What is active and passive immunity?

Your immune system protects you from a multitude of germs and harmful substances that can make you ill. It comprises a complex network of cells, tissues, and proteins located throughout your body.

It’s important to understand immunity during the COVID-19 pandemic. One reason for this is that your immune system has a memory. It can store information about germs (like viruses) that it’s encountered before. As a result, your immune system is able to respond more quickly if it has to fight off that germ again.

Researchers are working diligently to find out how long immunity lasts after having COVID-19. Understanding COVID-19 immunity also plays an important role in developing an effective vaccine that can protect you from the new coronavirus.

In this article, we’ll take a closer look at your immune system, the different types of immunity you can acquire, and what we know so far in relation to COVID-19.

Your immune system activates when it encounters foreign invaders like viruses, bacteria, and fungi that can cause illness, infection, or disease. These invaders are called antigens.

In the context of infectious disease, antigens are proteins that are found on the surface of viruses, bacteria, or fungi.

Your own cells also have surface proteins. An immune system that’s working normally doesn’t react to them, as they’re recognized as being a part of you.

An autoimmune response occurs when your immune system attacks healthy cells in your own body. Some examples of autoimmune diseases include:

Now let’s take a look at how your immune system responds to a potential threat. Since we’ll be discussing COVID-19, we’ll use a virus as an example.

There are two different arms of the immune response: innate and adaptive.

The innate response happens quickly. Innate immune cells circulating throughout your body detect the virus. Their response is broad spectrum and typically involves inflammation.

Cells of the innate response will also go on to alert the cells involved in the adaptive response. As such, the adaptive response occurs later on in the course of an infection.

While the innate response is broad, the adaptive response is specific. The cells involved are specialized. They have the ability to recognize very particular characteristics of this viral antigen, and can tell if they have encountered this virus before.

The adaptive response is what’s responsible for immune memory.

Innate and adaptive immune responses and COVID-19

Because the new coronavirus, known as SARS-CoV-2, is so new, scientists and researchers are still trying to learn how the immune system responds to it.

Some people with COVID-19 become very ill. One aspect of this severe illness is an excessive inflammatory response by the innate immune cells. It’s known as a cytokine storm. Researchers are investigating how and why this occurs in COVID-19.

The adaptive response also plays a role in fighting off the new coronavirus. It does this through your T cells. T cells are specialized cells that can kill virus-infected cells or help other immune cells in the response to an infection.

T cells specific to the virus that causes COVID-19 have been identified in people who have recovered. These T cells targeted several parts of the virus.

Active immunity happens when your immune system is triggered to produce antibodies to fight a disease.

Antibodies are proteins that are specific to an antigen (foreign invader), allowing the antigen to be targeted, neutralized, and destroyed.

Active immunity can be achieved in two different ways:

• Natural infection. You gain active immunity by getting an infection or illness.
• Vaccination. You get active immunity by receiving a weakened form of a pathogen via vaccination.

Now let’s look at some examples:

• Chickenpox. Active immunity to chickenpox is lifelong. This means if you get an infection with the chickenpox virus, reinfection is unlikely. If you had it as a child, you made antibodies to the virus. So, your immune system knows how to fight it off if it encounters it again.
• Flu. Active immunity to the flu isn’t lifelong. This is because reinfection with different flu viruses is possible. However, the active immunity gained through previous infections and annual flu vaccines can still provide protection, potentially reducing the length or severity of illness.

Active immunity and COVID-19

People who contract an infection with the new coronavirus develop antibodies within 2 weeks, but the protection provided by these antibodies isn’t well understood. It’s also still currently unclear whether a person can contract a second SARS-CoV-2 infection.

Many COVID-19 antibody studies focus on a type of antibody called a neutralizing antibody. Neutralizing antibodies bind directly to viral proteins, preventing the virus from infecting a cell.

A recent study looked at antibodies in 149 people who had recovered from COVID-19. High levels of neutralizing antibodies to SARS-CoV-2 weren’t found circulating in their blood.

However, sequencing efforts did detect antibody-producing cells (B cells) that could produce potent neutralizing antibodies to SARS-CoV-2.

But it seems that antibodies to the new coronavirus may not last long. Another recent study found that antibody levels dropped sharply in the months following COVID-19 recovery in more than 90 percent of both symptomatic and asymptomatic individuals.

Research into active immunity and COVID-19 is ongoing. There’s a lot we still need to learn.

Understanding the ins and outs of the antibody response to the new coronavirus will be vital in helping achieve active immunity through vaccination.

Passive immunity is when you’re given antibodies as opposed to producing them on your own. In the case of passive immunity, protection is immediate. However, unlike active immunity, it’s not long lasting.

Some examples of how passive immunity can be obtained include:

• Maternal antibodies. Maternal antibodies are passed from a mother to a developing fetus through the placenta during pregnancy. These antibodies can also be transferred from a mother to child during breastfeeding. They can protect a baby from some types of infections.
• Blood products. Various blood products can contain antibodies. One example is homologous human hyperimmune globulin. It comes from blood plasma and contains high levels of antibodies to a specific antigen.

Passive immunity and COVID-19

Researchers are investigating passive immunity as a potential treatment for those who are seriously ill with COVID-19. It involves the use of a blood product called convalescent plasma.

When someone recovers from COVID-19, they can choose to donate blood. Plasma, the yellow portion of the blood that contains their antibodies, can then be isolated. This plasma can then be given intravenously to someone who’s severely ill with COVID-19.

Studies are ongoing to determine how effective this type of therapy is for COVID-19. While some benefits have been reported, a recent review notes that larger, more complete studies are necessary.

Infectious diseases require susceptible individuals to continue to spread. If a high enough percentage of a population has immunity to a disease, it will be hard for that disease to spread. Because of this, fewer people will become ill.

This concept is called herd immunity. Herd immunity can help protect people who are particularly vulnerable to illness. Examples include the very young, older adults, and those with a weakened immune system.

Herd immunity is often discussed in relation to vaccination. High levels of immunity through vaccination can greatly limit the spread of various infectious diseases within our communities, protecting both yourself and those around you.

Herd immunity and COVID-19

Experts hope we can achieve COVID-19 herd immunity through vaccination. Since trials to evaluate potential COVID-19 vaccines are ongoing, we’re still several months away from a vaccine.

Herd immunity can also be achieved through natural infection. However, research estimates about 67 percent of the population would need to contract the new coronavirus and recover to obtain herd immunity in this way.

Given what we know about COVID-19, reaching herd immunity through natural infection could have serious consequences for many people. It would also create too much of a burden on our healthcare system.

This is why researchers are working harder than ever to develop an effective vaccine.

There are some strategies you can use in your everyday life to help keep your immune system robust and healthy.

However, it’s also important to note that although these strategies may help boost your immune health, they won’t specifically protect you against COVID-19.

To prevent contracting an infection with the new coronavirus, continue using infection control measures, like:

Your immune system is a complex network of cells, proteins, and tissues that helps protect your body from disease and infection. There are different types of immune responses and different types of immunity.

Researchers are currently working to understand how your immune responses and immunity can protect you from COVID-19. By having a better understanding of this, the scientific community can work to develop effective vaccines and other treatment strategies.

Although having a healthy immune system may help you fight off certain illnesses, it won’t protect you from contracting an infection with the new coronavirus.

To lower your risk for getting sick, it’s essential to practice preventive measures like physical distancing, handwashing, and wearing a face covering.