The topic of immunity can get very complex. We are only brushing the surface and it's important you keep that in mind as we learn about defense against infectious diseases the next few days.

read pages 226-237 in your tiger book


6.3.1 Define pathogen. An organism or virus that causes a disease.

6.3.2 Explain why antibiotics are effective against bacteria but not against viruses. Antibiotics block specific metabolic pathways found in bacteria. Viruses reproduce using the host cell’s metabolic pathways, which are not affected by antibiotics.

6.3.3 Outline the role of skin and mucous membranes in defence against pathogens.
  • Skin and mucous are non-specific immunity. They work to keep out all potential threats to the body.
  • Read over the roles of the skin and mucous in your book (pg. 227)

6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.

macrophage_ingesting_bacteria_copy.jpg
phagocyte engulfing bacteria



6.3.5 Distinguish between antigens and antibodies.
  • Antigenis a substance that provokes the production of one or more antibodies:
    • An antigen is a large biological molecule (protein, glycoprotein, lipoprotein or polysaccharide) many time found on the outer surface of a cell.
    • All living cells have these antigens as part of their cell membrane or cell wall.
    • The capsid proteins of viruses and even individual molecules can also be classed as antigens.
    • Their purpose is for cell communication, and cells from different individuals have different antigens, while all the cells of the same individual have the same antigens.
    • Antigens are genetically controlled, so close relative have more similar antigens than unrelated individuals.
    • Blood groups are an example of antigens on red blood cells, but all cells have them.
    The link with infection is that when a pathogen or toxin enters the body it is the antigen the immune system reacts against.


  • Antibodies are proteins secreted from lymphocytes that destroy pathogen and antigen infections
  • ** B-cells make antibodies.
    • An antibody (also called an immunoglobulin) is a protein molecule that can bind specifically to an antigen.
    • Antibodies all have a similar structure composed of 4 polypeptide chains (2 heavy chains and 2 light chains) joined together by strong disulphide bonds to form a Y-shaped structure.
    • The ends of the arms of the Y are called the variable regions of the molecule because different antibody molecules have different amino acid structure and therefore different shape.
    • These variable regions are where the antigens bind to form a highly specific antigen-antibody complex, much like an enzyme-substrate complex.
external image imuunoglobulin.gif
    • Each B-cell has around 10^5 membrane-bound antibody molecules on its surface and can also secrete soluble antibodies into its surroundings.
    • Every human has around 10^8 different types of B cell, each making antibodies with slightly different variable regions.
    • Between them, these antibodies can therefore bind specifically to 10^8 different antigens, so there will be an antibody to match almost every conceivable antigen that might enter the body.


6.3.6 Explain antibody production.
11.1.4 Explain antibody production. Limit the explanation to antigen presentation by macrophages and activation of helper T-cells leading to activation of B-cells which divide to form clones of antibody-secreting plasma cells and memory cells.
  • read in your book and here

11.1.2 Outline the principle of challenge and response, clonal selection and memory cells as the basis of immunity. Include polyclonal response.
  • B cells make antibodies.
  • The immune system can make 10^15 different types of antibodies (but not all at once).
  • A few of each type of B cell are produced and they "wait" until the body is infected with an antigen.
  • When challenged with a specific antigen B cells will multiply to form many clones; this is called clonal selection.
  • The clones of B cells can produce large amounts of antibodies specific to the antigen quickly and give immunity to a disease only after the immune system is challenged by a disease. This is called the challenge and response system. The immune system needs to be "challenged" by a disease, usually in the form of an antigen present. The immune system responds by producing clones of B cells which produce large amounts of antibodies to fight and eliminate the pathogen.
  • A polyclonal response is when the immune system reacts to more than one part of the same antigen (called an epitope), triggering the production of more than one specific B cell with their antibodies specific to the epitopes.
  • Memory cells are B cells that are produced from the clonal or polyclonal response that stay in the immune system, ready to respond to any successive challenge of the same antigen.
11.1.3 Define active and passive immunity.
Active immunity is immunity due to the production of antibodies by the organism itself after the body’s defence mechanisms have been stimulated by antigens.
Passive immunity is immunity due to the acquisition of antibodies from another organism in which active immunity has been stimulated, including via the placenta, colostrum, or by injection of antibodies.

11.1.6 Explain the principle of vaccination.

vaccination animations



11.1.5 Describe the production of monoclonal antibodies and their use in diagnosis and in treatment.
Description should be limited to the fusion of tumour and B-cells, and their subsequent proliferation and production of antibodies.
Limit the uses to one example of diagnosis and one of treatment.

6.3.7 Outline the effects of HIV on the immune system.
6.3.8 Discuss the cause, transmission and social implications of AIDS.
11.1.1 Describe the process of blood clotting. Limit this to the release of clotting factors from platelets and damaged cells resulting in the
formation of thrombin. Thrombin catalyses the conversion of soluble fibrinogen into the fibrous protein fibrin, which captures blood cells.
  • watch blood clotting
  • and this (be sure to remember the role of thrombin, fibrinogen, fibrin and capture of blood cells)
  • and this if you want to get a taste of how complicated it can get