Active immunity is one which results on the exposure to a disease organism triggers the immune system to produce antibodies to that disease.
Active immunity may be of two type natural active immunity and passive active immunity.
Natural Active Immunity
It results from either a clinical or an in apparent infection by a microbe. A person who have recovered from an attack of measles develops natural active immunity.
Such immunity is usually long-lasting but the duration varies with the type of pathogen. The immunity is life long following many viral diseases, such as chicken pox or measles. In some other viral diseases such as influenza and common cold, the immunity appears to be short lived.
In common cold the apparent lack of immunity is because the same clinical picture can be caused by infection with a large number of different viruses.
Influenza can be reoccur after a few months or a year but this is not so much due to lack of the immunization capacity of the virus as to its ability to undergo antigenic variation so that immunity following first infection is not effective against second infection caused by an antigenically novel virus.
The immunity following bacterial infection is generally less permanent then those of viral infection.
Artificial Active Immunity
It is the resistance induced by vaccines. Vaccines are preparations of the live or killed microorganisms or their products used for immunization.
Examples of vaccines are as follows:
- Bacterial vaccines
a. Live (BCG vaccine for tuberculosis)
b. Killed (cholera vaccine)
c. Subunit (Typhoid Vi antigen)
d. Bacterial products (Tetanus toxoid)
- Viral vaccines
a. Live (Oral polio vaccine – Sabin)
b. Killed (Injectable polio vaccine – Salk)
c. Subunit (Hepatitis B Vaccine)
Live vaccine initiate an infection without causing any injury or disease. The immunity following live vaccine administration therefore parallels that following natural infection though it may be of a lower order. The immunity last for the several years but booster dose may be necessary.
Killed vaccines are generally less immunogenicity than live vaccines, and protection lasts for a short period. They have, therefore, to be administered repeatedly, generally as least two doses being required for the production of immunity. The first is known as the primary dose and the subsequent doses as booster doses.
Natural Passive Immunity
It is the resistance passively transferred from mother to baby. In human infants, maternal antibodies are transmitted predominantly through the placenta, while in animals such as pigs, transfer of antibodies occurs mainly orally through the colostrum. The human colostrum, which is also rich in IgA antibodies resistant to intestinal diseases.
The human fetus acquired some ability to synthesis antibodies (IgM) from about the twentieth week of life but its immunological capacity is still inadequate at birth. It is only by about the age of three months that the infant acquires some measure of immunological independence. Until then, maternal antibodies give passive protection against infectious diseases to the infant.
By active immunity of mothers during pregnancy, it is possible to improve the quality of passive immunity in the infants.
Artificial Passive Immunity
It is the resistance passively transferred to a recipient by the administration of antibodies.
Passive immunization is indicated for immediate and temporary protection in a nonimmune host faced with the threat of an infection, when there is insufficient time for the active immunization of some infections.
Passive immunization may also be employed for the suppression of active immunity, when the latter may be injurious. An example is the use of Rh immune globulin during delivery to prevent immune response to the Rhesus factor in Rh negative women with Rh positive babies.