Cells (Immunity) - Adaptive Immune Response Flashcards
What is the difference between T and B cells?
T-cells respond to antigens presented on the surface of our own cells (antigen presenting cells). They have receptor proteins that can detect antigens and form antigen-receptor complexes.
B-cells respond to free antigens found in our blood plasma and tissue fluid. They display and secrete antibodies that can detect antigens and form antigen-antibody complexes.
What is the humoral response?
Produces antibodies that are complementary to the antigen. They destroy the pathogen and their toxins. Called ‘humoral’ because it is triggered by antigens in the body’s ‘humour’ (body fluids).
What are the phases of the immune response?
physical barrier -> phagocytosis -> cell-mediated immunity -> humoral immunity
What is immunity?
Immunity is the ability of organisms to resist infection by protecting against disease-causing microorganisms or their toxins that invade their bodies due to the presence of antibodies. It involved the recognition of foreign material (antigens).
What is an antigen?
An antigen is any part of an organisms or substance that is recognised as non-self (foreign) by the immune system and stimulates an immune response. Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms, or abnormal body cells, such as cancer cells. The presence of an antigen triggers the production of an antibody as part of the body’s defence system. The antibodies are specific to the antigen and react with it to make it harmless.
What is the difference between the specific and non-specific responses?
Immune responses such as phagocytosis are non-specific and occur whatever the infection. The body also has specific responses that react to specific antigens. These are slower in action at first, but they can provide long-term immunity.
What does the specific immune response depend on?
The specific immune response depends on a type of white blood cell called a lymphocyte. Lymphocytes are produced by stem cells in the bone marrow. There are two types of lymphocytes, each with its own role in the immune response.
What are B-lymphocytes (B cells)?
B lymphocytes (B cells) are so called because they originate and mature in the bone marrow. They are associated with humoral immunity, that is, immunity involving antibodies that are present in body fluids, or ‘humour’ such as blood plasma.
What are T-lymphocytes (T cells)?
T lymphocytes (T cells) are so called because they originate in bone marrow and mature in the thymus gland. They are associated with cell-mediated immunity, that is immunity involving body cells.
What do lymphocytes respond to?
Lymphocytes respond to an organism’s own cells that have been infected by non-self material from a different species, e.g. a virus. They also respond to cells from other individuals of the same species because these are genetically different. These therefore have different antigens on their cell-surface membrane from the antigens on the organism’s own cells.
How do T lymphocytes distinguish invader cells from normal cells?
- Phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogen’s antigens on their own cell-surface membrane.
- Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane.
- Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane.
- Cancer cells are different from normal body cells and present antigens on their cell-surface membranes.
What are antigen presenting cells?
Cells that display foreign antigens on their surface are called antigen-presenting cells because they can present antigens of other cells on their own cell-surface membrane.
What are the stages in the response of T lymphocytes to infection by a pathogen?
- Pathogens invade body cells or are taken in by phagocytes.
- The phagocyte acts as an APC and places antigens from the pathogen on its cell-surface membrane.
- Helper T cells are activated by antigen presenting cells. Receptors on a specific helper T cell fit exactly onto these antigens.
- This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.
- The cloned T cells:
- develop into memory cells that circulates in the blood and tissue fluid in readiness to respond to future infections by the same pathogen (provide long-term immunity)
- stimulate phagocytes to engulf pathogens by phagocytosis
- stimulate B cells to divide and secrete their antibody specific to the pathogen
- activate cytotoxic T cells to kill infected cells by perforin making holes in their cell-surface membranes
Helper T cells also produce cytokines that increase proliferation of these cells, and produce interferons which increase the immune response.
How do cytotoxic T cells kill infected cells?
- Cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens, by causing apoptosis.
- They produce a protein called perforin that binds to the cell-surface membrane of the target cell and perforates the membrane.
- These holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result.
- Before apoptosis, interferons are released. Interferons inhibit viral replication in the surrounding area, and increase the anti-viral defence mechanisms of other body cells.
