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:
- 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.
- 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 circumstances. 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.
- 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 in each of these sections:
Types of Immunity
People can be protected against or respond to infection in different ways, such as active immunity, passive immunity and community immunity. Active immunity is the gold standard. Find out why.
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, respectively, is most helpful.
Development of the Immune System
A baby’s immune system starts developing by the fourth week of gestation — around the same time that cells from the fetus cross the placenta and circulate in the maternal bloodstream. These maternal-fetal exchanges and interactions are important because the fetus is not genetically identical and could, otherwise, be attacked by the maternal immune system.
How the Immune System Works
The immune system functions differently based on whether the pathogen has previously been introduced, referred to as primary or memory immune responses. Other topics addressed on this page include: original antigenic sin, weakened immunity, and questions related to vitamins, zinc, medications and complementary and alternative medicine.
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.
Immune System and Vaccines
Questions related to vaccines and the immune system are answered here, including common concerns such as whether vaccines weaken or overwhelm the immune system and whether natural infection is better than immunization.
Reviewed on July 29, 2024