Human Immune System

Like the beating of our hearts and the breaths we take, our immune systems must function around the clock. At any time — virtually all the time, really — we are exposed to things that our bodies recognize as foreign. And, some of these can cause us to become ill. The job of our immune system is to protect us from these attacks.

Despite this important role, the immune system is less well understood than many other systems of our bodies. To be sure, we understand a tremendous amount about the immune system, but gaining this information has not been as straightforward for a few reasons:

  1. The immune system, while composed of a few organs and tissues, is not limited to a particular part of the body. It functions, by necessity, in every part of the body.
  2. Much of the work of the immune system is done by cells that move around and signaling chemicals, called cytokines, that change in levels based on circumstance. For example, when some cytokine levels increase, they cause others to decrease. Like a seesaw moving one child up while the other is going down, these chemicals are rarely present in an “all or nothing” situation. So, understanding their roles can be difficult.
  3. As our immune system learns how to effectively overcome disease-causing agents, or pathogens, these same pathogens often change or adapt, so that they can survive evolutionarily. Despite most disease-causing pathogens being known for thousands of years, humans have only successfully “beat” two of them — smallpox and rinderpest, a pathogen that did not infect humans, but was extremely fatal for cattle. Rinderpest, a virus similar to measles, led to death in about 8 of 10 infected animals and historically, led to starvation of populations of people when herds were infected. For all other infections, our immune systems need to be prepared if we are exposed.

So, how does our immune system work? Find out more about:

Types of immunity

People can be protected against or respond to infection in different ways. If the protection comes from a person’s own immune system, it is called “active immunity.” If the person is protected by immunologic agents generated in someone else’s immune system, it is called “passive immunity.” A third category of immunity, called community immunity, does not involve physical agents of immunity.

Find out more about each of these types of immunity and the different ways they can be achieved.

Parts of the immune system

The immune system is best understood from the cellular level of responses. While tissues and organs are part of this system, understanding the components of the first and second lines of defense, called innate and adaptive immunity, is most helpful.

Find out about the tissues and organs, innate immune system components, and adaptive immune system components.

Development of the immune system

A baby’s immune system starts developing by the fourth week of gestation. Likewise, cells from the fetus cross the placenta and can be found in maternal circulation starting around the same time.  The fetus will begin to have maternal antibodies that cross the placenta around 13 weeks of gestation. In this manner, mom and baby are closely intertwined. If this did not happen, the fetus may be attacked by the maternal immune system because it is not genetically the same. 

Find out more about the development of the fetal immune system.

How the immune system works

The immune system functions differently based on whether or not the pathogen has previously been introduced; these differences, referred to as primary and memory immune responses, are described. Special considerations, such as “original antigenic sin” and weakened immunity, as well as common questions related to vitamins, zinc, medications and complementary and alternative medicine are also discussed.

Find out more about different types of immune responses and get answers to common questions.

What happens when the immune system doesn’t work properly?

Sometimes the immune system does not function properly because of immune deficiencies present at birth, medications that suppress the immune system, autoimmunity, or immune responses that are unnecessarily activated or overzealous.

Find out more about the different reasons why the immune system might not work as expected and learn about some of the related conditions.

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.