- Many viruses are released during target cell apoptosis. Upon recognising a particular virus, the antibodies bind to it, and opsinate it. Opsination of a virus covers its attachment proteins which prevent it from binding to and infecting other cells.
How do B cells divide?
The first phase of the specific response to infection is the mitotic division of specific T cells to form a clone of the relevant T cells to build up their numbers. Some of these T cells stimulate B cells to divide. It is the B cells that are involved in the next phase of the immune response: humoral immunity.
Why is it called humoral immunity?
Humoral immunity is so called because it involves antibodies, and antibodies are soluble in the blood and tissue fluid of the body. An old-fashioned word for body fluids is ‘humour’.
How many types of B cells are there?
There are many different types of B cell, possibly as many as ten million, and each B cells starts to produce a specific antibody that responds to one specific antigen. When an antigen, for example, a protein on the surface of a pathogen, foreign cell, toxin, damaged or abnormal cell, enters the blood or tissue fluid, there will be one B cell that has an antibody on its surface whose shape exactly fits the antigen. The antibody therefore attaches to this complementary antigen.
How does the antigen enter the B cell?
The antigen enters the B cell by endocytosis and gets presented on its surface (processed).
What is clonal selection?
T cells bind to the processed antigens and stimulates the B cell to divide by mitosis to form a clone of identical B cells, all of which produce the antibody that is specific to the foreign antigen. This is called clonal selection and accounts for the body’s ability to respond rapidly to any of a vast number of antigens.
What are monoclonal antibodies?
In practice, a typical pathogen has many different proteins on its surface, all of which act as antigens. Some pathogens, such as the bacterium that causes cholera, also produce toxins. Each toxin molecule also acts as an antigen. Therefore many different B cells make clones, each of which produces its own type of antibody. As each clone produces one specific antibody, these antibodies are referred to as monoclonal antibodies. In each clone, the cells produced develop into one of two types of cell.
Are B cells produced in response to a pathogen?
B cells with the appropriate antibody to bind to antigens of a pathogen are not produced in response to the pathogen. They are present from birth. Being present, they simply multiply in response to the pathogen.
What are plasma cells?
Plasma cells secrete antibodies usually into blood plasma. These cells survive for only a few days, but each can make around 2000 antibodies every second during its brief lifespan. These antibodies lead to the destruction of the antigen. The plasma cells are therefore responsible for the immediate defence of the body against infection. The production of antibodies is known as the primary immune response.
What are memory cells?
Memory cells are responsible for the secondary immune response. Memory cells live considerably longer than plasma cells, often for decades. These cells do not produce antibodies directly, but circulate in the blood and tissue fluid. When they encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells. The plasma cells produce the antibodies needed to destroy the pathogen, while the new memory cells circulate in readiness for any future infection. In this way, memory cells provide long-term immunity against the original infection. An increased quantity of antibodies is secreted at a faster rate than in the primary immune response. It ensures that a new infection is destroyed before it can cause any harm - and individuals are often totally unaware that they have ever been infected.
Explain the role of B cells in immunity.
B cells are specific against a particular antigen and pathogen. Similarly, a particular B cell will be activated by a particular helper T Cell.
- The surface antigens of an invading pathogen are taken up by a B cell.
- The B cell processes the antigens and presents them on its surface.
- Helper T cells attach to the processed antigens on the B cell thereby activating the B cell.
- The B cell is now activated to proliferate via mitosis. They differentiate to give a clone of plasma cells.
- The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface.
- The antibody attaches to antigens on the pathogen and destroys them.
- Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response.
What is the structure of antibodies?
Composed of four polypeptide chains:
- 2 heavy chains
- 2 light chains
- joined by disulphide bonds
Form a Y-shaped structure:
- the stem = constant region
- the ends of the arms = variable regions that bind to the antigen
What does the variable region do?
The variable region binds to the antigen forming an antibody-antigen complex. Cloned B cells will only make antibodies with the same shape variable region - they are genetically identical.
