Health Hygiene & Sanitation

Lesson 11 : Host Defences

Acquired Immunity

Passive Immunity

When antibodies produced in one body (man or animal) are transferred to another to produce protection against disease it is known as passive immunity. For example passive immunity is transmitted to the infant from the mother through the placenta. The antibodies received from the mother are lost during the period of first 6 months of life. A large proportion of babies are free from infection upto 3 months of life. This protective biological shield is due to the presence of high levels of immunoglobulins – IgM and IgG in cord blood and plasma of infants, breast milk, fetal hemoglobin which protect the child against infections.
It may be induced by

1. administration of a preparation containing antibody (immunoglobulin or anti serum)
2. Transfer of maternal antibodies across the placenta
3. Human milk containing protective antibodies (IgA)
4. Transfer of lymphocytes

Difference between passive and active immunity

1. Active Immunity is rapidly established and long acting
2. Passive immunity takes long to establish and it is temporary.

Herd Immunity:

It is the level of resistance of community / group of people to a particular disease. It is the group protection beyond that afforded by the protection of immunized individuals.
Elements which contribute to herd immunity are -

1. Occurrence of clinical and sub-clinical infection in the herd
2. Immunization of the herd
3. Herd structure

E.g. Diphtheria, poliomyelitis