UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 16/05/2023 [SEARCH]

 Body defences: 6. The lymphatic system, lymphocytes, production of antibodies which destroy pathogens using our immune system

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INDEX of biology notes on the body's defence mechanisms against infections from pathogens

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(6) The lymphatic system, lymphocytes, production of antibodies which destroy pathogens

The lymphatic system

Your lymphatic system is part of your immune system and has many functions

These functions include:

protecting your body from infection - the lymphatic system is part of the immune system. It produces and releases lymphocytes (white blood cells) and other immune cells that monitor and then destroy the foreign pathogen invaders that may enter your body e.g. bacteria, viruses, parasites and fungi.

maintaining body fluid levels - the lymphatic system collects excess fluid that drains from cells and tissue throughout your body and returns it into your bloodstream, which is recycled through your body.

absorbing digestive tract fats - lymph includes fluids from your intestines that contain fats and proteins and transports it back into your bloodstream.

removing cellular waste - lymph fluids transports and removes waste products and abnormal cells.

Blockages, diseases or infections can impair your lymphatic systemís function.

The main parts of the Lymphatic system

1. Lymph (lymphatic fluid)

Lymph is a collection of the extra fluid that drains from cells and tissues (that is not reabsorbed into the capillaries) and contains other substances such as proteins, minerals, fats, nutrients, damaged cells, cancer cells and foreign invaders (bacteria, viruses, etc). Lymph also transports infection-fighting white blood cells (lymphocytes).

2. Lymph nodes

Lymph nodes are bean-shaped glands of lymphatic tissue, found throughout the lymphatic system (particularly the neck and armpits), that monitor and cleanse the lymph fluid as it filters through them.

Large numbers of lymphocytes are found in the lymph nodes. Tissues such as bone marrow, are associated with the lymphatic system and produce these lymphocytes which play an important role in defending the body against infections (e.g. pathogens).

The lymph nodes filter out the damaged cells and cancer cells and also store lymphocytes and other immune system cells that attack and destroy bacteria and other harmful substances in the fluid.

3. Lymphatic vessels and collecting ducts

Lymphatic vessels are the network of capillaries and a large network of fine tubes located throughout your body that transport lymph away from tissues back to the heart and connects with the blood system near the heart where lymph fluids is returned to the blood plasma.

Lymphatic vessels collect and filter lymph fluid (at the nodes, see above) as it continues to move toward larger vessels called collecting ducts. These lymphatic vessels operate very much like blood veins do, but work under very low pressure, and have a series of valves in them to keep the fluid moving in one direction. The collecting ducts return lymph to the bloodstream which helps to maintain normal blood volume and pressure and also prevents the excessive build up of fluid around the tissues.

4. Spleen

The spleen is the largest lymphatic organ is located under your ribs and above your stomach. The spleen filters and stores blood and produces white blood cells to fight infection or disease.

5. Thymus

This thymus organ is located in the upper chest beneath the breast bone and its function is to mature a specific type of white blood cell that fights off pathogens.

6. Tonsils and adenoids

These lymphoid organs trap pathogens from the food you eat and the air you breathe and they are your bodyís first line of defence against pathogens.

7. Bone marrow

Bone marrow is the soft, spongy tissue in the centre of certain bones, such as the hip bone and breastbone. White blood cells, red blood cells, and platelets are all made in the bone marrow.

8. Peyer's patches

Peyer's patches are small masses of lymphatic tissue in the mucous membrane that lines your small intestine and these lymphoid cells monitor and destroy bacteria in the intestines.

9. Appendix

Your appendix contains lymphoid tissue that can destroy bacteria before it breaches the intestine wall during absorption of food from digestion. The appendix may play a role in housing 'good bacteria' and repopulating our gut with good bacteria after an infection has cleared.

A note on transportation - function of the lymphatic system in the circulation of body fluids

The walls of the associated capillaries are very thin and water, so dissolved solutes and dissolved gases can diffuse in and out of them and pass through the walls from the plasma into the tissue fluid surrounding the cells

Cells can exchange materials such as water, oxygen, glucose, amino acids, carbon dioxide, mineral ions (e.g. K+, Na+, Cl-, Ca2+ etc.) out through their cell membranes into the tissue fluid surrounding them (either by diffusion, osmosis or active transport).

More fluid leaks out of the capillaries than is returned to them, and the excess leaked fluid surrounding the capillaries passes into the lymphatic system and becomes part of the lymph fluid.

Reminders:

Phagocytes (see part 5) can leave the bloodstream and squeeze through capillaries and enter tissues attacked by some invasive pathogen infection.

The phagocytes move to the pathogens (or toxins) and ingest them.

When we suffer from a disease that we recover from, the body makes memory cells that recognise the same infection if it enters the body again.

The memory cells produce antibodies to destroy the pathogen, hopefully to prevent us from feeling or being ill - we may be completely unaware that this has happened.

All invading cells have unique molecules ('molecular structure') on their surface called antigens.

When white cells encounter a 'foreign' antigen on a pathogen they don't recognise, they produce proteins called antibodies which lock onto the antigens of the pathogen making them more susceptible to phagocytosis - described above and also inhibit the pathogen from entering your cells.

Reminder: The pathogen can be a bacteria, virus or fungus.

The white blood cells that perform this task are called B-lymphocytes and the overall process is described using the diagram below.

These cells are involved with specific immune responses which can involves various mechanisms after the lymphocytes recognise pathogens and quickly reproduce to make lots of antibodies e.g.

The antibodies can cause pathogen cells to burst - a process called lysis, often due to an enzymic action.

The antibodies can bind to pathogens and destroy them.

The antibodies coat the pathogen, sticking them together so that phagocytes can ingest them.

1. Large numbers of B-lymphocyte white blood cells (grey) are always present in the blood and they can recognise OR not recognise, different types of pathogens - bacteria and viruses.

2. All invading pathogens (green O) have unique molecules on their surface called antigens (blue -, often proteins). If the surface of the lymphocyte detects the antigens (blue) on the surface of a 'foreign' pathogen they don't recognise, a response is triggered by the lymphocyte cell.

3. The lymphocyte cell begins to produce protein molecules called antibodies (black Y).

4. The antibodies move out of the cell to 'confront' the invading pathogen and will not lock onto any other pathogen.

5. The antibodies lock onto the antigens on the surface of the pathogen (e.g. invading bacteria cell).

6. The invasive pathogen is then more easily found and destroyed by another type of white blood cell - the phagocytes, which destroy them by phagocytosis - described in section (a) above.

The antibodies often cause the pathogens to clump together making it easier for the phagocyte cells to find and ingest them by phagocytosis.

The white blood cells that detect the pathogen then divide to produce more copies (clones) of the same white blood cell, which in turn make more of the antibody.

The antibodies are produced quite rapidly and move all around the body in the bloodstream to find other similar pathogens.

If exactly the same type of pathogen enters your body again, the lymphocyte cells recognise it immediately and make lots of antibodies to counteract it.

This the basis of immunity i.e. how you become immune from a disease and this is described in detail in the next section.


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