It is pathogen specific immunity. It is not present from birth and develops during an individual's lifetime. This type of immunity is acquired, either by encountering the disease or by vaccination. It has the following characters:
- Specificity: It has the ability to distinguish many different foreign molecules.
- Memory: It is a unique feature, which helps in producing an intensive secondary/ anamnestic response when the pathogen attacks the second time.
- Discrimination between self and non-self: This type of immunity is able to recognise and respond to foreign molecules (non-self) and can avoid response to those molecules that are present within the body (self).
Types of Acquired Immunity Acquired immunity can be classified as:
Active Immunity: It is the immunity developed by the body when it is exposed to the antigens which may be in the form of living or dead microbes or other proteins. Antibodies are produced by the body in this case. Introduction of pathogens or microbes either during immunisation or by any infection induces active immunity. It is slow, but long lasting and has no side effects. This immunity may be
- Artificial immunity developed by vaccination.
- Natural immunity developed during natural infection.
Passive Immunity: It is the immunity bestowed by antibodies that are directly given to the body. It is fast but lasts only for few days. Some examples of passive immunity are:
- Antibodies received by foetus from mother through placenta.
- Antibodies in the colostrum (IgA rich). Colostrum is a yellowish fluid secreted by mother during the initial days of lactation.