CELLS: The Immune System Flashcards
What are antigens?
Molecules that can generate an immune response when detected by the body.
They are usually proteins.
Where are antigens usually found?
What can they be used to identify?
Usually found on the surface of a cell.
Used by immune system to identify:
- Pathogens
- Abnormal body cells
- Toxins
- Individuals of the same species (organ transplants)
What are pathogens?
Microorganisms that cause disease.
What is a phagocyte?
A type of white blood cell that carries out phagocytosis.
They are the first cell to respond to an immune system trigger inside the body.
Some travel in the blood but can move out of the blood vessels into other tissues.
What is phagocytosis?
The process of pathogens being engulfed by phagocytes in the immune system.
Describe the process of phagocytosis:
- Chemical products of pathogens act as attractants, causing phagocytes to move towards the pathogen.
- Receptors on the phagocyte recognise and attach to the antigens on a pathogen.
- Cytoplasm of the phagocyte moves round the pathogen, engulfing it.
- Pathogen is now contained in phagocytic vacuole/phagosome in the cytoplasm of the phagocyte.
- Lysosome fuses with the phagocytic vacuole.
- Released lysozymes destroy ingested bacteria by hydrolysis of their cell walls.
- Soluble products of pathogen are absorbed into the cytoplasm.
- Phagocyte then becomes antigen-presenting activating other immune system cells.
The presence of an antigen does what?
Triggers the production of an antibody as part of the body’s defence system.
What type of immune responses can the body have?
Give examples.
- Non-specific, eg. phagocytosis which occurs whatever the infection.
- Specific, eg cell-mediated and humoral immunity which uses specific lymphocytes.
What is a benefit of specific immune responses?
Whilst they can be slower in action at first, they often provide long-term immunity.
What are lymphocytes?
White blood cells, used in specific immune resonses, that are produced by stem cells in the bone marrow.
What are the two types of lymphocytes?
- B lymphocytes - mature in the bone marrow.
- T lymphocytes - mature in the thymus gland (in the chest, between the lungs).
What type of immunity are T cells associated with?
Cell-mediated immunity.
What type of immunity are B cells associated with?
Humoral immunity.
What are the different types of T cells?
What is the function of each?
- Helper T-cells - release chemical signals that activate and stimulate phagocytes. Also activate B-cells which secrete antibodies.
- Cytotoxic T-cells - kill abnormal and foreign cells.
How does a T cell become activated?
Receptor proteins on its surface bind to complementary antigens presented by phagocytes.
Receptor proteins are only specific to one type of antigen and therefore there is a vast number of different types of T cells.
Why is the work done by T cells known as cell-mediated immunity?
Because T lymphocytes will only respond to antigens that are presented on a body cell, not just antigens within the body fluids.
Describe cell-mediated immunity:
- Pathogens invade body cells or are taken in by phagocytes.
- Phagocyte becomes antigen-presenting.
- Receptors on a specific helper-T cell are complementary to this antigen.
- This attachment activates the T cell to divide by mitosis and form a clone of genetically identical cells.
- Cloned T cells:
- Develop into memory cells
- Stimulate phagocytes to engulf pathogens by phagocytosis
- Stimulate B cells to divide and secrete antibodies
- Activate cytotoxic T cells, which kill the pathogen.
How to cytotoxic T cells kill infected cells?
- Produce a protein called perforin that makes holes in the cell-surface membrane.
- Holes mean the cell membrane becomes freely permeable to all substances.
- The cell dies.
What are B cells?
White blood cells that are covered with specific antibodies
Describe the process of humoral immunity:
- Surface antigens of an invading pathogen are taken up by a B cell.
- B cell processes the antigens and becomes antigen-presenting.
- Helper T cells attach to processed antigens on B cell and activate the B cell - this is known as clonal selection.
- B cell is activated to divide by mitosis to give a clone of plasma cells.
- Cloned plasma cells produce and secrete specific antibodies.
- Antibody attaches to antigens on pathogen and destroys them.
Some B cells develop into memory cells, which respond to future infections by the same pathogen.