2.4 T Lymphocytes and cell-mediated immunity & B lymphocytes and humoral immunity Flashcards
What is immunity?
The ability of organisms to resist infection by protecting against disease-causing microorganisms or their toxins that invade their bodies. It involves the recognition of foreign material (antigens)
What is an antigen?
• Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.
• Antigens are usually glycoproteins, sometimes glycolipid or polysaccharide 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
• Immune system recognises as ‘self’ or ‘nonself’ = enables identification of cells from other organisms of same species, pathogens, toxins and abnormal body cells
What are B lymphocytes?
- They 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?
- They mature in the thymus gland
- They are associated with cell-mediated immunity, that is immunity involving body cells
What are lymphocytes produced by?
Stem cells in the bone marrow
What are non-specific and 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. This depends on a type of white blood cell called a lymphocyte
How can 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 membranes
- 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 because they can present antigens of other cells on their own cell-surface membrane
What is cell mediated immunity/cellular response?
T lymphocytes will only respond to antigens that are presented on a body cell, rather than to antigens within the body fluids
What is the role of the receptors on T cells?
- The receptors on each T cell respond to a single antigen
- It follows that there is a vast number of different types of T cell, each one responding to a different type of antigen
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 places antigens from the pathogen on its cell-surface membrane
- 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:
a. develop into memory cells that enable a rapid response to future infections by the same pathogen
b. stimulate phagocytes to engulf pathogens by phagocytosis
c. stimulate B cells to divide and secrete their antibody
d. activate cytotoxic T cells
How do cytotoxic T cells kill infected cells?
- Cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens,
- by producing a protein called perforin that makes holes in the cell-surface membrane.
- These holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result.
- This illustrates the vital importance of cell-surface membranes in maintaining the integrity of cells and hence their survival
- The action of T cells is most effective against viruses because viruses replicate inside cells. As viruses use living cells in which to replicate, this sacrifice of body cells prevents viruses multiplying and infecting more cells.
What is the role of B cells in humoral immunity?
- 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 divide by mitosis 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 humoral immunity?
It involves antibodies which are soluble in the blood and tissue fluid of the body. There are many different types of B cell, possibly as many as 10 million, and each B cell starts to produce a specific antibody that responds to one specific antigen.
What is the response of B lymphocytes to a foreign antigen?
When an antigen, for example, a protein on the surface of a pathogen, foreign cells, 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, that is, they are complementary. The antibody therefore attaches to this complementary antigen. The antigen enters the B cell by endocytosis and gets presented on its surface.
