Cell recognition and immune response

What are antigens?

Antigens are specific molecules, usually glycoproteins or proteins, found on the cell surface membrane, which triggers an immune response if identified as non-self or foreign

What is antigen variability and why is it bad?

• Pathogenic DNA can randomly mutate

• If the mutation affects the gene coding for the antigen, the antigen changes

• Existing lymphocytes cannot bind because they are no longer complementary

• Memory cells recognise the old antigen, so previous immunity is lost or ineffective as there is no secondary immune response

Describe the process of phagocytosis

• Chemo-taxis: Chemicals released by pathogens or damaged cells attract phagocytes to the infection site.

• Attachment and recognition: Phagocytes detect non-self antigens on the pathogen using receptor proteins on their surface.

• Engulfment: The phagocyte membrane extends around the pathogen, engulfing it via endocytosis to form a phagosome.

• Phagolysosome formation: The phagosome fuses with a lysosome, forming a phagolysosome, where enzymes hydrolyse the pathogen into soluble components.

• Antigen presentation: The phagocyte presents the pathogen’s antigens on its surface to activate the specific immune response.

Describe the cellular response

• T lymphocytes recognise APCs after phagocytosis because they have a foreign antigen

• Specific T helper cells with receptor molecules complementary to specific antigens on antigen-presenting cells bind to the antigens

• This activates specific T helper cells and causes them to divide rapidly by mitosis to form clones

What do the clones produced from T-helper cells do?

Differentiate into:

• B cells (start the humoral response)

• Cytotoxic T cells (T killer cells)

• Stimulate pathogens to be engulfed more by phagocytosis

• Produces more T-lymphocytes

• Memory T cells

Describe the humoral response

• B cells binds to the antigen presenting cell or free antigen which is complementary to its receptor protein

• It is stimulated by the binding of the T helper cell which releases cytokines that activates the cell

• The activated B cell divides by mitosis → produces a clone of identical B cells

• Some become plasma cells, some become memory B cells.

• The plasma cells produce antibodies

• The memory cells remain in the blood → and provide rapid, stronger secondary responses if the same antigen is encountered again

How do cytotoxic T-cells work?

• Bind to specific infected or abnormal cells displaying a specific complementary antigen to its receptors

• Release perforin, which forms pores in the cell membrane

• Membrane becomes freely permeable, therefore it is unable to regulate the movement of substances in and out of the cell

• Leads to cell lysis and so it dies

Describe the primary immune response

• First exposure to a pathogen with a specific antigen.

• Slow response as few B and T cells are complementary to the antigen.

• Clonal selection and expansion occurs → plasma cells produce antibodies; T cells activated.

• Lower antibody concentration produced over a longer time.

• Memory T cells are formed for future infections.

Describe the secondary immune response

• Second or subsequent exposure to the same antigen.

• Rapid response because memory B and T cells are already present.

• Clonal selection occurs faster, producing many plasma cells quickly.

• Higher antibody concentration is produced more quickly.

• Cytotoxic T cells are stimulated faster and respond faster

• Memory cells are maintained for future infections

• Pathogen is often destroyed before symptoms appear, giving effective immunity.

Define antibodies

Globular proteins with a quaternary structure secreted by plasma cells in response to a specific antigen

What are structures present in antibodies?

• Variable region

• Constant region

• Hinge region

• Heavy and light chain

• Disulfide bridges

What is the structure of the variable region

• Variable regions are the specific amino acid terminals at the ends of both heavy and light chains

• This region has a specific tertiary structure that differs in amino acid sequence between other variable regions on other antibodies

• Hence, a single antibody is complementary to a specific antigen only to form an antigen-antibody complex

What is the structure of the constant region

• Region of the heavy chain with the same amino acid sequence within an antibody class

• Binds to receptors on phagocytes, activating phagocytosis (increasing the rate of phagocytosis)

• Provides stability and forms the stem

What is the structure of the hinge region

• Flexible region which allows the 2 antigen binding sites to move independently

• Allows binding to two antigens simultaneously, even if spaced differently

What is the structure of the heavy chain

• Polypeptide chains forming the central stem and part of the arms

• Contribute to constant and variable regions.

• Determine antibody class.

• Provide structural stability to the antibody

What is the structure of the light chain

• Polypeptide chains in the arms of the antibody.

• Part of the antigen-binding site

• Hence contribute to the variable region hence help determine antigen specificity

What is the function of the disulfide bridges

• Covalent bonds between sulfur atoms in cysteine amino acids in polypeptide chains.

• Hold heavy and light chains together.

• Maintain overall antibody shape while allowing hinge flexibility

What are the functions of antibodies

• Neutralisation

• Agglutination

• Opsonisation / Phagocyte activation

Describe how antibodies carry out neutralisation

• Antibodies bind specifically to antigens on pathogens or toxins using their variable regions

• This blocks the pathogen or toxin from attaching to receptors on host cells, preventing entry into cells

• Therefore preventing infection or damage.

Describe how antibodies carry out agglutination

• Antibodies have two antigen-binding sites, so one antibody can bind to two antigens at the same time

• This links multiple pathogens together, causing them to clump (agglutinate)

• Clumping reduces their mobility and makes them easier for phagocytes to engulf and destroy

Describe how antibodies carry out phagocyte activation

• The constant region binds to receptors on the surface of phagocytes

• This makes the pathogen easier to recognise and triggers the phagocyte to engulf and destroy it by phagocytosis

• This increases the rate of phagocytosis

Describe the concept of herd immunity

• High proportion of population immune.

