Unit 4 - Immunology and Public Health CfE Higher Human Biology Specific Cellular Defences

Learning Intentions I can state that one group of T-lymphocytes destroy infected cells by inducing apoptosis. Another group of T-lymphocytes secrete cytokines that activate B lymphocytes and phagocytes. When pathogens infect tissue, some phagocytes capture the pathogen and display fragments of its antigens on their surface. These antigen presenting cells activate the production of a clone of T-lymphocytes that move to the site of infection under the direction of cytokines. I can state that each B-lymphocyte clone produces a specific antibody molecule that will recognise a specific antigen surface molecule on a pathogen or a toxin. Antigen- antibody complexes may inactivate a pathogen or toxin or render it more susceptible to phagocytosis. In other cases the antigen-antibody complex stimulates a response which results in cell lysis. B-lymphocytes activated by antigen presenting cells and T-lymphocytes produce a clone of B-lymphocytes that secrete antibodies into the lymph and blood where they make their way to the infected area. I can describe immunological memory. I can state that some T- and B-lymphocytes produced in response to antigens by clonal selection survive long-term as memory cells. A secondary exposure to the same antigen rapidly gives rise to a new clone of lymphocytes producing a rapid and greater immunological response

specific cellular defences-lymphocytes(the third line of defence). The third line of defence is brought about by lymphocytes derived from stem cells in bone marrow. The thymus gland is a gland in the chest cavity and some lymphocytes pass to this gland to develop into T lymphocytes(T cells). Those that stay and mature in bone marrow become B lymphocytes (B cells).

T-lymphocytes There are two different types of T- lymphocytes or T-cells. - helper T cells (T H cells) - cytotoxic T cells (T C cells) Helper T cells become activated when they have come into contact with a foreign antigen. They secrete cytokines activate phagocytosis, Tc cells and B cells. Cytotoxic T cells cause apoptosis of an infected cell.

T H Cells Antigen presenting cell

.

T H Cells As seen before, there is a vast pool of different T H cells with different forms of antigen receptors. One of them will be specific to the antigen. When the phagocyte has engulfed the foreign cell and has presented the foreign antigen, one of the T H cells will be able to bind with it.

T H Cells Once one version of the T H cell is activated, it goes on to multiply to give either; - further clones of activated T H cells - clones of memory T H cells The activated T H cells release cytokines which will stimulate cytotoxic T cells (T c cells) and also stimulate B cells To see animation about Th click herehere

Antigen presenting cell. When a phagocyte has captured and destroyed an invading pathogen it usually presents fragments of the pathogen’s antigens on its surface. This kind of phagocyte is called an antigen presenting cell. Other cells in the body also become antigen presenting cells.

Antigen presenting cell.

T C cells Cytotoxic T cells are also activated in a similar way, where the specific antigen receptor binds to the antigen on the presenting cell. They go on to multiply to give either; - clones of the activated T C cells - clones of memory T C cells The activated T C cells go on, under the action of cytokines from T H cells, to destroy infected cells

Action of cytotoxic cells. In the body’s pool of cytotoxic T cells there is a type of Tc cell that has copies of one type of antigen receptor on its surface that are specific to and are able to bind to the type of foreign antigen on the surface of an antigen- presenting phagocyte. This binding process results in the Tc cell becoming activated by the antigen presenting cell and then proliferating and differentiating. A clone of activated Tc cells and a clone of memory Tc cells are made. The activated Tc cells go to the site of the infection under the influence of cytokines released by helper T cells and attack infected cells. Some types of Tc cells kill the infected antigen presenting cells by inducing them to undergo apoptosis. Once a Tc cell has killed a target cell it leaves the cell and moves on to another infected cell. An infected cell shrinks but its membrane is not destroyed by apoptosis but its cell contents stay enclosed and are not dispersed. A phagocyte engulfs and digests the infected cell. Other Tc cells recognise antigens on the surface of cancer cells and attack them.

Action of cytotoxic cells.

Destruction of infected cells Apoptosis is where cells are programmed to self destruct. The T C cells bind to an infected cell as they have copies of the foreign antigen on their surface. The T C cells release chemicals which perforate the cell membrane allowing further chemicals access to the cell causing the DNA and proteins to be broken down. The cell now slowly shrinks and dies, allowing a phagocyte to engulf and digest it. This means the cell remains whole so the foreign pathogen is not released to infect more cells.

T C cells and cancer T C cells are also able to recognise antigens found on the surface of cancer cells. They are then able to bring about the lysis of large cancer cells.

B-lymphocytes B lymphocytes are able to recognise foreign antigens and engulf them. Some will become clone antibody-producing B cells, others become cloned memory B cells They then display the antigens on their surface waiting for a T H cell to release cytokines and activate it.

