this issue.2
Another way of augmenting subunit vaccines immunogenicity is by conjugating them with immunogenic proteins that will convert T-independent immunity to a T-dependent one. It is the case of Haemophilus influenzae conjugated vaccine. Since the polysaccharide vaccine failed to protect children aged two years or less (in whom the disease was highly prevalent), conjugation through covalent binding of its main capsular polysaccharide PRP (polyribosylribitol phosphate) to immunogenic proteins proved more efficient. The proteins used in this conjugation were tetanus toxoids, diphtheria toxoids and meningococcal outer membrane protein complex. Polysaccharide vaccines are still used in older children and in adults.12 The same concept was used for the pneumococcal vaccine of which exist both conjugated and polysaccharide vaccines.1-3,10
Likewise, adjuvants can aid in elevating immunogenicity. They will enhance immune responses in individuals that react poorly to vaccines such as infants, the elderly and immunocompromised hosts. Some work by releasing the antigen slowly, therefore maintaining a long stimulation of the immune system. Some will activate certain PRRs ascertaining more cellular engagement.1-3,13 Even the use of certain chemokines is being considered in order to attract immune cells to the site of injection, this will help generate a mucosal immunity and a port-of-entry protection.2
Regarding the hepatitis B vaccine, it is composed of the virus’s surface antigen or HbSAg. It was first derived from the sera of patients who had chronic hepatitis B. This circulating antigen has no pathogenic consequence for it has no viral DNA. These molecules were later used as a vaccine after isolation and appropriate purification. Plasma-derived subunitary hepatitis B vaccine has been replaced by recombinant DNA vaccines. Through recombinant DNA technology, cloning the gene encoding HbSAg into yeast cells had rendered the process much easier.2,14
Since we are venturing into the world of genetic engineering, let us talk a little bit about recombinant vector vaccines.
The idea behind recombinant vector DNA is to incorporate the gene coding for a pathogen’s antigen into a live attenuated vaccine. Preferably, one that has already been approved for use. Therefore, in vaccinating against the main pathogen we could obtain immunity against the inoculated antigen at the same time. This has been tested with the yellow fever vaccine who had been genetically engineered to express West Nile Virus antigen. Using the vaccine of typhoid fever to express vibrio cholera antigens, has the benefit of stimulating the synthesis of IgAs in intestinal mucosal surfaces, targeting thus the pathogen’s port of entry.
DNA vaccines on the other hand, use this concept: a plasmid DNA encoding for certain antigenic glycoproteins is injected directly into the muscle of a person. The muscle cells and DCs will take it up, express it over a long period of time because it will be integrated into their DNA. This ingenious idea can be problematic due to the little quantity of CMH expressed by muscle cells and the low numbers of dendritic cells in muscle tissue. A stronger immune reaction can be induced if extremely immunogenic DNA fragments of certain pathogens are included in the vaccine. Do you remember how we said earlier that certain PRRs have the capacity of identifying microbial DNA? Toll Like Receptors or TLRs are a family of PRRs that can do this. So, in adding DNA motifs that stimulate specific TLRs and amplify the immune response, we increase the antigen’s chances of being processed by DCs and presented to LTs.2
Singularities of infants
As cited above, infants have certain characteristics that renders their immunization challenging. Infants’ immune system is activated by only a handful of vaccines (mainly BCG and hepatitis B vaccine).10,16 As to why it responds to some antigenic stimulations and not to others, remains a mystery. It is speculated that immune immaturity is due to the high tolerogenic environment (high concentration of Tregs and inhibitory cytokines) that is much needed during the pregnancies. Additionally, infants have low PRRs activation levels and are unable of initiating T-independent immune responses, this is why conjugating polysaccharide vaccines as well as adding adjuvants is particularly interesting in this group of age.10 Moreover, newborns are under the protection of passively-acquired maternal antibodies. Maternal IgGs passing through the placenta (IgMs are too large to be transferred through the placenta and lack the specific receptor) at high rates during the last trimester of pregnancy, as well as maternal IgA and IgGs passing through breastfeeding confer a certain protection to the newborn.17 This protection however can intervene with immunization schedules when specific antibodies neutralize antigens contained in vaccines.10,17
By looking at the full half of the glass, scientists put this obstacle into use. As a matter of fact, by immunizing pregnant women against pathogens to which their neonates are susceptible of contracting, we will protect the infant through the passive immunity bestowed by his mother and will prevent his mother from being the person from which he contracted the disease. This would be quite effective in preventing Group B streptococcal infections (a pathogen living in the genital system of women often transmitted to newborns during delivery), but also SRV infections (that cause bronchiolitis in children), pertussis (the vaccine administration is delayed to 2 months of age due to adverse effects in newborns) and the flu.10,17-19
In conclusion, vaccination schedules, administration and adverse effects are well-studied domains. The fact that vaccinating is directed toward healthy individuals in the purpose of preventing disease, holds their manufacturing at the utmost standards of safety.20 Moreover, the arising epidemics of measles and influenza only further advocate their efficacy and necessity. Even so, vaccinating an individual can, despite all efforts, fail to immunize him against diseases (in a minority of cases). Consequently, the community immunity induced by immunizing a sufficient number of people can protect these individuals by reducing the number of human reservoirs and receptive hosts and thwarting further transmission. This is called the ‘Herd immunity’ effect.1,2,20 One of numerous benefits vaccination bestows upon us. That is why vaccinating is our shared responsibility and a main public health resolve.
