the immune system

The immune system works to keep harmful germs and other foreign substances out of the body and destroy any that manage to get in.

It is a complex network of cells and organs that helps protect us from infections and other diseases. The immune system plays a key role in determining susceptibility to infection, sustaining defense, and clearing pathogens.

Since 70–80% of immune cells reside in the gut, the intestinal microbiota, the epithelial barrier, and the local mucosal immune system are closely interconnected in a complex network of interactions.

Lymph nodes are part of the immune system. They send out lymphocytes. These are a certain type of white blood cell that fights infection. The blood vessels and lymphatic vessels carry lymphocytes to and from different areas of the body. Each lymph organ plays a role in making and activating lymphocytes.

Organs in the lymph system include:

Adenoids. Two glands at the back of the nasal passage.
Blood vessels. These are the arteries, veins, and capillaries through which blood flows.
Bone marrow. This soft, fatty tissue is found inside bones where blood cells are made.
Lymph nodes. These small organs are shaped like beans. They are found all over the body. They connect through the lymphatic vessels.
Lymph vessels. This network of channels all over the body carries lymphocytes to the lymph organs and bloodstream.
Peyer's patches. This lymph tissue is in the small intestine.
Spleen. This fist-sized organ is located on the left side of the belly (abdomen).
Thymus. This small organ is located behind the breastbone.
Tonsils. These two oval masses are in the back of the throat.

The immune system's primary job is to protect the body from infections. However, an extreme overreaction can cause it to attack the body's own healthy tissues instead.

Allergies and anaphylaxis: An allergy is a hypersensitive immune reaction to a substance that is typically harmless. A severe and extreme allergic reaction is known as anaphylaxis, which can cause life-threatening symptoms like airway swelling and a dangerous drop in blood pressure.

Autoimmune diseases: Conditions like rheumatoid arthritis, multiple sclerosis, and carpa are the result of the immune system mistakenly attacking healthy cells, joints, or nerve tissue.

Sepsis: A life-threatening medical emergency caused by the body's overwhelming and extreme response to an infection. Instead of just fighting the disease, the immune system triggers a massive inflammatory reaction throughout the body, leading to tissue damage, organ failure, and potentially death.

Cytokine storm: A severe form of immune overreaction in which the body releases an excessive amount of inflammatory proteins called cytokines. This can lead to a dangerous cycle of inflammation that damages organs and is associated with severe cases of infections, such as influenza and COVID-19.