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Immunity to a given pathogen can be acquired in more ways than one. In most cases, you will acquire immunity simply by being exposed to the pathogen, whether by an infection, or via vaccination. This isn’t the only way to acquire immunity, however.
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Active immunity is acquired when your body mounts an immune response to an antigen – typically one derived from a bacterial or viral pathogen (or perhaps a parasite, an allergen, or some other protein).
The development of active immunity means that immunological memory has been acquired. If you encounter that same pathogen a second time, your immune system will “remember" and quickly mount an immune response to eliminate the source of the infection – often before you even have symptoms of illness.
Alternatively, active immunity may occur as a result of vaccination. Effective vaccination involves the development of memory, to prevent any infection with the relevant pathogen. Vaccinations are typically developed to serious diseases – polio, smallpox, and diphtheria, for example, have been eliminated from many countries (and in the case of smallpox, all but eradicated from the entire world) following the development of effective vaccines.
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But what of a newborn baby? It is not considered safe to vaccinate a newborn – their immune system is still developing, and vaccinating too early can cause the system to develop abnormally, and may even cause disease, depending on the type of vaccine used. So how can a baby be protected from disease in those first few months before vaccination?
The answer lies in passive immunity. Both during pregnancy and as a result of breast-feeding following birth, a baby can acquire passive immunity to infections its mother has been exposed to.
This occurs due to the presence of antibodies in the mother’s blood and breast milk. Antibodies present in blood can be transferred to the developing fetus across the placental barrier – and these can provide protection for a limited time after birth. More important is the protection provided to the baby after it has been born, in the form of antibodies which are present in the milk of its mother. Breastfeeding, therefore, is important for the health of the baby not only in terms of nutrition, but also in terms of providing protection from disease.
The type of immunity this provides is termed passive because it does not involve the baby developing its own immune response – the protection is derived not from the baby’s own immune system, but from that of its mother.