How the Immune System Works

Its complex functions work to protect you from infection

Your immune system protects your body from infectious germs. Through highly complex and adaptive processes, a healthy immune system is always at work, protecting you from infections by identifying and destroying harmful microorganisms. Your immune system also helps you build immunity so that when you encounter certain invading germs again, you can fight them faster the next time around, often without even getting sick at all.

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Recognizing Infectious Organisms

To do its job, the immune system must understand the difference between a foreign substance and the cells of your own body.

Foreign substances can be referred to as invaders or pathogens and may include microorganisms, such as bacteria, viruses, fungi, and parasites.

The cells and tissues of your body have proteins called self-antigens. Likewise, living organisms that can cause infections do too, though their antigens are not the same. Your immune system "flags" foreign antigens to quickly target the invading microorganisms and destroy them, protecting you from harm.

White Blood Cells (Leukocytes)

White blood cells (WBCs), the cornerstone of your immune system, are called leukocytes. There are a variety of types of leukocytes, each with unique features that work together to protect you from infections.

Depending on the leukocyte, it may help the "seek and destroy" function of the immune system by:

  • Rapidly recognizing germs
  • Binding to germs
  • Engulfing and surround germs
  • Using chemicals contained within to destroy germs

Others take time to recognize and respond to infectious microorganisms.

Macrophages, Neutrophils, Mast Cell, Basophils

Macrophages are leukocytes that circulate throughout the blood and tissues, while neutrophils are leukocytes that circulate in the blood, patrolling for new foreign antigens.

Invading germs and microorganisms enter the body through different entry points, such as the nostrils or a cut on the surface of the skin. When these particular leukocytes recognize such infectious threats, they send chemical signals that attract other leukocytes to surround, absorb, and destroy these harmful substances.

Macrophages and neutrophils, along with other leukocytes, such as mast cells and basophils, secrete toxins that damage or kill foreign microorganisms, and then they engulf the cellular debris to "clean it up."

Lymphocytes: T- and B-Cells

Lymphocytes are a subset of leukocytes. They take longer than other leukocytes to mount a response to an infection, and they build your long-term immunity. The two types of lymphocytes are T-cells and B-cells, and they each have different jobs.

B-cells are largely responsible for creating specific proteins called antibodies. The antibodies bind to the antigen on the surface of a foreign invader and mark it for destruction by the immune system. B-cells are useful in protecting you against bacterial infections.

  • Antibodies: Your body can produce a variety of antibodies. The different types of antibodies work against various types of infections, such as infections of the skin or gastrointestinal system. Antibodies bind to antigens, forming an immune complex that is destroyed by the body's leukocytes and their associated chemicals.
  • Autoantibodies: Problems occur when the immune system mistakenly manufactures autoantibodies, which are antibodies that fight your own body. This is the hallmark problem of autoimmune diseases, such as thyroid disease, and it happens when the immune system misidentifies self-antigens—your own cells, tissues, and organs—as foreign bodies.

T-cells identify antigens on the surface of your own cells. When a tiny microorganism, such as a virus, enters into your cells, your body’s major histocompatibility complex (MHC) can change the surface of your cells, adding new antigens to your own cells. Passing T-cells are alerted to the presence of the infection within your cell because of these altered antigens. T-cells themselves are useful in destroying viruses and cancer cells.

The MHC is quite sophisticated. A tiny microorganism "hiding" inside a human cell would not be recognized—and can wreak havoc. The MHC can bind to fragments of microorganisms within a human cell and carry these fragments to the surface of the cell so that they can be recognized by their new antigens.

The antigen molecules on an infected cell and a responding T-cell bind together to form co-stimulatory molecules, which mediate an immune response.

Cytokines and Chemokines

Lymphocytes can release chemicals called cytokines, which are signaling molecules. There are several types of cytokines involved in the immune response, including:

  • Chemokines
  • Interferons
  • Lymphokines
  • Interleukins

These immune-mediated cytokines can affect lymphocytes, as well as other nearby cells that are not part of the immune system. In doing so, they stimulate an inflammatory response, as well as repair of tissues that may have been harmed by an infectious microorganism.

Immune Complexes and the Complement System

Part of the body’s immune activity involves the complement system, which is a group of specialized molecules that work in a variety of ways to destroy invaders. For example. the complement system can form a structure called the membrane attack complex, which punctures the microorganism to destroy it from within by inserting toxic chemicals.

Autoimmune Disease and Allergies

You may have recurrent inflammation and an immune response even at times when you do not have an infection. Autoimmune diseases, such as thyroid disease, lupus, or multiple sclerosis, occur when the body's immune system attacks itself. In some types of hypothyroidism, for example, the body can attack cells that produce thyroid hormone, interfering with the production and function of the hormone.

Allergies are an inflammatory response to a non-threatening substance, like pollen or certain foods. These illnesses can develop at least partially as the result of genetic factors, but it is not always clear why someone develops these conditions.

Your genes are the blueprint for your body’s cells and tissues. That same blueprint patterns your immune function, including your T-cell receptors, the type of MHC molecules produced, and your antibody response. An overactive immune system can cause recurrent pain, swelling, and may even cause life-threatening allergic reactions.

A Word From Verywell

Given the intricacy of the immune system and the important roles it plays, it's in your best interest to do all you can to foster healthy immune function.

If your healthcare provider thinks that you might have an infection or an autoimmune condition, you might need to have a blood test to see if your white blood cell count is increased or decreased, and which white blood cells are most reactive. This can help guide your healthcare provider in knowing which type of condition you have, guiding treatment.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."