5.1, 5.2, 5.3 Cell recognition and the immune system

Infection is an interaction between the pathogen and the body's various defence mechanisms

  • sometimes the pathogen is overwhelmed as the individual recovers. Then the body is better prepared for a second infection - immunity

RECOGNISING YOUR OWN CELLS

Each type of cell has specific molecules or antigens on its surface that identify it. Antigens made of proteins - large variety and highly specific tertiary structure.

Identify:

  • pathogens
  • non-self material
  • cells from other organisms of the same species
  • toxins
  • abnormal body cells such as cancer

TISSUE AND ORGAN TRANSPLANTS

Transplants are recognised as non-self. Body rejects and attempts to destroy the transplant

Donor tissues for transplants are matched as closely as possible to those of the recipient.

Immunosuppressant drugs to reduce the level of the immune response.

  • there are around 10 million different lymphocytes present in the body, each with a different chemical shape.
  • lymphocytes that match the body cells are destroyed by apoptosis so they do not kill self cells - lymphocytes only respond to foreign material.
  • lymphocytes produced in the bone marrow.

PHAGOCYTOSIS

Phagocytes ingest and destroy pathogens before they can cause harm. This is if the first lines of defence (physical barriers such as skin) fail and pathogens gain entry into the body. Non-specific response
Phagocytes travel in the blood and move in and out of body tissues.

  1. Attractants - chemical products of pathogens or abnormal cells cause phagocytes to move towards the pathogen
  2. Receptors on the phagocyte cell-surface membrane recognise and attach to the pathogen.
  3. they engulf the pathogen to form a vesicle known as a phagosome
  4. lysosomes fuse with the vesicle
  5. lysozymes in the lysosome hydrolise the pathogen into small soluble molecules
  6. molecules are absorbed into the cytoplasm of the phagocyte.

CELL-MEDIATED IMMUNITY AND HUMORAL IMMUNITY

Specific immune response caused by lymphocytes.

Antigens any part of an organism or substance that is recognised by the immune system and stimulates an immune response. Usually proteins as part of the cell-surface membranes or cell wall. Triggers production of an antibody.

Lymphocytes are produced by stem cells in the bone marrow.

  • B lymphocytes: mature in the bone marrow. Humoral immunity involving antibodies.
  • T lymphocytes mature in the thymus gland. Cell mediated immunity involving body cells.

CELL MEDIATED IMMUNITY

Antigen-presenting cells present the antigen of the pathogen on their own cell surface membrane. T helper cell receptors fit onto the antigen. T helper cell activates T cell to divide rapidly. T cells develop into memory cells / or stimulate phagocytosis / stimulate B cells to divide and secrete antibodies / activate cytotoxic T cells

Cytotoxic T cells kill abnormal cells and infected cells by producing a protein (perforin) that make holes in the cell surface membrane. Cell becomes freely permeable to all substances and it dies. Most effective against viruses.

HUMORAL IMMUNITITY

When a B cell which is complementary to an antigen attaches to a pathogen, the antigen enters the B cell through endocytosis and gets presented on its surface. T helper cells bind to these antigens and stimulate B cells to divide by mitosis. These B cells become either plasma cells or memory cells. Plasma cells secrete antibodies that destroy pathogens or infected cells.
Memory cells are responsible for the secondary immune response. Live for decades to increase speed of response in the case of re infection. Provides long term immunity.

ANTIBODIES

  • synthesised by B cells
  • specific binding sites complementary to antigen
  • four polypeptide chains. 1 pair heavy chains, 1 pair light chains.
  • can cause agglutination of bacterial cells (clumps of bacteria are formed so phagocytes can find them more easily)
  • Can serve as markers to stimulate phagocytes to engulf bacterial cells

monoclonal antibodies A single type of antibody isolated and cloned outside of the body.

  • Used to target medication to specific body cells by attaching a therapeutic drug to an antibody. Used for cancer cells. Very specific so less side effects. Uses smaller doses, therefore cheaper.
  • used for medical diagnosis. Influenza, hepatitis, chlamydia. Diagnosing cancers.
  • pregnancy testing. Monoclonal antibodies on a test strip are linked to coloured particles. If hCG (pregnancy) is present in the urine, it binds to these antibodies and the complex moves along the strip until it is trapped by another antibody and creates a visible coloured line.

ETHICS:

  • production involves mice (antibodies and tumour cells)

VACCINATION

Passive immunity - antibodies introduced to a body from an outside source, no direct contact with pathogen or antigen. Immediate. No memory cells formed so no long term immunity.
Eg. when snake bite victims are given anti-venom.

Active immunity
Stimulating the production of antibodies by the individuals own immune system. Can be natural active immunity when an individual becomes infected with a disease under normal conditions, or artificial active immunity when an individual is vaccinated and immune response is induced without them suffering the symptoms of the disease.

Vaccination is the introduction of disease antigens into the body to stimulate an immune response. Memory cells are produced and remain in blood to allow a greater and faster response if the individual becomes infected with an active form of the same pathogen. Can more easily be overcome with less symptoms.

Vaccination is a precautionary measure.
A successful vaccination programme has a number of factors:

  • suitable vaccine economically available in sufficient quantities to immunise most of the population
  • few side effects
  • easy to produce, store and transport
  • appropriate means of administering the vaccine properly at the appropriate time.
  • must be possible to vaccinate the vast majority of the population to produce herd immunity.

Herd Immunity when sufficiently large proportion of the population has been vaccinated to make it difficult for a pathogen to be spread. Herd immunity is important because it is never possible to vaccinate everyone in a population eg babies and young children because their immune system is not fully functional. If the vast majority of the population is immune, it is unlikely that an unprotected individual will come into contact with an infected person. Even people who are not immune, are still protected.
A disease may not be eradicated even if herd immunity is achieved. Vaccination can not work on some people. Some people can get the disease immediately after vaccination so their immune system still cannot cope. Pathogens can mutate regularly (antigenic variability)

HIV

Human immunodeficiency virus (HIV) causes acquired immune deficiency syndrome (AIDS). Can lay dormant in the body for years.

  • Structure:
  • lipid envelope
  • with attachment proteins
  • inside is a protein layer called a capsid
  • inside that is two single strands of RNA and some enzymes (including reverse transcriptase)

REPLICATION
Uses genetic material to instruct a host cell to produce components to make a new HIV.

  • enters blood
  • proteins on HIV bind to protein called CD4 in T helper cells.
  • protein capsid fuses with cell-surface membrane and RNA and enzymes enter the helper T cell.
  • HIV reverse transcriptase converts the virus's RNA into DNA.
  • DNA moves into helper T cell's nucleus, inserted into the cells DNA.
  • HIV DNA creates messenger RNA using the cell's enzymes. Instructions for new viral protiens.
  • mRNA passes out of the nucleus. Protein synthesis creates HIV particles.
  • HIV particles break away from the helper T cell using its cell-surface membrane as a new lipid envelope.

As it attacks helper T cells, causes AIDS by interfering with the normal immune response. Reduces amount of helper T cells. Body is unable to produce a sufficient immune response to infections and cancer. These secondary diseases ultimately cause death.

ELISA test

  • enzyme linked absorbent assay.
  • antibodies to detect presence of a protein in a sample, and the quantity.
  1. sample applied to surface. Antigens will attach.
  2. wash the surface to remove unattached antigens.
  3. add specific antibody, leave to bind to antigens.
  4. wash to remove excess antibody
  5. add second antibody that binds to first antibody (with an enzyme attached)
    6 add colourless substrate of the enzyme. Let enzyme act to change substrate into coloured product.
  6. the amount of antigen present is relative to the intensity of the colour that develops.