Everything about Immunity Medical totally explained
Immunity is a medical term that describes a state of having sufficient biological defenses to avoid
infection,
disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of pathogens to stop infection by micro-organisms before they can cause disease. Other components of the
immune system adapt themselves to each new disease encountered and are able to generate pathogen-specific immunity.
Adaptive immunity is often sub-divided into two major types depending on how the immunity was introduced.
Natural immunity occurs through contact with a disease causing agent, when the contact wasn't deliberate, whereas
artificial immunity develops only through
deliberate actions. Both natural and artificial immunity can be further subdivided, depending on the amount of time the protection lasts.
Passive immunity is short lived, and usually lasts only a few months, whereas protection via
active immunity lasts much longer, and is sometimes life-long. The diagram below summarizes these divisions of immunity.
A further subdivision of adaptive immunity is characterized by the cells involved;
humoral immunity is the aspect of immunity that's mediated by secreted antibodies, whereas the protection provided by
cell mediated immunity involves T-lymphocytes alone. Humoral immunity is active when the organism generates it’s own antibodies, and passive when antibodies are transferred between individuals. Similarly, cell mediated immunity is active when the organisms’ own T-cells are stimulated and passive when T cells come from another organism.
History of theories of immunity
The concept of immunity has intrigued mankind for thousands of years. The prehistoric view of disease was that it was caused by supernatural forces, and that illness was a form of
theurgic punishment for “bad deeds” or “evil thoughts” visited upon the soul by the gods or by one’s enemies. Between the time of
Hippocrates and the 19th century, when the foundations of the scientific method were laid, diseases were attributed to an alteration or imbalance in one of the
four humors (blood, phlegm, yellow bile or black bile). Also popular during this time was the
miasma theory, which held that diseases such as
cholera or the
Black Plague were caused by a miasma, a noxious form of "bad air". The term “immunes”, is also found in the epic poem “
Pharsalia” written around 60 B.C. by the poet
Marcus Annaeus Lucanus to describe a North African tribe’s resistance to
snake venom.) written by the
Islamic physician
Al-Razi in the 9th century. In the treatise, Al Razi describes the clinical presentation of smallpox and measles and goes on to indicate that that exposure to these specific agents confers lasting immunity (although he doesn't use this term).
In 1888
Emile Roux and
Alexandre Yersin isolated
diphtheria toxin, and following the 1890 discovery by
Behring and
Kitasato of antitoxin based immunity to
diphtheria and
tetanus, the
antitoxin became the first major success of modern therapeutic Immunology. Passive immunity provides immediate protection, but the body doesn't develop memory, therefore the patient is at risk of being infected by the same pathogen later.
Naturally acquired passive immunity
Maternal passive immunity is a type of naturally acquired passive immunity, and refers to
antibody-mediated immunity conveyed to a
fetus by its mother during pregnancy. Maternal antibodies (MatAb) are passed through the
placenta to the fetus by an
FcRn receptor on placental cells. This occurs around the third month of
gestation. IgG is the only antibody
isotype that can pass through the placenta. It is also used in the treatment of several types of acute infection, and to treat
poisoning. Subsequently the practice of vaccination would increase with the spread of war.
There are four types of traditional
vaccines:
- Inactivated vaccines are composed of micro-organisms that have been killed with chemicals and/or heat and are no longer infectious. Examples are vaccines against flu, cholera, bubonic plague, and hepatitis A. Most vaccines of this type are likely to require booster shots.
- Live, attenuated vaccines are composed of micro-organisms that have been cultivated under conditions which disable their ability to induce disease. These responses are more durable and don't generally require booster shots. Examples include yellow fever, measles, rubella, and mumps.
- Toxoids are inactivated toxic compounds from micro-organisms in cases where these (rather than the micro-organism itself) cause illness, used prior to an encounter with the toxiod. Examples of toxoid-based vaccines include tetanus and diphtheria.
- Subunit -vaccines are composed of small fragments of disease causing organisms. A characteristic example is the subunit vaccine against Hepatitis B virus.
Most vaccines are given by
hypodermic injection as they're not absorbed reliably through the gut. Live attenuated
Polio and some
Typhoid and
Cholera vaccines are given
orally in order to produce immunity based in the
bowel.
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