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Passive Immunization involves delivering preformed antibodies to an individual at risk of infection. You are taking the humoral immunity from one individual or animal and transferring it to another individual. It has an instant effect as the immunoglobulins are preformed and the body doesn’t need to “waste” any time figuring out how to make antibodies of its own that are targeted at the pathogen. However, passive immunity has a short duration of action because it only works for the lifespan of the transferred antibodies. This happens naturally in infants. IgA is transferred from mother to infant through breast milk. IgG is also transferred from the mother to the fetus through the placenta. Through these mechanisms an infant that does not yet have a mature immune system can be protected. Purified antibodies made by horses are also treatment options for tetanus, botulism and rabies. Following an infection with one of these organisms traditional (active) immunization is not an option, because it takes too long and the patient could die before it kicks in.
Usually when you are discussing vaccination, you are referring to Active Immunization. This involves helping the host make their own antibodies against a pathogen. It takes longer to start working than passive immunity, but its affects are more potent and can last for many years. The rest of this section will discuss active immunity.
Through the activation of adaptive immunity our bodies have immunologic memory which allows us to react faster and with higher potency the second time we see a particular pathogen. This works really well for relatively benign pathogens. For example, you get a cold once and then your immune system will be better at fighting the causative pathogen the next time it is exposed to it. However, this trial by fire method doesn’t work well for infections that have significant morbidity or mortality. We need a prophylactic way to prime the immune system. Vaccinations/immunizations are the way we gain immunologic memory without having an active infection. This involves exposing the patient to an antigen on the pathogen of interest in a way that is not dangerous. This “dress rehearsal” lets the immune system “practice” fighting the pathogen in a situation where the patient isn’t actually infected. After being exposed to the epitope present in the vaccine the patient will react much faster and with greater potency if they ever see the actual pathogen.
Live/Attenuated Vaccination involves giving the patient a small amount of the living pathogen you are trying to immunize for. Attenuation is the process of altering a pathogen to diminish its virulence (for example growing the pathogen outside of a human host for many generations so that it adapts to a non-human environment). These attenuated pathogens can grow inside of the patient, but they grow very slowly and have lost their ability to cause disease. These attenuated pathogens look similar enough to the infectious pathogen that the immune response against the vaccine will also work against the real pathogen. The immunologic memory induced can last for decades meaning less booster shots are needed. The immune response is strong because the live pathogen induces humoral, mucosal and cell-mediated immunity. In rare cases the attenuated pathogen can mutate back into the virulent pathogen and cause the very disease you are trying to immunize against. Examples include MMR (Measles, Mumps & Rubella), Sabin (Oral Polio Vaccine), and the Varicella Zoster Vaccine.
Killed/Inactivated Vaccination involves giving the patient a pathogen that has been “killed” with heat or formaldehyde. These dead pathogens still have surface antigens that are recognized by the immune system creating a relatively weak, short term humoral immunity. Booster shots are needed more frequently with this type of immunization. Because there is no live pathogen, this type of vaccination cannot cause infection. Examples include the Salk (Injected Polio Vaccine), Rabies Vaccine and Pertussis Vaccine. The Influenza Vaccine either contains full inactivated virus particles or fragments of inactivated virus particles.
A Toxoid is a bacterial exotoxin that has been inactivated with heat or formaldehyde. This removes the toxic effect of the toxin so that it can be given in a vaccine without causing the disease. This type of immunization create immunologic memory against the noxious toxin instead of the pathogen itself with is relatively innocuous. Toxoid vaccines are available for Tetanus and Diphtheria. DPT is a combination vaccine that has tetanus and diphtheria toxoid combined with whole inactivated pertussis cells. Toxoids are highly immunogenic proteins that activate T-Cell Dependent immunity. Therefore, they are often covalently bound to carbohydrate antigens to create vaccines. A carbohydrate (such as a fragment of a bacteria carbohydrate capsule) is not immunogenic enough to create strong immunologic memory. Helper T-cells only recognize protein antigens, but if you connect a carbohydrate antigen to a protein toxoid the T-Cell Dependent portions of the adaptive immune system will be able to recognize it. This method of combining a carbohydrate immunogen with a carrier toxoid protein is referred to as a Conjugate Vaccine. Using this method you can create vaccines against the polysaccharide capsules of bacteria like Type B H. Flu and Neisseria Meningitidis.