Immune system. Organs

The immune system is a system of cells, tissues, organs and molecules that carries out the organism's immune response. It keeps the body in immune homeostasis by detecting, identifying and eliminating extrinsic genetic agents, such as microbes, viruses and other foreign substances. Because of this role, the immune system becomes the core of initiation, progress and outcome of allergic, malignant and autoimmune diseases.

This topic covers the basic principles of immunity, the two arms of the immune response, and the organs of the immune system: which organs are primary and which are secondary, and what each one does.

The foundation of the immune system is its capacity to distinguish non-self molecules, which have the potential to cause harm, from self molecules. This capacity rests on a delicate balance between tolerance of self and response or rejection of non-self. When this balance is disrupted, the result is either autoimmunity, a response against the body's own tissues, or allergy, a response against harmless environmental substances.

The immune system uses two complementary sets of defence mechanisms against potentially pathogenic microorganisms.

Innate immunity

Innate immunity is the first line of defence and gives an immediate, quick response.

• Its specificity is based on pattern recognition: it responds to molecules that are shared by multiple microbes but are not present in the host.
• It consists of physiological barriers such as the skin and mucosa, phagocytic cells, and plasma proteins such as the complement system and natural antibodies.
• The inflammatory reaction is considered the main form of innate immunity.

Adaptive immunity

Adaptive immunity develops more slowly but gives a long-lasting and highly specific protection.

• It is designed to recognise and distinguish a large number of different molecules.
• It can amplify the response with repeated exposures to the same pathogen or molecule.
• It manifests as two types: cell-mediated immunity, which is mainly related to antigen-presenting cells and T and B lymphocytes, and humoral immunity, which is based on specific immunoglobulins (antibodies).

The immune system consists of two kinds of organs. Primary organs are the sites where T and B lymphocytes mature. Secondary organs are the sites where mature lymphocytes are gathered and where immune responses are started.

Bone marrow

• The bone marrow produces stem cells. These are pluripotent cells, from which all the blood cells derive.
• Maturation of B lymphocytes takes place mainly in the bone marrow.

Liver

• During the embryonal period of mammals, the liver has a crucial significance for haemopoiesis and lymphopoiesis, so it is counted among the primary lymphoid organs.
• During fetal life, B cells mature in the liver. Later in life, the bone marrow takes over this role.

Thymus

• T cell progenitors are derived from the common lymphoid progenitor cells. They leave the bone marrow through the bloodstream and enter the thymus gland.
• The T cells remain in the thymus, where the precursor cells mature into T cells. They emerge with distinct surface antigen receptors and functional characteristics.

Lymph nodes and spleen

Lymph nodes and the spleen are the secondary organs of the immune system. Lymph nodes are placed along the lymphatic vessels, and the spleen is situated within the circulation.

Mucosal and skin lymphoid tissue

Lymphoid tissue is also found in the mucosa and in the skin. It is known as mucosa-associated lymphoid tissue (MALT). Its parts include the gut-associated lymphoid tissue (GALT), such as Peyer's patches in the small intestine and the appendix, and the bronchus-associated lymphoid tissue (BALT) of the airways. The tonsils and adenoids, which lie around the entrance of the pharynx, are part of this lymphoid tissue as well.