It’s the immune system’s job to recognise and identify threats to the body, remove them as quickly as possible and retain a ‘memory’ of that threat should we ever encounter it again.
There are two main branches of the immune system: the innate immune system and the adaptive immune system.
The innate immune system
We are born with innate immunity – a layer of defence that is activated immediately once a pathogen (or disease-causing organism) attacks. Its main purpose is to immediately prevent the spread and movement of foreign invaders throughout the body. The innate immune system reacts to the presence of antibodies to provide a generalised, non-specific layer of defence against common pathogens. It triggers inflammation and increases blood flow to the area in order to recruit other immune cells to the site and limit the spread of the invader.
The innate immune system also includes physical barriers to infection such as the skin, mucus, bile and stomach acid.
The adaptive immune system
By contrast, the adaptive immune system – also known as acquired immunity – is highly specific to the type of pathogen it’s targeting. It responds more slowly than the non-specific innate immune system, but its advantage lies creating a tailored response (antigens) that perfectly matches the threat. This allows the immune system to launch a more effective, coordinated and longer-lasting attack. We develop adaptive immunity as our body is exposed to new pathogens. What’s more, the adaptive immune system retains a ‘memory’ of pathogens it encounters, so if it were to appear a second time, the immune system is already primed to quickly remove it, often before we even know we’re sick.
Working together, the innate and adaptive immune responses provide all-round protection, dealing with both the everyday threats and pathogens as well as targeting the one-in-a-million cell that has a dangerous, cancer-causing mutation.
Components of the immune system
Organs and tissues
The cells of both the innate and adaptive immune system are found in various organs of the body, including:
Adenoids - Two glands located at the back of the nasal passage.
Bone marrow - The soft, spongy tissue found in bone cavities.
Lymph nodes - Small organs shaped like beans, which are located throughout the body and connect via the lymphatic vessels.
Lymphatic vessels - A network of channels throughout the body that carries lymphocytes to the lymphoid organs and bloodstream.
Mucous membranes - A layer of protective mucus lining various body cavities that traps pathogens. Found in the nose, throat, lung, eyelids, stomach, bladder and genitals
Peyer's patches - Lymphoid tissue in the small intestine.
Spleen - A fist-sized organ located in the abdominal cavity.
Thymus - Two lobes that join in front of the trachea behind the breastbone.
Tonsils - Two oval masses in the back of the throat.
Immune cells & related molecules
B-cells - B-cells (or B-lymphocytes) are a type of white blood cell and are an important component in the adaptive immune response. They are responsible for producing antibodies to target specific pathogens or infected cells. B-cells are produced in the bone marrow.
T-cells - T-cells are a type of immune cell produced in the thymus and play a central role in recognising antigens – a key condition for initiating an immune response. There are several kinds of T cells based on the type of immune response initiated.
NK (natural killer) cells - Rather than killing pathogens directly, NK cells kill infected or compromised cells. When a cell is infected by something like a virus, it can send signals to NK cells so they can be removed before the infection spreads.
NKT cells - NKT cells are a type of immune cell that has properties of both T cells and NK cells.
MAIT (mucosal associated invariant T) cells - MAIT cells are a type of T cell that responds to bacteria and microbial pathogens by producing cytokines that aid the immune response. They are predominantly found in mucosal membranes in the body such as the nose, lungs and gut lining.
Antibodies - Antibodies are Y-shaped proteins that bind to a specific antigen on the surface of a cell. Each antibody is unique for the antigen it targets. When an antibody binds to an antigen, it ‘tags’ the cell, signalling it for removal by the immune system. Antibodies are part of the adaptive immune response and made within B-cells.
Antigens - Antigens are molecules on the surfaces of cells and microorganisms that can be recognised by the immune system. Antigens are how the immune system recognises what is helpful and what is harmful to the body. Cells and other microorganisms labelled as harmful are destroyed by specialised immune cells.
Cytokines - Cytokines are signalling molecules produced by cells in the immune system that stimulate immune responses such as inflammation. They often function as messenger molecules, helping recruit immune cells to a specific site in the body.
Dendritic cells - Dendritic cells are a type of immune cell that is found throughout the body. They are often called the ‘sentinels of the immune system’ because one of their main functions is to scour the body for signs of infection and disease, and present this information to the rest of the immune system. They act as mediators between the innate and adaptive immune responses.
Phagocytes - Phagocytic “eating cell” cells is the term used to describe several classes of immune cells. When a phagocytic cell encounters something marked for destruction, they engulf the offending cell, bacteria, virus and destroy them.
Macrophages - Macrophages are a type of phagocyte that scours the body hunting for pathogens. Their ability to roam the body freely makes them especially efficient hunters.
Eosinophils - Eosinophils are granulocytes that target multicellular invaders like parasites. When they encounter a parasite like a worm, they release highly toxic proteins and chemicals (called free radicals) to damage and kill the offending parasite.
Neutrophils - Neutrophils are another type of phagocytic cell called granulocytes that are particularly toxic to bacteria and fungi and are produced in large quantities in the bone marrow. They are often the first immune cells to arrive at the site of an infection.
Basophils - A less common type of granulocyte, basophils are not yet fully understood. Part of their function is to release histamines, and they play a role in protecting against parasitic infection, and also help initiate repair mechanisms in the skin
Mast cells - Mast cells are commonly found in mucous membranes and connective tissue. They play an important role in wound healing and defending the body from pathogens. They do this by releasing cytokines which cause localised inflammation, which also serves to recruit other immune cells to the area to deal with the threat.