• Fewer susceptible hosts available.

• Reduced transmission of pathogen.

• Protects vulnerable or non-immune individuals.

What is the structure of HIV

• Lipid envelope derived from host cell membrane

• Attachment proteins embedded in envelope which bind to CD4 receptors on T helper cells

• Protein capsid inside envelope

• Contains two single strands of RNA

• Contains enzymes like reverse transcriptase

How does HIV replicate?

• HIV is carried in the blood until it reaches a T helper cell.

• The HIV attachment protein binds to the CD4 receptor on the cell surface membrane on a helper T cell

• The viral envelope fuses with the host cell membrane, and the capsid is released and uncoats, releasing viral RNA and reverse transcriptase into the cytoplasm

• Reverse transcriptase converts RNA into DNA, which enters the nucleus and integrates into the host DNA

• The viral DNA can remain dormant

• The host cell produces viral RNA and proteins, which assemble into new HIV particles

• The virus buds from the cell, taking part of the host cell membrane to form its lipid envelope, causing the cell to die

• It then goes and infects other helper T cells

How is AIDS formed?

• HIV infects T helper cells by binding to CD4 receptors

• The virus replicates inside these cells and destroys them when new viruses bud off

• As T helper cells are lost, the immune system becomes weakened because fewer B cells and cytotoxic T cells are activated

• This reduces the body’s ability to fight infections and cancers

• The progressive loss of immune function leads to the symptoms of AIDS, including frequent infections and opportunistic diseases

What are monoclonal antibodies?

Monoclonal antibodies are antibodies produced from a single group of genetically identical clones of B cells/plasma cell

Why are monoclonal antibodies specific to one molecule or antigen

• They bind only to a specific complementary antigen or target molecule

• This is because they have a binding site/variable region with a specific tertiary structure

• Only a complementary antigen will bind to form an antigen-antibody complex with this tertiary structure

• So monoclonal antibodies will only recognise and bind to that one specific antigen

• This makes them highly specific for diagnostic or therapeutic use

How are monoclonal antibodies used in cancer cell treatment?

• Cancer cells have a specific antigen called tumor markers which binds a chemical called growth factor (which stimulates it to grow and divide)

• Antibodies can be produced which have a specific tertiary structure complementary to the tumor markers on cancer cell surface

• Hence it can bind to the tumor markers form an antibody-antigen complex and prevents the growth factor from reaching the cell (blocking the growth factor) hence prevents uncontrolled growth division.

• A cytotoxic drug can be attached to the monoclonal antibody which will deliver the drug directly to the cancer cell

• This is because the monoclonal antibody is complementary to receptor proteins on the cancer cell surface

• So it will accumulate where it is needed and doesn't damage normal body cells.

How are monoclonal antibodies used in pregnancy testing?

• Pregnancy tests detect the hormone hCG which is excreted in their urine

• Monoclonal antibodies can be made which have a specific tertiary structure with an antigen binding site complementary to the hCG hormone

• The application area contains mobile monoclonal antibodies that are attached to fluorescent dyes or beads, which bind to hCG to form hCG antibody complexes

• The urine and antibodies move up the test strip carrying any complexes present by capillary action to the test region

• The test region contains immobilised antibodies that are also complementary to hCG, which bind to hCG as well and trap the hCG antibody complexes causing the coloured dye/beads to accumulate in an area

• This creates a coloured line that indicates pregnancy

• Further up the strip is a control test, which traps the mobile antibodies that are not bound to hCG

• Hence a second line is formed, showing test is working regardless of whether hCG is present

How are monoclonal antibodies used to locate tumors or blood clots?

• Specific antigens are present only on tumour cells or blood clots.

• Monoclonal antibodies are produced with a tertiary structure complementary to these antigens

• The antibodies are labelled with a fluorescent dye or radioactive isotope and injected into the bloodstream

• They accumulate at sites with high antigen concentration, and a specialised scanner detects the dye or radiation

• This allows doctors to determine the size and location of the tumour or clot

Describe the ELISA test for antigens

• HIV antigens are bound to the bottom of the reaction vessel

• A sample of the patient's blood is added to the bottom of the reaction vessel

• Any HIV-specific antibodies that are present in the blood plasma bind to the HIV antigens stuck at the bottom of the reaction vessel

• These HIV antibodies are called primary antibodies

• The well is now washed to remove any unbound antibodies

• A secondary antibody is added to the reaction vessel with an enzyme attached

• The secondary antibodies bind to the primary antibodies. The well is washed out again to remove any unbound secondary antibodies

• A solution is added which contains a substrate that reacts with the enzyme attached to the secondary antibodies

• If there are any secondary antibodies present, a coloured precipitate is formed, causing the reaction vessel to change colour.

• This indicates that the patient has HIV-specific antibodies in their blood, hence are infected with HIV.

Describe the ELISA test for antibodies

• Antigen-specific antibodies are attached to the bottom of the reaction vessel.

• Patient blood is added and any complementary antigens bind to the immobilised antibodies, forming antigen–antibody complexes.

• The well is washed to remove unbound substances, preventing false positives.

• An enzyme-linked secondary antibody, specific to the antigen–antibody complex, is added and binds to the complex.

• The well is washed again to remove unbound secondary antibodies.

• A colourless substrate is added; if the enzyme is present, it reacts to produce a coloured product, indicating the antigen is present