1- Tortora GJ, Funke BR, Case CL. Microbiology: An Introduction. Thirteenth Edition. Boston : Pearson; 2019. Chapter 18: Practical Applications of Immunology; pp 500-506.
2- Owen JA, Punt J, Stranford SA. Kuby Immunology. Seventh Edition. New York NY: North American Edition; 2013. Chapter 17: Infectious diseases and vaccines; pp574-588.
3- Clem A. Fundamentals of vaccine immunology. Journal of Global Infectious Diseases. 2011;3(1):73.
4- Owen JA, Punt J, Stranford SA. Kuby Immunology. Seventh Edition. New York NY: North American Edition; 2013. Chapter 5: Innate Immunity; pp 141-182.
5- Owen JA, Punt J, Stranford SA. Kuby Immunology. Seventh Edition. New York NY: North American Edition; 2013. Chapter 11: T-Cell Activation, Differentiation, and Memory; pp 357-383.
6- Owen JA, Punt J, Stranford SA. Kuby Immunology. Seventh Edition. New York NY: North American Edition; 2013. Chapter 12: B-Cell Activation, Differentiation, and Memory; pp 385-414.
7- Gläser R, Harder J, Lange H, Bartels J, Christophers E, Schröder J. Antimicrobial psoriasin (S100A7) protects human skin from Escherichia coli infection. Nature Immunology. 2004;6(1):57-64.
8- Owen JA, Punt J, Stranford SA. Kuby Immunology. Seventh Edition. New York NY: North American Edition; 2013. Chapter 6: The Complement System; pp 187-221.
9- Netea M, Joosten L, Latz E, Mills K, Natoli G, Stunnenberg H et al. Trained immunity: A program of innate immune memory in health and disease. Science. 2016;352(6284):aaf1098-aaf1098.
10- Nicoli F, Appay V. Immunological considerations regarding parental concerns on pediatric immunizations. Vaccine. 2017;35(23):3012-3019.
11- Tortora GJ, Funke BR, Case CL. Microbiology: An Introduction. Thirteenth Edition. Boston: Pearson; 2019. Chapter 24:Microbial Diseases of the Respiratory System; pp701.
12- Acharya Nanduri S, Sutherland AR, Gordon LK, Santosham M. Haemophilus influenzae Type b Vaccines. In: Plotkin SA, Orenstein WA, Offit PA, Edwards KM. Plotkin’s Vaccines. 7th Edition. Philadelphia, PA : Elsevier, 2018. pp441-468.
13- Tregoning J, Russell R, Kinnear E. Adjuvanted influenza vaccines. Human Vaccines & Immunotherapeutics. 2018;14(3):550-564.
14- Van Damme P, Ward JW, Shouval D, Zanetti A. Hepatitis B Vaccines. In: Plotkin SA, Orenstein WA, Offit PA, Edwards KM. Plotkin’s Vaccines. 7th Edition. Philadelphia, PA : Elsevier, 2018. pp342-374.
15- Bresee JS, Fry AM, Sambhara S, Cox NJ. Inactivated Influenza Vaccines. In: Plotkin SA, Orenstein WA, Offit PA, Edwards KM. Plotkin’s Vaccines. 7th Edition. Philadelphia, PA : Elsevier, 2018. pp684-737.
16- Ota M, Vekemans J, Schlegel-Haueter S, Fielding K, Whittle H, Lambert P et al. Hepatitis B immunisation induces higher antibody and memory Th2 responses in new-borns than in adults. Vaccine. 2004;22(3-4):511-519.
17- Marchant A, Sadarangani M, Garand M, Dauby N, Verhasselt V, Pereira L et al. Maternal immunisation: collaborating with mother nature. The Lancet Infectious Diseases. 2017;17(7):e197-e208.
18- Abu Raya B, Edwards K, Scheifele D, Halperin S. Pertussis and influenza immunisation during pregnancy: a landscape review. The Lancet Infectious Diseases. 2017;17(7):e209-e222.
19- Heath P, Culley F, Jones C, Kampmann B, Le Doare K, Nunes M et al. Group B streptococcus and respiratory syncytial virus immunisation during pregnancy: a landscape analysis. The Lancet Infectious Diseases. 2017;17(7):e223-e234.
20- IOM (Institute of Medicine). 2013. The childhood immunization schedule and safety: Stakeholder concerns, scientific evidence, and future studies. Washington, DC: The National Academies Press.