The lymphatic and immune system
The lymphatic system is one component of the immune system. Its vessels carry lymph, its lymphocytes provide cellular and humoral immunity, and its nodules and organs house those cells. The immune system as a whole defends the body through innate and adaptive immunity.
Lymph and the lymphatic vessels
The lymphatic system is one component of the immune system, and it defends the body and maintains tissue homeostasis. It includes a network of lymphatic vessels that carry lymph, a fluid similar in composition to interstitial fluid. It returns interstitial fluid to the general circulation, prevents local variations in the composition of tissue fluids, and provides specialised defences against infection.
Lymphocytes
An antigen is any large molecule or small organism whose entry into the body provokes the synthesis of an antibody or immunoglobulin, that is, an immune system response.
Lymphocytes are the cells of the lymphatic system and comprise three distinct populations. T cells attack invading, infected or abnormal cells and are responsible for cellular immunity. B cells are concerned with the production of antibodies, which are complex immunoglobulin molecules produced by lymph tissue in response to an antigen, and they provide humoral immunity. Natural killer cells detect the presence of abnormal antigens on cell membranes and are responsible for immunological surveillance and the removal of cancer cells from normal tissues.
Lymphatic nodules and organs
A lymphatic nodule is a dense aggregation of lymphocytes in an area of loose connective tissue, usually beneath an epithelium. The pharyngeal (adenoid), palatine and lingual tonsils are large lymphatic nodules embedded in the walls of the pharynx. Peyer's patches are lymphatic nodules beneath the epithelium of the small intestine. Large lymphatic nodules are also found beneath the epithelium of the appendix and large intestine.
Lymphatic organs have a clear internal organisation and are surrounded by a dense fibrous capsule. They include the thymus, the lymph nodes and the spleen. The thymus lies within the mediastinum behind the sternum, reaches its maximal size during puberty and slowly shrinks thereafter. Lymph nodes are encapsulated masses of T cells and B cells, with blood vessels, nerves and lymphatics. Lymph arrives at a node through lymphatics and flows through a network of spaces where lymphocytes and macrophages monitor its contents as it proceeds toward the lymphatic ducts and the venous system. The spleen contains the largest mass of lymphatic tissue in the body. It lies in the upper part of the abdominal cavity, and its shape is determined mainly by the structures adjacent to it. Its cellular components make up the pulp: red pulp contains large numbers of red blood cells, whereas white pulp resembles lymphatic nodules.
The thymus is where T lymphocytes mature early in life. Epithelial cells scattered among its lymphocytes produce the hormone thymosin, which stimulates the division of stem cells in the thymus. The thymus produces and secretes these hormones, the thymosins, and thymosin promotes immunity. T lymphocytes become immunologically active when they interact with thymosins either inside the thymus or in the bloodstream. If the thymus does not develop, or is removed during childhood, the immune system will not develop completely. If the thymus does not develop, as in DiGeorge syndrome, the immune system is mildly to moderately deficient.
Innate and adaptive immunity
The vertebrate immune system can be divided into two categories of defence, innate immunity and adaptive immunity, although the distinction is not mutually exclusive.
Innate immunity is the first line of host defence against pathogenic micro-organisms. Innate immune cells such as macrophages and neutrophils recognise a limited set of danger signals and microbial molecular determinants shared by different pathogens. On recognising these threats, they act within minutes by engulfing pathogens and inducing inflammation and further immune-cell recruitment. Some innate cells can directly kill certain pathogens. Others take up antigens from pathogens, tumours and vaccines and present them to T cells to activate adaptive immunity.
Adaptive immune cells, notably T and B lymphocytes, provide the second line of defence, generally at a later stage of infection. Unlike innate immunity, adaptive immunity is antigen-specific: it recognises a particular type of pathogen through a specific antigen. It can also provide the immune memory that protects against re-infection with the same pathogen. Every encountered pathogen expresses or contains antigens, each of which can activate a specific lymphocyte. When an antigen-presenting cell activates a T lymphocyte, the T lymphocyte proliferates to form clones of T cells that recognise and eliminate infected or malignant host cells containing the specific antigen. Activated B lymphocytes similarly proliferate to give clones of B cells that differentiate into antibody-producing cells. The antibodies these cells produce recognise and neutralise pathogens or toxic products expressing the particular antigen. After eliminating the pathogen, the adaptive immune response regresses but leaves memory lymphocytes that can quickly proliferate to attack or produce antibodies against the same pathogen, even decades later, if it is encountered again.
From a naive state, T cells differentiate and diversify, adapting after each encounter with a new antigen. Some become memory T cells, which, enabled by the cytokine interleukin-7, are equipped for future specialised combat. T cells can be categorised into three types. Cytotoxic (CD8+) T cells attack cancer cells, other pathogens ("non-self") and damaged cells that could disrupt the balance of health. Helper (CD4+) T cells activate cytotoxic T cells and other lymphocytic functions. Regulatory T cells (Tregs), also called suppressor T cells, prevent unchecked autoimmunity by suppressing immunity and protecting "self".
Humoral and cellular immunity work together to defeat the pathogens that attack the body. Nonspecific interferons or antibodies produced by B lymphocytes take part in this defence, as do T lymphocytes or macrophages, which kill the pathogen directly.