Antibodies

Define antibodies

Proteins with specific binding sites complementary to antigens of non-self material. They bind to these antigens, helping immune system destroy the pathogen. Secreted by plasma cells.

Explain why antibodies are specific

• Antibodies are proteins, so have a specific primary structure.

‣ Therefore have a specific tertiary structure.

‣ So antigen-binding site is only complementary to one antigen.

‣ Antibody can only form antigen-antibody complex with one specific antigen.

Describe structure of antibodies

• Antibodies are made up of 4 polypeptide chains - chains of 1 pair are long and called ‘heavy chains’ whereas chains of other pair are short & known as ‘light chains’

• Rest of antibody known as constant region which binds to receptors on cells e.g B-lymphocytes

• Each antibody has two identical complementary binding sites

  

How do antibodies work to destroy pathogens?

• Each antibody has two antigen binding sites.

‣ So each antibody binds two pathogens at a time.

‣ Results in agglutination of pathogens.

‣ Increases efficiency of phagocytosis by allowing multiple pathogens to be phagocytosed at once.

Define monoclonal antibodies

• ‣ Antibodies with the same tertiary structure

  produced from cloned B cells.

Monoclonal antibodies: ethical issues

• Use of animals to produce the cells which produce the monoclonal antibodies.

‣ Risk of serious side effects when using monoclonal antibodies to treat disease.

Role of antibodies in an ELISA test

• First antibody binds to complementary antigen.

• Well is washed to remove unbound antibody.

• Second antibody with an enzyme attached is added.

• Second antibody binds to first antibody.

• Well is washed to remove unbound second antibody (important to prevent a FALSE POSITIVE).

• Substrate solution added and colour changes to indicate a positive result.

The above method, which uses two antibodies, is an indirect ELISA.

A direct ELISA test only uses one antibody, which has the enzyme attached to it.

Uses of monoclonal antibodies in medical

treatments & diagnosis

Medical treatments:

‣ To carry a drug to a specific cell.

‣ To block a particular receptor on a cell.

‣ Medical diagnosis:

‣ Pregnancy tests.

‣ Test for a specific cancer.

‣ ELISA test (e.g. HIV test)

Describe use of monoclonal antibodies in pregnancy tests

1. Placenta produces the hormone HCG present in mother’s urine

2. Monoclonal antibodies present on test strip linked to coloured particles

3. Binds to HCG if present

4. HCG-antibody colour complex moves along strip until trapped by a different antibody, creating a coloured line

Give 3 ethical issues of monoclonal antibodies

• Production of tumour cells involves inducing cancer to mice

• Some deaths associated with use in treatment of multiple sclerosis - indicating serious side effects

• Treatments can be expensive, limiting access for some patients.

Antibodies: How is their structure related to their function?

• Variable region forms antigen-binding sites.

‣ Shape of variable region is specific to an antigen (complementary shape).

‣ Hinge region allows flexibility in binding.

‣ Constant region allows antibody to bind to receptors on cells e.g. phagocytes.

‣ Disulphide bonds hold polypeptide chains together.

Primary vs Secondary Immune Response: Graph

Primary vs Secondary Immune Response: Differences

Primary immune response

‣ Slower response due to time taken for clonal selection, clonal expansion & production of antibodies (results in a lag period).

‣ Produces memory cells.

‣ Secondary immune response

‣ Faster (steeper gradient on graph) and stronger (higher concentration of antibodies produced).

‣ Memory cells divide quickly to form plasma cells which secrete antibodies.

‣ Allows more antibodies to be produced to the antigen (and antibodies are produced more rapidly).