Cell recognition and the immune system

• Molecules lymphocytes identify


  - Pathogens (bacteria, fungi, viruses)

  - cells from other organisms of same species (transplants)

  - abnormal body cells (tumour cells)

  - toxins (released from bacteria
• Antigens


  - Proteins on the cell-surface membrane

  - trigger an immune response when detected by lymphocytes
• Antigentic variability


  - When pathogenic DNA mutates causing a change in shape of antigen

  - previous immunity is no longer effective as memory cells don't recognise new shape of antigen

  - specific antibody no longer binds to new antigen
• Physical barriers


  Anatomical barriers to pathogens

  - skin

  - stomach acid

  - lysozymes in tears
• Phagocytes


  - Non-specific immune response

  - phagocytes become antigen presenting cells after destroying pathogen
• T lymphocytes


  - Made in bone marrow and mature in thymus gland

  - involved in cell-mediated immune response

  - respond to antigen-presenting cell
• Antigen-presenting cells


  Any cell that presents a non-self antigen on their surface

  - infected body cells

  - macrophage after phagocytosis

  - cells of transplanted organ

  - cancer cells
• Role of T helper cells


  - Have receptors on their surface that attach to antigens on APCs

  - become activated - clonal selection
• Role of cloned T helper cells


  - Some remain as helper T cells & activate B lymphocytes

  - stimulate macrophages for phagocytosis

  - become memory cells for that shaped antigen

  - become cytotoxic killer T cells
• Cy0toxic T cells


  - Destroy abnormal / infected cells by releasing perforin

  - so that any substances can enter or leave the cell and this causes cell death
• B lymphocytes


  - Made in bone marrow and mature in bone marrow

  - involved in humoral immune response

  - involves antibodies
• Humoral response


  - Antigen Presenting Cells activates B cell

  - B cell undergoes clonal selection and expansion - - rapid division by mitosis.

  - differentiate into plasma cells / memory B cells

  - plasma cells make antibodies
• B memory cells


  - derived from B lymphocytes

  - remember specific antibody for particular antigen - will rapidly divide by mitosis and differentiate in plasma cells upon secondary encounter

  - resulting in large numbers of antibodies rapidly
• Antibodies


  - Quaternary structure proteins made of four polypeptide chains

  - different shaped binding site = variable region

  - complementary to a specific antigen
• Antibody structure


  - Two parallel pairs of polypeptide chains

  - One pair of heavy chains

  - One pair of light chains

  - Each chain contains:

  - Constant segments

  - Variable segments

  
• Agglutination


  - Antibodies have two binding sites and are flexible - clumps pathogens together

  - it easier for phagocytes to locate and destroy pathogen
• Passive immunity


  - Antibodies introduced into body

  - plasma and memory cells not made as no interaction with antigen

  - short-term immunity

  - fast acting
• Active immunity


  - Immunity created by own immune system - antibodies made

  - exposure to antigen

  - plasma and memory cells made

  - long term immunity

  - slower acting
• Natural active immunity


  - After direct contact with pathogen through infection

  - body creates antibodies and memory cells
• Artificial active immunity


  - Creation of antibodies and memory cells following introduction of an attenuated pathogen or antigens

  - vaccination
• Vaccinations


  - Small amounts of dead or attenuated pathogens injected / ingested

  - humoral response activated

  - memory cells are able to divide rapidly into plasma cells when re-infected
• Primary vs secondary response


  - Primary = first exposure to the pathogen

  - longer time for plasma cell secretion & memory cell production

  - for the secondary response, memory cells divide rapidly into plasma cells

  - so a large number of antibodies made rapidly upon reinfection
• Herd immunity


  - When enough of the population is vaccinated so pathogen is not transmitted and spread easily

  - provides protection for those without vaccine
• Monoclonal antibodies


  - A single type of antibody that can be isolated and cloned

  - antibodies that are identical - from one type of B lymphocyte

  - complementary to only one antigen
• Uses of monoclonal antibodies


  - Medical treatment - targeting drugs by attaching antibody complementary to tumour cell antigen

  - medical diagnosis - pregnancy tests
• Pregnancy test


  - ELISA test which uses 3 monoclonal antibodies and enzymes to test for hCG
• Purpose of ELISA test


  - Detect the presence and quantity of an antigen

  - used for medical diagnosis. Eg., HIV
• Ethical issues with monoclonal antibodies


  - Requires mice to produce antibodies and tumour cells

  - requires a full cost-benefit analysis
• HIV structure


  - Core = RNA and reverse transcriptase

  - capsid = protein coat

  - lipid envelope taken from hosts cell membrane

  - attachment proteins so it can attach to Helper T cells
• HIV replication


  - Attaches to CD4 receptor on helper T cells

  - protein fuses with membrane allowing RNA + enzymes to enter

  - reverse transcriptase makes DNA copy and this is inserted into nucleus

  - nucleus synthesises viral proteins
• Auto Immunodeficiency Syndrome (AIDs)


  - When HIV has destroyed too many T helper cells, host is unable to produce adequate immune response to other pathogens

  - host susceptible to opportunistic infections
• Role of antibodies in ELISA


  - First antibody added is complementary to antigen in well - attaches

  - second antibody with enzyme added which attaches to first antibody as complementary.

  - when substrate solution added enzyme can produce colour change
• Why vaccines may be unsafe?

  - Inactive virus may become active - viral transformation

  - non-pathogenic virus can mutate and harm cells

  - side effects of immune response

  - people may test positive for disease