Biology Unit 1 AQA Immunity

Immunity - disease is the interaction between a pathogen & the body, sometimes the pathogen overwhelms the body & the person dies, sometimes the bodies defences overwhelm the pathogen & they recover, the bodies defences are then better prepared for a second infection.

Defence Mechanisms:

Non-specific - do not distungish between one type of pathogen and another, but respond to all in the same way.

• a barrier to the entry of the pathogen
• phagocytosis

Specific - distungish between different pathogens, less rapid but provide long lasting immunity. Responses involve white blood cell lymphocyte:

• Cell mediated response, T lymphocytes
• Humoral responses, B lymphocytes

Non-Specific -

Barriers to entry

• Protective covering, the skin provides a barrier.
• Epithelia covered in mucus.
• Hydrochloric acid in the stomach, low pH denatures the enzymes of the pathogens.

Phagocytosis

• Chemicals on the pathogen attract the phagocyte towards it.
• Phagocyte attaches itself to the surface of the pathogen.
• Phagocyte engulfs the pathogen, forming a phagosome.
• Lysosomes within the phagocyte migrate towards the phagosome.
• Lysosomes release their lytic enzymes, and break down the pathogen.
• The broken down products are absorbed by the phagocyte.

Specific -

Non-specific is the initial response, the second response determines immunity.

T Lymphocytes - Cell Mediated Immunity

• Formed from the stem cells in the bone marrow, mature in the thymus gland.
• Respond to own cells that have been invaded by foreign material (cancer cells, virus or transplanted tissues) not cells in body fluids.

They can distinguish these cells because: phagocytes that have engulfed a pathogen present the pathogens antigens on their cell surface membrane, body cells invaded by a virus or cancer cells can also present viral antigens. Called antigen presenting cells.

• Pathogens are taken in by phagocytes and present their antigens on the cell surface membrane.
• Receptors on the T helper cells fit onto these antigens.
• Other T cells divide and clone.
• These T cells: develop into memory cells - rapid response to future infections. Stimulate phagocytes to engulf pathogens. Stimulate B cells to divide. Kill infected cells.

B lymphocytes - humoral immunity

Involves antibodies, these are soluble in the body fluids. The B cells will have a specific antibody on its surface that is complementary to a certain antigen.

• Surface antigens take up the antigens of pathogen
• The B cells present the antigens on their surface
• T helper cells attach to the antigens, and activate them
• The B cells divide into plasma cells or memory cells

Plasma cells - produce antibodies that fit the antigens on the pathogen and destroy the pathogen. Primary immune response. Memory cells - respond to future infections by the same pathogen by dividing rapidly into plasma cells and memory cells. Secondary immune response. Antigenic Variability - some antigens change on the viruses, therefore their antigens will not correspond to the antibodies or the memory cells, memory cells cannot stimulate plasma cells to produce the right antibodies therefore primary immune response will have to take place - very slow.

Antibodies - made from proteins, four polypeptide chains.

• Heavy chains - long
• Light chains - short

They can change shape to fit around the antigen. Antigen-antibody complex, the binding site is different on each antibody - variable region. The rest of the antibody is the same - constant region - binds to the B cells.

Monoclonal antibodies

Each antigen will induce a different B cell to divide, each of these clones will produce a different antibody - polyclonal antibodies.

Medical value to produce antibodies outside the body, a single type of antibody being cloned is a monoclonal antibody. These can help science:

• separation of chemicals from mixtures
• immunoassay - calculating amount of substance in a mixture - pregnancy tests
• cancer treatment - monoclonal antibodies can be made to attach onto cancer cells - destroyed
• transplant surgery - usually rejected due to the T cells action, but monoclonal antibodies can knock out these T cells.

Ethical Issues related to monoclonal antibodies -

• involves the use of mice - induces cancer in the mice deliberately.
• monoclonal antibodies have helped people, but some have also died.