Causes of Actively Acquired Immunity

The following points highlight the two main causes of actively acquired immunity. The causes are: 1. Infectious Agents 2. Vaccination.

Actively Acquired Immunity: Cause # 1. Infectious Agents:

Infectious Agents of two types:

(a) Some Infections:

Diphtheria, whooping cough, small pox and mumps induce a long lasting immunity,

(b) Some Infections:

Common cold and influenza confer immunity lasting for a short time.

Actively Acquired Immunity: Cause # 2. Vaccination:

Vaccination is an artificial process which stimulates the lymphoid tissue and is adapted to meet practical requirement. It is under human control. Several such artificial processes are used for artificial immunity, vaccines are cheapest and most effective means of controlling deadly, infectious diseases.

(A) Vaccine Prepared from Living Attenuated Organisms:

Active specific immunity may be acquired artificially similar to subclinical infection, by means of injection of living attenuated (weakened) organisms in the human body which cannot cause a severe infection e.g. Bacille Calmette Guerin (BCG) vaccine; small pox vaccine; Anti- Rabies vaccine—vacca, Latin (for cow).

i. BCG Vaccination:

Calmette and Guerin, French scientists, developed a BCG vaccine consisting of live tubercle bacilli of low virulence for human being as an immunizing agent against tuberculosis.

ii. Small Pox Vaccination:

It is carried out by inoculation of living cowpox virus into the human body which could give protection against small pox virus because of their antigenic relationship. The vaccine was available in freeze dried form, when small pox was in existence. In 1979, WHO declared the world free of smallpox.

iii. Anti-Rabies Vaccine:

Sample vaccine is prepared from the brain of sheep infected with the fixed rabies virus. Infected sheep brain suspension inactivated by 0.5 per cent phenol vaccine may cause paralysis in small proportion (1: 4,000 to 1: 10,000) of human beings, because of the presence of nerve cell in the vaccine (allergic type of encephalitis).

iv. Avianised Rabies Vaccine:

To avoid the nervous symptoms the vaccine is prepared from virus cultivated repeatedly in successive live embryonated avian eggs, inactivated by formalin, ultraviolet irradiation or phenol and, finally, the vaccine is dehydrated. This vaccine is reconstructed, used safely and effectively than sheep brain tissue vaccine.

(B) Vaccine Prepared from Dead Organisms:

The active artificial immunity can be produced by another method i.e. by injecting the actual organisms that cause the disease into the body. The body reacts and produces the antibodies,. In this method, the organisms are dead or killed by heat or chemical disinfectant.

Many types of bacterial vaccines are used. Typhoid vaccine and Whooping cough (Pertussis) vaccine are very common. These vaccines are prepared by submitting the bacterial suspensions to a heat of 60°C for 1 hour to kill the living bacteria. It is customary to give three successive injections of the vaccines at intervals of one month and an annual booster dose.

i. Mixed Vaccines:

Vaccines containing more than one kind of organisms is known as mixed vaccines e.g. Triple vaccine (Diphtheria, Pertussis, Tetanus, DPT) vaccine is not only effective, but it saves many extra pricks.

ii. Sensitized Vaccines:

Organisms meant for the preparation of vaccine are sometimes treated before use with the serum of a person or animal immune to the bacteria. These bacteria combine with the antibody in the serum, so that they are acted upon quickly by the blood tissues or phagocytes of the person into whom they are infected and, ultimately, immunity is produced rapidly and effectively. Such vaccines are called sensitized vaccines.

iii. Viral Vaccines:

Killed viral vaccines are the vaccines against Influenza and Salk vaccine against Poliomyelitis.

(C) Injection of Modified Bacterial Exotoxin:

i. Toxoid:

Certain bacteria damage the body tissue by elaborating the toxin. When these toxins are detoxified by formaldehyde, they can be used as immunizing agents against the diseases e.g. Tetanus toxoid (TT) against tetanus, diphtheria toxoid against diphtheria.

Passively Acquired Immunity:

Administration of immune serum from another species e.g. horse—diphtheria antiserum, tetanus antiserum (ATS) and gas gangrene antiserum were used therapeutically until the advent of antibiotics.

Still now, it is used. Passive immunity due to maternal antibody across the placenta in some species, man and rabbit—where a particular part of immunoglobulin polypeptide chain has been found necessary for effecting the transfer.

In other species, such as dogs and rats, antibodies are transmitted through the colostrum via the intestine and via placenta; the other animals, lamb and calf receive this form of immunity only by means of the colostrum. Pooled human immunoglobulin is also used as a source of antibody in a number of infections (measles, small pox and serum hepatitis) during the period of incubation to modify or prevent the infection.