Immune System and Immunity

The 3 Types of Immunity

We have three types of immunity: active, passive, and community. Learn about each type of immunity and what they do to protect us from disease.

3 Types of Immunity

Have you ever thought about how immunity works? If so, you might have realized that immunity keeps us from becoming sick in different ways. Two types of immunity exist: active and passive. Both different immunities can be gained in different ways. A third category, community immunity, does not involve physical components of the immune system for protection.

What is active immunity?

Individuals rely on active immunity more so than passive immunity. Active immunity is created by our own immune system when we are exposed to a potential disease-causing agent (i.e., pathogen). Most of the time, we are exposed to these potential pathogens naturally throughout the course of our day, in the air we breathe, the food we eat, and the things we touch. Luckily, most of these exposures are two agents that will not result in disease, because they are harmless or because our immune system works to neutralize them.

Besides “fighting off” these pathogens, active immunity is important because it lasts a long time as immunologic memory. Immunologic memory comprises B and T cells that can recognize a particular pathogen. These cells circulate at low levels in our bodies and if “activated” by recognizing that pathogen in their travels, they quickly multiply and signal other elements of the immune system to activate as well. Memory cells are crucial for two reasons. First, they allow our immune systems to respond quickly. Second, they are specific for the pathogen, so the immune response is ready the moment the pathogen is encountered.

Because we don’t know about most of the work, our immune system does. We rarely think about how busy it is. But like our hearts and lungs, our immune system is constantly working to keep us healthy. This effort is evidenced because our immune system generates grams of antibodies every single day!

Vaccines contribute to active immunity by providing us with a controlled way to create an immune response. When a vaccine is introduced, our immune system treats it like any other exposure. It works to stop the “assault” and immunologic memory develops. Because vaccines are designed such that they do not cause illness, we gain the benefits of the exposure without the risks associated with fighting off a natural infection. In this way, vaccines offer our immune systems a chance to “train” for a future encounter and provide us with a “shortcut” to protection. We gain the immunity that follows surviving a natural infection without having to pay the price of a natural infection.

Examples of active immunity

  • Natural–Producing antibodies in response to exposure to a pathogenic infection (I.e. challenge and response)
  • Artificial–Producing antibodies in response to the controlled exposure to an attenuated pathogen (i.e. vaccination) 

What is passive immunity?

Passive immunity, or immunity gained in a way other than from one’s own immune system, can occur in a few ways and can be lifesaving. However, passive immunity is short-lived because the antibodies are not continually replenished, as they would be in an individual whose immune system is responding directly. Passive immunity can occur in a couple of ways;

Maternal antibodies

Unborn and newly born babies are protected by antibodies from the maternal immune system. These antibodies are shared in two ways: across the placenta and in breast milk.

Immunoglobulin treatments

In certain situations, antibodies got from animals, from other people, or synthesized in a laboratory can treat individuals at risk of infections. For example, infants born to women infected with hepatitis B are treated with antibody preparations besides being vaccinated to protect them from also becoming infected with hepatitis B. In another example, people bitten by some poisonous snakes may be treated with antivenom, a mixture of antibodies against the type of snake venom to which the person was exposed.

Examples of passive immunity

  • Natural–Receiving antibodies from another organism (e.g. to the foetus via the colostrum or a newborn via breast milk)
  • Artificial–Receiving manufactured antibodies via external delivery (e.g. blood transfusions of monoclonal antibodies)

What is community immunity?

Community immunity occurs when people are protected by those around them. This type of protection is indirect because it does not involve physical components of immunity, such as antibodies, but results when a pathogen is less likely to infect a susceptible person because of the high numbers of protected people around them. Because this immunity is not based on “products” of the immune system, it is the least reliable. However, for some in our communities, such as those too young to be immunized or those with weakened immunity because of illness or treatment, community immunity is the only way they can be protected.

We talk about community immunity from two perspectives, that of the community, commonly referred to as herd immunity, and that of the individual, commonly known as cocooning.

Herd immunity

When enough people in a community have been exposed to a pathogen, it cannot spread as easily. As more people become immune, the pathogen has a smaller pool of people to infect. The result is that the community overall will have fewer outbreaks. Because not all pathogens spread with the same efficiency, the community levels of immunity necessary to benefit from herd immunity vary. For example, because measles is one of the most contagious pathogens known, a community requires almost everyone to be immune in order to stop its transmission. Or said another way, it is much more difficult for an individual to benefit from herd immunity to measles than from most other infectious agents.

Vaccines have made it easier for society to reap the benefits of this type of protection. Before vaccines, diseases continued to have susceptible pools of individuals, most often infants and young children not previously exposed to the disease. Therefore, childhood diseases and deaths were so common.

Cocooning

This type of passive immunity is like herd immunity, but is more often aimed at protecting a particular individual rather than a community. Ensuring that everyone around a young infant is immune to a disease like pertussis (whooping cough) is an example of this type of indirect immunity. Another example is ensuring that everyone who visits or cares for a person being treated for cancer is healthy, so that the cancer patient whose immunity is weakened by treatment is less likely to be exposed to a pathogen.

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