Types of immunity

Types of immunity

Immunity may be described as active or passive. Both types may be acquired naturally or artificially. Providing immunity artificially is called Immunization. 

Natural active immunity

   This is the kind of immunity which is obtained as a result of an infection. The body manufactures its own antibodies when exposed to an infectious agent. Because memory cells, produced on exposure to the first infection, are able to stimulate the production of massive quantities of antibody when exposed to the same antigen again, this type of immunity is most effective and generally persists for long time, sometimes even for life.

Artificial active immunity  (vaccination)

This is achieved by injection (or less commonly administering orally) small amounts of antigen, called the vaccine, into the body of an individual. The process is called vaccination. If they whole organism is used as the vaccine, it is first made safe by being killed or attenuated (see below). The antigen stimulates the body to manufacture antibodies against the antigen. Often a second, booster injection is given and this stimulates a much quicker production of antibody which is longer lasting and which protects the individual from the disease for a considerable time. Several types of vaccine are currently in use.                                                                                     

·        Toxoide.  Toxins (poison) produced by tetanus and diphtheria bacteria are detoxified with formaldehyde, yet their antigen properties remain therefore vaccination with the toxoid will stimulate antibody prediction without producing symptoms of the disease.

·        Killed organisms.  Some dead viruses and bacteria are able to provoke normal antibody responses. An example is the flu vaccine which contains dead flu viruses.

·        Live vaccines (attenuated organisms).  An attenuated organism is one which has been ‘crippled’ in some way so that it cannot cause disease. Often it can only grow and multiply slowly. Attenuation may be achieved by culturing the organisms at higher temperatures than normal, or by adding specific chemicals to the culture medium for long period of time. Alternatively, The attenuated organism may be a mutant variety with the same antigens but lacking the ability to cause disease. Attenuated vaccine for the bacterial disease tuberculosis and for measles, mumps, rubella, and polio are in general use.

·        With small pox,  which is now extinct, a live virus vaccine was used. However, the virus in the vaccine was not an attenuated virus but a closely related and harmless form.

·        New vaccine.  Vaccine development and made little progress for many years, but new approaches to vaccine design are now possible using modern techniques of molecular biology and genetics engineering. Antigens are very often proteins and proteins are code for by genes. If the genes for and antigen is transferred into a bacterium, using standard techniques describe in chapter 12, the bacterium can be use as a ‘factory’ for producing large quantities of the antigen for use in vaccines. Cholera, typhoid and hepatitis B vaccines have been prepared in this way. Some vaccine can be made safer in this way. For example the whooping coughs vaccine. An alternative approach is to synthesise antigens artificially froe amino acid once their amino acid sequences are known. 

Vaccination is a common experience in develop countries and is one of the weapons which have help to reduce the incidence of infectious disease so dramatically in those countries. Other weapons have been social and environmental improvement such has treatment to clean water for drinking, better nutrition and sewage treatment.

  An example of the use of vaccination is the recommendation that all children in the UK should have the MMR vaccine at 2 year of age. This protect against measles, mumps and rubella. Also three dose of another trip vaccine, DTP (diphtheria, tetanus, and pertussis (whooping cough)) are recommended at virus age. In some countries vaccination is a legal requirement. Smallpox has been made extinct by vaccination and some childhood diseases such as diphtheria, polio and measles have become extremely rare. Polio may become extinct in the near future. The world health organization, with support from virus organization like UNICEF and the World Bank, has targetted six major diseases in the developing world in its EPI programme (expanded programme on immunization). The diseases are diphtheria, whooping cough, tetanus, polio, measles and TB. Although not feared so much anymore in develop countries, these are still killer disease worldwide. Over 80% of children in develop countries now receive vaccination against these disease. Hepatitis B is also being targeted.

    Improvements to some vaccines, such as the flu vaccine, are still needed and there is still no vaccine against some disease, notable cancers, leprosy, malaria and AIDS, despite intensive research.

                              Passive immunity

In passive immunity antibodies from one individual are passed into another individual. They give immediate protection, unlike active immunity which takes a few days or weeks to build up. However, it only provides protection against infection for a few week, for the antibodies are broken down by the body’s natural processes, so their numbers slowly fall and protection is lost.

                            Natural passive immunity

Passive immunity may be gained naturally. For example, antibodies from a mother can cross the placenta and enter her fetus. In this way they provide protection for the baby until its own immune system is fully functional. Passive immunity may also be provided by colostrum, the first secretion of the mammary glands. The baby absorbs the antibodies through its gut.

                    Artificial passive immunity

Here antibodies which have been formed in one individual are extracted and then injected into the blood of another individual which may or may not be of the same species. They can be use for immediate protection if a person has been or is likely to be, exposed to a particular disease. For example, specific antibodies used for combating tetanus and diphtheria used to be culture in horse and injected into humans. Only antibodies of human origin are now used for humans. Antibodies against rabies and some snake venoms are also available. Antibodies against the human rhesus blood group antigen are used for some rhesus negative mothers when carrying rhesus positive babies.

                 A summary of the different types of immunity is given in table 01.

                                              Active                                             Passive

                                           Antigens received                          antibodies received

Natural                             Natural active                                  natural passive

                                           e.g fighting infection,                     from mother via

                                            Rejecting transplant                       milk of placenta

Artificial                             Artificial active                                artificial passive   

                                            Vaccination (injection                    injection of antibodies

                                            of antigen)