The lymphatic system, which comprises of lymph, lymphatic vessels, and lymphatic tissue, has an important role within the body. It:
- drains excess fluid and protein from the interstitial tissue back into the blood
- transports fat from the GI tract to the blood
- produces and circulates lymphocytes that help in keeping the body protected
Lymph
Lymph is the same as the interstitial fluid. Fluid that bathes the cells is referred to as interstitial fluid, while when it flows through the lymphatic vessels, it is called lymph.
Lymphatic Vessels
The lymphatic vessels are microscopic vessels in between the cells spaces known as lymph capillaries. They are slightly larger than blood capillaries and more permeable. Whilst we can find lymphatic capillaries throughout the whole body, they are not found in the avascular tissue, CNS, splenic pulp and bone marrow.
Lymph capillaries connect through larger lymph vessels known as lymphatics, which converge into two main channels, namely the thoracic duct and the right lymphatic duct. Lymphatics have thinner walls and valves (more valves than veins). Lymph nodes can be found at various intervals.
Drainage of the Lymphatic System
The left side of the head, neck, chest, upper left extremities and the entire body below the ribs all drain into the thoracic duct.
The upper right side of the body drains into the right lymphatic duct.
Lymphatic Tissue
Lymphatic tissue is rich in lymphocytes and accessory cells such as macrophages and reticular cells. It is scattered in the linings of the GI tract, the respiratory tract, the urinary tract, the reproductive tract, and in the stroma a.k.a. core of multiple organs. Lymphatic tissue can also be found surrounded by a capsule within the lymphatic organs a.k.a. lymph nodes, spleen, and the thymus gland.
Lymph nodes
Lymph nodes are oval structures measuring between 1-25mm in length, commonly found in groups, located along the lymph vessels’ pathway.
Afferent Lymphatic Vessels => Sinuses => Efferent Lymphatic Vessels
- Lymph passes through the nodes and is filtered from foreign substances by the reticular fibres within the node
- Macrophages destroy foreign substances through phagocytosis
- T Cells destroy foreign substances through the release of various products
- B Cells produce antibodies to destroy them
Lymphatic ORgans: Tonsils
Tonsils are a pair of soft tissue masses located at the pharynx. Their location helps protect against the invasion of foreign substances through the production of lymphocytes and antibodies.
Lymphatic Organs: Spleen
The spleen is an oval shaped organ measuring around 12cm in length which is made up of lymphatic tissue. It is located in the upper left side of the abdomen, next to the stomach and behind the left ribs. Functions of the spleen include:
- B Lymphocyte Production – eventually develop into antibody-producing plasma cells
- Phagocytosis – of bacteria and damaged or worn-out red blood cells and platelets
- Blood Storage and Release – in cases such as haemorrhage
NOTE: the spleen does not filter lymph.
Lymphatic Organs – Thymus Gland
The thymus gland is found in the superior mediastinum, between the lungs and behind the sternum. It reaches it’s maximum size during puberty, after which it starts to break down. The main function of the thymus gland is to produce T Lymphocytes.
Body Defences
The human body aims to maintain haemostasis by counteracting pathogens or related toxins in the environment.
Resistance = the body’s ability to keep off disease
Susceptibility = the body’s inability to resist disease
Body defences can be divided in two groups: Non-Specific Defence and Specific Defence a.k.a. Immunity
Non-Specific Defences
The non-specific defence mechanism provides an immediate response to protect the body from foreign substances. Components of the non-specific defence mechanisms include:
- SKIN & MUCOUS MEMBRANES – MECHANICAL FACTORS include the epidermis‘ anatomy i.e. made up of closely packed cells, continuous layering and the presence of keratin; mucous membranes that secrete mucus to prevent cavities from drying up whilst trapping microbes at the same time (eg. in nose through hairs and in the upper respiratory tract through cilia); lacrimal apparatus; saliva which helps prevent microbe colonisation; epiglottis which helps prevent microbes from entering the lower respiratory tract; CHEMICAL FACTORS include sebum which forms a protective film over the skin’s surface and inhibits bacterial growth; perspiration which flushes microbes from the skin; gastric juice produced by the stomach glands which is highly acidic due to being made of hydrochloric acid, enzymes and mucus, all of which help preserve the stomach’s sterility whilst destroying bacteria and most bacterial toxins; lyzozyme (found in perspiration, tears, saliva, nasal secretions and tissue fluids) which is an enzyme that can break down cell walls of various bacteria;
- ANTI-MICROBIAL SUBSTANCES – INTERFERON (IFN) (alpha, beta & gamma) which are produced by lymphocytes and other leucocytes and fibroblasts; COMPLEMENT – a group of 11 proteins found in normal blood serum which complements immune and allergic reactions involving antibodies – once activated, destroys microbes; PROPERDIN – a protein found in the serum which together with COMPLEMENT causes the destruction of several types of bacteria, enhances phagocytosis, and triggers inflammatory responses;
- PHAGOCYTOSIS– the ingestion and destruction of microbes or other foreign particulate matter by phagocytes through the Adherence process and the Ingestion process;
- INFLAMMATORY PROCESS – when cells are damaged by microbes, inflammation is triggered, characterised by redness, pain, heat, swelling, and loss of function; during inflammation, vasodilation increases permeability of blood vessels, neutrophils migrate to the injured area within one hour, nutrients are released to help support defensive cells and increased metabolic reactions of the affected cells, fibrin formation, and pus formation;
- FEVER – inhibits microbial growth and speeds up body reactions which help the body to heal
HIGH FEVER => HIGH BODY TEMPERATURE => INCREASED RATE WITH WHICH THE BODY WORKS TO FIGHT OFF INFECTION, BACTERIA OR VIRUSES.
Specific Defences a.k.a. immunity
Immunity involves the production of a specificy cell type or molecule a.k.a. antibody that can destroy a particular antigen.
An antigen is a chemical substance which causes the body to produce specific antibodies which can react with the antigen. Antigens have immunogenicity (can stimulate the formation of specific antibodies) and reactivity (can react specifically with the produced antibodies). Antigens with both immunogenicity and reactivity are called complete antigens eg. proteins, nucleoproteins, lipoproteins, glycoproteins, and some polysaccharides. Non-microbial antigens include pollen, egg white, incompatible blood cells, and transplanted tissues and organs.
An antibody is a protein produced by the body in response to antigen presence. An antibody can combine with the antigen.
Antibodies belong to a group of proteins called globulins, hence the name immunoglobulins.
Cellular immunity & Humoral Immunity
The body can defend itself against bacteria, toxins, viruses and foreign tissues thanks to 2 components:
- CELLULAR IMMUNITY A.K.A. CELL-MEDIATED IMMUNITY – includes the formation of specially sentisised lymphocytes which can attach to the foreign agent and destroy it; T cells are responsible for cellular immunity
- HUMORAL IMMUNITY A.K.A. ANTIBODY-MEDIATED IMMUNITY – includes the formation of circulating antibodies able to attack an invading agent; B cells are specialised plasma cells which produce antibodies and provide humoral immunity
The Immune Response
The immune response of the body, be it cellular or humoral, is more intense after a second or subsequent exposure to an antigen than after initial exposure, as illustrated below…
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