T and B lymphocytes

Question 1

What is an antigen?

Answer 1

An antigen is any organism or substance that is recognised as non-self by the immune system and stimulates an immune response

Question 2

What is an antigen presenting cell?

Answer 2

• A host cell that has been invaded by a pathogen and displays its antigen on its cell surface membrane

Question 3

Where do T lymphocytes mature?

Answer 3

• T lymphocytes mature in the thymus gland and carry out cell-mediated immunity

Question 4

What are the two most common antigen presenting cells?

Answer 4

• Macrophages
• Dendritic cells

Question 5

What is the role of antigen presenting cells?

Answer 5

1. Receptors on specific helper T cells fit exactly on to antigens on an antigen presenting cell
2. The helper T cell then activates a cell mediated response and the T cell divides rapidly by mitosis producing clones with genetically identical antigen receptors
3. The cloned cells then develop into:
   - memory cells
   - Cells that stimulate phagocytes to engulf pathogens
   - Cells that stimulate B cells to divide
   - Cells that activate cytoxic T cels

Question 6

What is a cytoxic T cell?

Answer 6

• Cytoxic T cells kill abnormal cells by producing perforin which makes holes in cell membranes, killing pathogens

Question 7

What is a B lymphocyte?

Answer 7

• They carry out humoral immunity which involves antibodies that are souble in the humour of our body

Question 8

Describe the process of humoral immunity

Answer 8

• the surface antigens of an invading pathogen are taken up by a B cell
• the B cell processes the antigens and presents it on its surface
• Helper T cells attach to the antigens and activate the B cell to divide by mitosis
• This produces cloned plasma cells which secrete the specific antibody to the antigen on the pathogens surface
• the antibody attaches to antigens on the pathogen and destroys them
• some B cells develop into memory cells, these can respond rapidy to infections by the same pathogen by dividing rapidly into plasma cells that produce antibodies. This is a secondary immune response

Question 9

What is the role of a memory cell?

Answer 9

• memory cells carry out secondary immune responses and when they circulate the blood and tissue fluid and locate the same antigen, they rapidly divide into plasma cells and more memory cells
• this therefore provides long term immunity
• memory cells are also a secondary immune response
• the clones produced by a memory cell dividing are referred to as monoclonal antibodies

Question 10

What is the role of plasma cells?

Answer 10

• plasma cells secrete antibodies into blood plasma
• these antibodies only last a few days and are produced rapidly
• these are primary immune responses