Activated B lymphocytes Activated lymphocytes can produce a protein specific to the antigen called antibodies. The antibodies are able to bind to the antigen creating an antigen-antibody complex which makes it more susceptible to phagocytosis An antibody is a y shaped protein molecule where each arm has a receptor binding site specific to a particular antigen.

Antibody action The binding of the antibodies causes the inactivation of the pathogen (or the toxin it produces) and pathogen to become more susceptible to phagocytosis. In some cases the formation of the complex itself stimulates the activation of proteins that bring about lysis of the pathogen.

Production of antibodies-like a protein factory. 1.Each clone of B cells produce one type of antibody. 2.These antibodies will be specific to one type of surface antigen molecule on the surface of a pathogenic cell or toxin. 3.Each B cell produces about 2000 antibody molecules per second during its 4-5 day life span. 4.Once released into the blood and lymph system the antibodies are transported around the body and head to the infected area.

Immunological memory When a pathogen infects the body, it takes time to raise an immune response and select the correct T and B cells (clonal selection) and produce antibodies. This is called the primary response, and very often it is too slow to prevent the pathogen causing illness. However, if the person survives, and they come across the pathogen a second time they have memory T C, T H and B cells.

Memory Cells When the foreign antigen is recognised by these memory cells, they very quickly proliferate and form many clones of both T cells and antibody- producing B cells. This means that their are more antibodies produced faster and for a longer duration. This is the secondary response.

Primary and secondary response and immunological memory. On first being exposed to a pathogen it takes a certain length of time for antibodies to it to appear in the blood stream. This is called the primary response. Sometimes there are not enough antibodies made to prevent the person from suffering the disease. If the person survives and meets this same pathogen again then the production of antibodies will be much more rapid and of a higher concentration that is maintained for a longer time. This is called the secondary response. As a result the disease is usually prevented. The secondary response is possible due to the presence of memory cells. These are B and T lymphocytes specific to the antigen and produced in response to it by clonal selection following the body’s first exposure to it. When exposed a second time the memory cells proliferate and differentiate producing clones of T cells and B cells and antibody forming B cells.

Antigen engulfed and presented on its surface Formation of clone memory T C cell Formation of cloned activated T C cells Formation of clone memory T H cell Formation of cloned activated T H cell Formation of clone memory B cells Formation of cloned antibody producing B cells T C cell activatedT H cell activatedB cell activated Produce cytokines to stimulate Infected cell destroyed Antibodies produced

Questions 7) How does a T C cell lead to the destruction of an infected cell? 8) a) What does an activated B cell produce? b) How do these molecules bring about destruction of a pathogen? 9) What term is used to describe the first infection of the body by a pathogen? 10) If the body is re-infected at a later date, what is this known as? 11) Which cells are central to being able to fight off re- infection? 12) How does the immune response to the second infection compare to the first?

Chapter 22 specific cellular defences. 1.Give another name for the third line of defence against pathogens in the body. 2.Name the two types of lymphocytes involved in this line of defence and say where each type originates from. 3.From which cells did all of these lymphocytes originally develop from? 4.What is meant by ‘immune surveillance’? 5.What is an ‘antigen’? 6.Name 5 things that can all act as antigens. 7.What is an ‘antigen receptor’? Say what is special about it using the words ‘selected’, ‘clonal population’, and ‘clonal selection’. 8.Describe why the process of ‘weeding out’ any lymphocyte with an antigen receptor that would fit a body cell surface protein is very important. 9.What is autoimmunity and name 2 diseases of this type. 10.When would the body exhibit an allergic reaction?

Chapter 22 specific cellular defences continued. 1.Name the two groups of T lymphocytes produced by the human body. 2.Describe in general how each type of T cell operates-top paragraph page What is an antigen presenting cell? Figure page 321 will help you describe it. 4.Look at figure page 321 and describe numbered stages of the activation of a Th cell. 5.Name the cells that stimulate other cells including Tc cells and B cells. 6.Look at figure page 322 and describe numbered stages of the activation of a Tc cell. 7.Look at figure page 322 and describe numbered stages of the destruction of infected cells by Tc cells. 8.Describe how other Tc cells recognise and destroy cancer cells. 9.The presence of what triggers the production of antibodies by B lymphocytes? 10.Describe an antibody and then draw and label one. 11.Describe the indirect way in which antibodies are produced by B cells and with the help of Th cell by listing the numbered stages from figure page Describe the action of antibodies by listing numbered stages from figure22.20 page 324(production of antibodies). 13.Write about immunological memory.