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The Science of Immunity and Vaccines


Immunity is the capacity of the human body to shield itself from infectious diseases. The body's defensive mechanisms are diverse and include:


  • Innate (non-specific or non-adaptive)
  • Acquired (specific or adaptive)

Innate Immunity:

Innate or non-specific immunity is present from birth which contains physical barriers (e.g., preserved skin and mucous membranes), biological barriers (e.g., gastric acid, digestive enzymes, and bacteriostatic fatty acids in the skin), phagocytic cell, and the complement method.

Acquired Immunity:

Acquired immunity is usually unique to a particular individual or a group of closely associated species.

2 basic mechanisms for acquiring immunity:

  • Active
  • Passive


Active immunity is a defense that is provided by an individual immune system and is normally long-lasting. In general, such immunity requires cellular reactions, serum antibodies, or a mixture acting against one or more antigens in the infecting organism. Active immunity can be obtained by natural illness or by the vaccine.


  • Antibody-mediated (responses produced by B lymphocytes or B cells)
  • Cell-mediated (responses produced by T lymphocytes or T cells)


Passive immunity defends against the spread of antibodies from resistant individuals, most often through transfusion of blood or blood components, including immunoglobulin, via the placenta.

The defense offered by the cross-placental transmission of antibodies from mother to child is more effective against such infections.

This defense is brief – usually only for a few weeks or months.

How vaccines generally work?

Vaccines typically offer immunity close to that offered by a normal infection, but without the possibility of illness or complications. Compared to exposure in the natural world, vaccines offer precise stimuli with a limited number of antigens (any foreign body).

Vaccines have a beneficial effect by stimulating active immunity and supplying immunological memory. Immunological memory helps the immune system to recognize and respond easily to exposure to normal infection later, thus avoiding or altering the disease. Antibodies may be found in blood or serum or other body fluids, but immunological memory may also be present even in the absence of detectable antibodies.

Traditional Vaccines:

Traditional vaccines are made from entire pathogens, either inactivated (killed) or attenuated live species, secreted materials, like toxins or pieces of the pathogen skeleton, either as virus-like particles or subunit vaccines.

Newer Vaccines:

Newer vaccines are being produced and manufactured using recombinant viral vectors to transmit the antigen (Ewer et al. 2016). These viral vectors may be either replicating vectors where there is local replication and thus multiplication in the receiver, like live attenuated vaccines, or non-replicating vectors, or replicating defective vectors, which can only reproduce in some cell lines used for production, and more analogous to inactivated vaccines.

The newest forms of vaccination use the genetic code of the pathogen as a vaccine, and then uses the host cell machinery (including enzymes and ribosomes) to transform proteins that then serve as an intracellular antigen and activate the immune response (van Riel and de Wit, 2020). These DNA or RNA vaccines also use a lipid outer shell to assist cell entry and may have modified nucleotides or nucleosides to delay destruction by host cell machines and to modulate the necessary components of the immune system (Verbeke et al 2019). In any of these vaccinations, the gene sequence will code for self-replication within the host cell to produce much more antigen and thus cause a more vigorous response. The mRNA is a normal part of the body, does not reach the nucleus, and is fully absorbed in the cytoplasm.

Examples of Vaccines:

  • Inactivated poliomyelitis virus (IPV) contains inactivated bacteria or viruses.
  • Tetanus and Diphtheria vaccines contain inactivated toxins (toxoids)
  • Influenza vaccine contains a surface protein called haemagglutinin
  • Pneumococcal vaccine contains the polysaccharide from the capsule.
  • Live attenuated vaccines include yellow fever; measles, mumps, and rubella (MMR); and Bacillus Calmette–Guérin (BCG).

Vaccines failure:

No vaccine gives 100% protection, and a proportion of individuals get affected even vaccinated. Vaccines can fail in two major ways – known as primary or secondary failures of the vaccine.

Primary failure failure happens when a person fails to react to the vaccine with an initial immune response. Infection can also occur at any stage after vaccination.

Secondary loss happens as the person reacts initially, but then the defense reduces over time. As a result, the rate of secondary vaccine deficiency increases over time.

Individuals who become affected after vaccines may have a changed, milder type of disease and are less likely to have severe symptoms than people who have never been vaccinated.

How vaccines are made?

Vaccines are typically produced by growing colonies of the target virus or bacteria. Viruses need to develop in tissues, so vaccine viruses are mostly grown in eggs (e.g., influenza) or mammalian-derived cells, like humans.

This culture medium contains a broad variety of nutrients and growth factors that may have been extracted from materials of animal origin, such as serum, milk and milk compounds, gelatin, meat extracts, or extracts from other muscle tissues. These elements are used in the early stages of development and cannot be present or only present in trace quantities (residues) in the final vaccinations.

Animal enzymes are still used during the manufacturing of vaccine viruses, however subsequent cleaning, purification, and dilution measures are separated from the final vaccine. One example is trypsin, generally obtained from pigs, which is commonly used in the manufacturing of vaccines and is usually applied to the final cell culture to activate the vaccine virus. Trypsin is also used in the manufacturing of other medicinal products, e.g., insulin and heparin. Trypsin is then extracted during the next phase of the production process (e.g., by washing and filtration).

Cell lines are used for vaccines!

The mammalian cell lines used to develop the vaccine virus would usually be extracted from the primary cell culture of a single animal liver, which has been propagated extensively in the laboratory for several decades. For example, measles vaccines are grown in chick embryo cells and polio vaccines are grown in a mouse cell line. Another animal cell line, now used to develop an egg-free flu vaccine, was derived from a cocker spaniel kidney in 1958.

The best-known human cell line is MRC5; these cells are extracted from the lung of a 14-week-old male fetus from birth, which ended in 1966 for medical reasons. This cell-line is used to produce viruses for rubella, chickenpox, and hepatitis A vaccines. Some fetal cell lines have been used for other vaccines, including measles and some of the latest COVID-19 vaccines.

Ingredients of Vaccines

As vaccines are typically complex biological products, a variety of substances are used to ensure the consistency of the finished product.


  • Highly modified animal substance derivatives (Gelatin) are rarely used in the finished vaccine formulation and are known as excipients.
  • Adjuvants – are used to enhance the immune response of vaccines. The most widely used adjuvants are aluminum salts. Aluminum salts, such as aluminum hydroxide, aluminum phosphate, and aluminum potassium sulfate, which have been safely used in vaccinations for more than 70 years.

Population Immunity or Herd Immunity

The main purpose of vaccines is to protect the person who receives the vaccine. Vaccinated people are therefore less likely to be a cause of illness for others, which decreases the likelihood that unvaccinated people may be susceptible to illness. This ensures that people who cannot be vaccinated will also benefit from the routine vaccine program. This term is known as population (or 'herd') immunity.