7.2.3. Active and Passive Immunity

When a host is exposed to antigens, which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body. This type of immunity is called active immunity. Active immunity is slow and takes time to give its full effective response.

Active immunity is the body's own immune response to an antigen. When you come into contact with a pathogen, your immune system takes time to detect it and produce specific antibodies to fight it. This can occur naturally, such as when you get an infection, or artificially, through vaccinations which introduce weakened or inactive parts of the pathogen to stimulate an immune response without causing disease.

Think of active immunity like training for a sport. When you first start, it takes time to learn the skills and get better. But once you are trained (like your immune system making antibodies), you can respond more quickly and effectively in future games (future exposures to the pathogen). For instance, when you get a vaccination, it prepares your immune system by teaching it to recognize the pathogen without having to fight the disease itself.

Injecting the microbes deliberately during immunisation or infectious organisms gaining access into the body during natural infection induce active immunity.

Active immunity can develop in two main ways: through natural infection and vaccination. Natural infection occurs when a person is exposed to and contracts a disease, prompting the immune system to fight it off and remember the pathogen for future defense. Vaccinations involve deliberately introducing a harmless form of the pathogen, activating the immune system without causing illness, and allowing the body to build a memory of the immunity.

You can compare this to a fire drill at school. The first time you practice, it can be slow and people may forget what to do. However, after practicing several times (like natural infections or vaccinations), everyone remembers the steps quickly in case of a real fire, just as your immune system remembers how to deal with real infections.

When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity.

Passive immunity refers to the temporary immunity gained through the transfer of antibodies from another person or animal. For example, when a mother breastfeeds her newborn, she provides them with antibodies (specifically IgA) through her milk, giving them immediate protection against infections. Unlike active immunity, passive immunity does not involve the immune system producing its own antibodies and is short-lived—lasting only as long as the antibodies remain in the body.

Consider passive immunity like borrowing a book from a friend. You don’t have to write it or create it yourself; you just get to use it temporarily. Similarly, when a baby gets antibodies from breast milk, it's like temporarily borrowing a protective shield without needing to learn how to build one.

Mother’s milk is considered very essential for the new-born infant. The yellowish fluid colostrum secreted by mother during the initial days of lactation has abundant antibodies (IgA) to protect the infant. The foetus also receives some antibodies from their mother, through the placenta during pregnancy.

Passive immunity can happen in situations where immediate protection is required. When a mother breastfeeds, she passes her antibodies to her baby. Additionally, during pregnancy, a mother transfers some of her antibodies to her fetus via the placenta. This ensures that the baby has a protective defense against infections during a critical early stage of life when their own immune system is still developing.

Imagine a child sitting on a carousel, going round and round, where the mother ensures they are safe by holding on to them. Breastfeeding and placental transfer offer similar safety by giving the newborn immediate immunity, making sure they are protected while their own system learns to defend itself.

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