How the Immune System Works

The first time we encounter a pathogen, our bodies generate a primary immune response. Subsequent exposures, on the other hand, benefit from secondary (or memory) immune responses. As described in the "Types of Immunity" section, these responses differ in that memory responses are generated more rapidly and are specific. Let’s take a closer look.

Primary immune responses

The first time we encounter a pathogen, our B and T cells are “naïve,” meaning that they do not have any specificity, or preference, for the pathogen. That’s because they’ve never seen it before. Using our police department example (see “Parts of the Immune System” section), the responding officers do not have information about the suspect. They respond according to protocol, but it may take longer because the suspect is not already “in the system.” The same is true for primary immune responses. For a pathogen that has not been encountered before, it will take several days — even weeks — for the complete immune response to develop. This is not to say it will take weeks to recover, but depending upon the infection, some symptoms may linger or you may just not “feel completely back to yourself.” Generally speaking though, as our immune system responds, we feel a little better each day.

Memory immune responses

After the first encounter with a pathogen, our immune systems generate a small number of cells that remain for a long time, are specific for that pathogen, and circulate in the blood, spleen and lymph nodes to keep watch for another encounter. Thinking of the police force (or any job for that matter), the officers who are the best at their job will likely be kept around for longer.

Because our memory cells are “experienced,” we will respond to a second (or subsequent) encounter more quickly. Whereas in a primary encounter, antibodies are not generated for several days, during a memory response antibodies appear in just a few days. Because these antibodies are generated by the cells that were most effective during the initial infection, they are also highly efficient.

Memory and “original antigenic sin”

Some pathogens, like influenza virus, vary. The type of virus or bacteria to which you are first exposed is the version your immune system will tend to remember the best. So when you are exposed to a second or third type of the pathogen, your immune system will react as if it is still recognizing the first type. This is called “original antigenic sin.” For many infections, this doesn’t matter because the response is “good enough,” but for some infections, the lower efficiency can make a difference.

Influenza virus is a perfect example. Most people are aware that influenza virus changes regularly, requiring us to get annual flu vaccines. However, the vaccine tends to be less effective for older people than for children. This happens in part because of original antigenic sin and, in part, because of an aging immune system. However, even if the vaccine is less effective for some people, they will still have some protection, and if exposed to influenza, will likely suffer a shorter, less severe infection. If we pick up our police force example again, while an officer who typically responds to crime scenes following robberies can respond to a vehicular accident, the officer may not be as efficient as a traffic cop would be.

While original antigenic sin can be problematic, it is not always the case. For example, during the 1918 influenza pandemic, older adults were less likely than young adults to die from their infection. It was determined that the immunologic memory of older adults was more effective against the pandemic strain than that of young adults. In attempting to overcome the deficiency, the immune systems of some young adults generated too strong of an immune response, tragically leading to fast and harsh deaths.

Weakened immunity

Our immune system can respond to confrontations on different fronts at the same time. However, this doesn’t mean we should not help ourselves. When ill, it is best to stay home, rest, and drink plenty of fluids. This allows our immune system to most effectively fight the infection.

Further, because infections diminish our immune resources, we are more susceptible to new infections in the period during and immediately after an infection. Some pathogens rely on this for their evolutionary survival. These are called “opportunistic pathogens,” and they wait for an opportunity to cause infection when the immune system is weak. Some examples include:

  • Varicella zoster — This virus causes chickenpox and shingles. When someone has chickenpox, a small amount of virus remains behind in nerve cells after the infection. As a person ages, or has weakened immunity caused by stress or illness, the virus will reactivate in the nerve cells. As it reproduces, the new viral particles infect other nerve cells in the region. This is why the rash tends to look like a line or stay localized in a region, such as from the back to the side or by the eye.
  • Pneumococcus — Many elderly people who get influenza seem to be recovering only to become ill again. While this scenario is often thought of as a long illness, in many cases, what really happened is that pneumococcus took advantage of the weakened immunity caused by the influenza infection. Often, the latter infection leads to pneumonia, and sadly, every year, thousands of elderly adults die in the winter from pneumonia as a result of a virus or bacteria taking advantage of weakened immunity caused by influenza. This is why the influenza and pneumococcal vaccines are so important for older adults.

Other questions you might have

Do vitamins strengthen our immune system?

Some vitamins play a role in the immune system; however, in general, taking vitamin supplements is not considered useful in protecting an individual from disease. Also, vitamins never induce immune responses that are specific for a particular pathogen. In most cases, following a nutritious, balanced diet, exercising, and getting enough sleep are likely more effective.

Find out more about taking vitamins to prevent heart disease and cancer from the Choosing Wisely campaign.

Does zinc help treat colds?

Studies have shown that if zinc is started early during a cold, it may decrease the length of illness by about one day and have a minor to moderate effect on the severity. However, nausea and negative effects on taste are known side effects.   

Do complementary and alternative treatments help our immune systems fight infections?

Since a variety of different types of complementary and alternative treatments are available, this question is difficult to answer. If you are interested in trying a non-traditional treatment, it is important to research whether controlled studies evaluating the effectiveness of the treatment you are considering have been completed. Talk with your doctor, and be wary of relying on anecdotal accounts of improvement.

Any treatment can be assessed in controlled studies to determine its effectiveness. If it is found to be effective, it will likely move from being an alternative treatment to a mainstream one.

How do medications affect our immune response?

Different types of medications have different effects on the immune response. Let’s take a closer look:

Over-the-counter medications (OTC)

Often when someone is ill, their first instinct is to take an OTC medication to “feel better.” It is important to realize that many of the symptoms we experience during infections are not caused by the virus or bacteria, but rather are evidence that our immune system is working. For example, fever not only allows our immune system to work more effectively, but it also makes our body a less favorable host for the infectious agent. For these reasons, if someone takes a medication to reduce fever, called an antipyretic, they are inadvertently dampening the immune response to infection. As an example, one can look to studies using antipyretics, such as acetaminophen or ibuprofen, before vaccination to prevent children from developing a fever. These studies found that children who were pre-medicated with an antipyretic developed lower antibody responses than children who were not. The good news was that responses were still sufficient to protect children, but these studies offer evidence that medications used to treat symptoms of fever result in changes to the immune response.

If someone is extremely uncomfortable, using a medication to stay comfortable makes sense, but if symptoms are manageable, it is best to allow the immune system to respond with watchful eye. If symptoms worsen quickly, become severe, or do not respond to medications, seek medical treatment. Likewise, those who are ill should drink enough liquids to remain well-hydrated.

Prescription medications

Because some medications are only available when prescribed by a healthcare provider, it is appropriate to discuss why a medication is being used and what potential side effects it will have. For example, it is inappropriate to take an antibiotic for a viral infection because antibiotics are only effective against bacteria. Different classes of medications work in different ways, so talking to your healthcare provider or pharmacist about any potential effects on your immune system would be the most prudent approach.

Special situations

In some cases medications are prescribed to treat conditions that directly compromise the immune system or when the immune system is lacking important components. For example, some people are born with a condition called agammaglobulinemia. This condition causes a problem with a person’s B cells, so they do not produce antibodies. In order to make up for this deficiency, patients are often treated with immunoglobulins, which are preparations of antibodies from blood donors.

Reviewed on April 22, 2019

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.