The immune system is made up of special organs, cells and chemicals that fight infection (microbes). The main parts of the immune system are: white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus, and the bone marrow.
The overall function of the immune system is to prevent or limit infection. An example of this principle is found in immune-compromised people, including those with genetic immune disorders, immune-debilitating infections like HIV, and even pregnant women, who are susceptible to a range of microbes that typically do not cause infection in healthy individuals.
The immune system can distinguish between normal, healthy cells and unhealthy cells by recognizing a variety of “danger” cues called danger-associated molecular patterns (DAMPs). Cells may be unhealthy because of infection or because of cellular damage caused by non-infectious agents like sunburn or cancer. Infectious microbes such as viruses and bacteria release another set of signals recognized by the immune system called pathogen-associated molecular patterns (PAMPs).
When the immune system first recognizes these signals, it responds to address the problem. If an immune response cannot be activated when there is sufficient need, problems arise, like infection. On the other hand, when an immune response is activated without a real threat or is not turned off once the danger passes, different problems arise, such as allergic reactions and autoimmune disease.
The immune system is complex and pervasive. There are numerous cell types that either circulate throughout the body or reside in a particular tissue. Each cell type plays a unique role, with different ways of recognizing problems, communicating with other cells, and performing their functions. By understanding all the details behind this network, researchers may optimize immune responses to confront specific issues, ranging from infections to cancer.
All immune cells come from precursors in
the bone marrow and develop into mature cells through a series of
changes that can occur in different parts of the body.
Skin: The skin is usually the
first line of defense against microbes. Skin cells produce and secrete
important antimicrobial proteins, and immune cells can be found in
specific layers of skin.
Bone marrow: The bone marrow
contains stems cells that can develop into a variety of cell types. The
common myeloid progenitor stem cell in the bone marrow is the precursor
to innate immune cells—neutrophils, eosinophils, basophils, mast cells,
monocytes, dendritic cells, and macrophages—that are important
first-line responders to infection.
The common lymphoid progenitor stem cell
leads to adaptive immune cells—B cells and T cells—that are responsible
for mounting responses to specific microbes based on previous
encounters (immunological memory). Natural killer (NK) cells also are
derived from the common lymphoid progenitor and share features of both
innate and adaptive immune cells, as they provide immediate defenses
like innate cells but also may be retained as memory cells like adaptive
cells. B, T, and NK cells also are called lymphocytes.
Bloodstream: Immune cells
constantly circulate throughout the bloodstream, patrolling for
problems. When blood tests are used to monitor white blood cells,
another term for immune cells, a snapshot of the immune system is taken.
If a cell type is either scarce or overabundant in the bloodstream,
this may reflect a problem.
Thymus: T cells mature in the thymus, a small organ located in the upper chest.
Lymphatic system: The lymphatic
system is a network of vessels and tissues composed of lymph, an
extracellular fluid, and lymphoid organs, such as lymph nodes. The
lymphatic system is a conduit for travel and communication between
tissues and the bloodstream. Immune cells are carried through the
lymphatic system and converge in lymph nodes, which are found throughout
Lymph nodes are a communication hub
where immune cells sample information brought in from the body. For
instance, if adaptive immune cells in the lymph node recognize pieces of
a microbe brought in from a distant area, they will activate,
replicate, and leave the lymph node to circulate and address the
pathogen. Thus, doctors may check patients for swollen lymph nodes,
which may indicate an active immune response.
Spleen: The spleen is an organ
located behind the stomach. While it is not directly connected to the
lymphatic system, it is important for processing information from the
bloodstream. Immune cells are enriched in specific areas of the spleen,
and upon recognizing blood-borne pathogens, they will activate and
Mucosal tissue: Mucosal surfaces are prime entry points for pathogens, and specialized immune hubs are strategically located in mucosal tissues like the respiratory tract and gut. For instance, Peyer’s patches are important areas in the small intestine where immune cells can access samples from the gastrointestinal tract.
Common Disorders of the Immune System
Disorders of the immune system can be due to overactivity or underactivity.
Overactivity of the immune system can take many forms, including:
- allergic diseases – The immune system makes an overly strong response to allergens ( the substances that trigger allergy such as
pollen, grasses, dust, and some medications ) . Allergic diseases are very common. They include allergies to foods, medications or stinging insects, anaphylaxis (life-threatening allergy), hay fever (allergic rhinitis), sinus disease, asthma, hives (urticaria), dermatitis and eczema
- autoimmune diseases – In this condition, the immune system mounts a response against normal components of the body. Autoimmune diseases range from common to rare. They include multiple sclerosis, autoimmune thyroid disease, type 1 diabetes, systemic lupus erythematosus, rheumatoid arthritis and systemic vasculitis.
Underactivity of the immune system, also called immunodeficiency, can:
- be inherited – examples of these conditions include primary immunodeficiency diseases such as common variable immunodeficiency (CVID), x-linked severe combined immunodeficiency (SCID) and complement deficiencies
- arise as a result of medical treatment – this can occur due to medications such as corticosteroids or chemotherapy
- be caused by another disease – such as HIV/AIDS or certain types of cancer.
An underactive immune system does not function correctly and makes people vulnerable to infections. It can be life threatening in severe cases. People who have had an organ transplant need immunosuppression treatment to prevent the body from attacking the transplanted organ.
Immunisation works by copying the body’s natural immune response.
A vaccine (a small amount of a specially treated virus, bacterium or toxin) is injected into the body. The body then makes antibodies to it.
If a vaccinated person is exposed to the actual virus, bacterium or toxin, they won’t get sick because their body will recognise it and know how to attack it successfully. Vaccinations are available against many diseases, including measles and tetanus. The immunisations you may need are decided by your health, age, lifestyle and occupation. Together, these factors are referred to as HALO, which is defined as:
- health – some health conditions or factors may make you more vulnerable to vaccine-preventable diseases. For example, premature birth, asthma, diabetes, heart, lung, spleen or kidney conditions, Down syndrome and HIV will mean you may benefit from additional or more frequent immunisations.
- age – at different ages you need protection from different vaccine-preventable diseases. Australia’s National Immunisation Program sets out recommended immunisations for babies, children, older people and other people at risk, such as Aboriginal and Torres Strait Islanders. Most recommended vaccines are available at no cost to these groups.
- lifestyle – lifestyle choices can have an impact on your immunisation needs. Travelling overseas to certain places, planning a family, sexual activity, smoking, and playing contact sport that may expose you directly to someone else’s blood, will mean you may benefit from additional or more frequent immunisations.
- occupation – you are likely to need extra immunisations, or need to have them more often, if you work in an occupation that exposes you to vaccine-preventable diseases or puts you into contact with people who are more susceptible to problems from vaccine-preventable diseases (such as babies or young children, pregnant women, the elderly, and people with chronic or acute health conditions). For example, if you work in aged care, childcare, healthcare, emergency services or sewerage repair and maintenance, discuss your immunisation needs with your doctor. Some employers help with the cost of relevant vaccinations for their employees.
Read: The Nervous System