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Objectives – What you will need to know from this section
The Immune System
Role of lymphocytesB and T cell types.
Role of B cells in antibody production.
Role of T cells as helpers, killers, suppressors, and memory T cells.
B cells produce antibodies.
Each B cell produces just one type of antibody.

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T cells act in one of four processes:
as helper T cells which recognise antigens, and stimulate the production of B cells
as killer T cells which attack cells containing a foreign antigen,
as suppressor T cells which stop immune responses
as memory T cells which can memorise the immunity.

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The human body is designed to protect itself from foreign cells or invaders, and we have two lines of defence -- general and specific.
Our Specific Defence System involves an antigen-antibody response, involving lymphocytes and monocytes.

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LYMPHATIC SYSTEM
1. Drainage – returns leaked fluid to the blood.
2. Transport – lipids (fats) are absorbed from the small intestine and carried to the skin or other organs for storage.
3. Defence – filters blood. Germs that invade the body will enter the blood or tissue fluid and eventually get filtered through the lymph and lymph nodes, where white cells attack and destroy them.

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Our Specific Defence System involves :
an antigen-antibody response, involving speciliased white blood cells, called lymphocytes and monocytes [macrophages].
1. Lymphocytes - Stored in lymphatic system (spleen, lymph nodes, tonsils, adenoids & thymus gland), have large nucleus and make antibodies. (25% of white cells)
lymphocyte
2. Monocytes – digest bacteria, have kidney shaped nucleus & live for 6-9 days (5% of white cells)

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Monocytes [Macrophages]
  Monocytes are phagocytic white blood cells – also called macrophages
  These engulf bacteria and viruses upon contact.
“Eating” germ
Germ
Monocyte
  Others secrete chemicals that stimulate general defence and cause fever to destroy microbes at high temperatures.

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Monocyte engulfs an invader
Monocyte engulfs a bacterium

8. LEARNING CHECK What is meant by a “specific defence system”?
Name two types of White cell involved in this system.
What is a lymphocyte?
What is a Monocyte?
What does “engulf” mean?
Can you name an animals that engulfs its prey?

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Germ (antigen)
Antibody
 Antigens are any foreign molecules capable of triggering an antibody response.
“Eating ” germ
monocyte
An antibody is a protein, made by a lymphocyte, which sticks to an antigen and marks it for destruction by other defence cells [monocytes].
 Antigens may be found in bacterial cell walls, viral coats, foreign cells or produced in cancerous cells.

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Lymphocytes are made in the bone marrow and then move to the lymph nodes before or soon after birth.
Monocytes engulf an invader, digest it and then display some of the invader’s antigen on its own cell membrane.
Body cells do the same with viruses that get into them.
Lymphocytes are continually inspecting monocytes and body cells for evidence of ‘foreign’ antigen.
When detected, the lymphocytes are stimulated to divide into huge numbers of cells.

11. Two types of lymphocytes T-lymphocytes B-lymphocytes
THE IMMUNE RESPONSE
Two types of lymphocytes
T-lymphocytes
B-lymphocytes

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T lymphocytes
They attack cells that have invading antigen on their cell membrane.
T cells do not produce antibodies.
Instead they act as

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helper T cells
recognise antigens, enlarge, and secrete chemicals such as interferon that stimulate production of B cells
killer T cells
attack cells containing a foreign antigen,
secrete a chemical called perforin that perforates the membranes of cells ;
destroy virus-infected cells, tumour cells and transplant tissue.

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suppressor T cells
stop the immune response after the pathogen has been destroyed.
memory T cells
Memory cells that form during the first response to an antigen do not engage in that first battle.
They circulate in the body for years, and can intercept antigens far quicker.
So they memorise the immunity, even for life.

15. LEARNING CHECK Distinguish between antigen and antibody.
Distinguish between lymphocyte and monocyte.
What is the thymus gland?
Name 4 types of T-lymphocyte and a function of each.
Distinguish between interferon and perforin.

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B lymphocytes
B lymphocytes mature in the bone, before moving to the lymph nodes, where they acquire their receptor molecules.
B because they are made in the bone marrow
B lymphocytes attack antigens in the blood or body fluids, by producing antibodies that surround them.

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When a B cell encounters matching antigen, they divide into plasma and memory cells.
The plasma cells produce antibody molecules, that bind to pathogens and toxins that are circulating in tissues or body fluids, and mark them for destruction by monocytes.
Each B cell produces just one type of antibody.

19. Interleukins are made by helper T cells stimulate B cells to divide into plasma cells and memory B cells.

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Memory B and T cells that form during the first response to an antigen, do not engage in that first battle.
They circulate in the body for years, and can intercept antigens far quicker.
This is how we achieve immunity against many diseases – either by getting the disease once or by preventing them using vaccines.

21. Monocyte
Monocyte

22. LEARNING CHECK What are B-lymphocytes?
What is an activated B-lymphocyte?
What is a plasma cell?
Explain why antibodies are specific.
Explain why vaccination gives us long term immunity.