6.3 Defence Against Infectious Disease

6.3.1  Define pathogen

A pathogen is a disease-causing micro-organism, virus or prion

6.3.2  Explain why antibiotics are effective against bacteria but not against viruses

  • Antibiotics are substances or compounds that kill or inhibit the growth of bacteria by targeting the metabolic pathways of prokaryotes
  • Specific prokaryotic features that may be targeted by antibiotics include key enzymes, 70S ribosomes and the bacterial cell wall
  • Because eukaryotic cells do not have these features, antibiotic can kill bacterial cells without harming humans (or viruses)
  • Virus do not carry out metabolic reactions themselves but instead infect host cells and take over their cellular machinery
  • Viruses need to be treated with specific antiviral agents that target features specific to viruses (e.g. reverse transcriptase in retroviruses)

The first line of defence against infection are the surface barriers that prevent the entry of pathogenic substances

These surface barriers include the skin and mucous membranes

Summary of Surface Barriers

6.3.3  Outline the role of skin and mucous membranes in defence against pathogens


  • Protects external structures (outer body areas)
  • A dry, thick and tough region made of predominantly dead surface cells
  • Contains biochemical defence agents (sebaceous glands secrete chemicals which inhibit the growth of some bacteria)
  • The skin also releases acidic secretions to lower pH and prevent bacteria from growing

Mucous membranes

  • Protect internal structures (externally accessable cavities and tubes, such as trachea, vagina and urethra)
  • A thin region containing living surface cells that release fluids to wash away pathogens (mucus, tears, saliva, etc.)
  • Contains biochemical defence agents (secretions contain lysozyme, which can destroy cell walls and cause cell lysis)
  • Mucous membranes may be ciliated to aid in the removal of pathogens (along with physical actions such as coughing or sneezing)

The second line of defence against pathogenic invasion are the non-specific defence mechanisms

Non-specific mechanisms do not differentiate between types of microorganisms and always invoke the same response

Examples of non-specific defence mechanisms include phagocytic leucocytes, inflammation, fever and anti-microbial proteins

Non-specific Immunity

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

  • Phagocytic leucocytes (macrophages) circulate in the blood but may move into body tissue (extravasation) in response to infection
  • They concentrate at sites of infection due to the release of chemicals (such as histamine) from damaged body cells
  • Pathogens are engulfed when cellular extensions (pseudopodia) surround the pathogen and then fuse, sequestering it in an internal vesicle
  • The vesicle may then fuse with the lysosome to digest the pathogen
  • Some of the pathogens antigenic fragments may be presented on the surface of the macrophage, in order to help stimulate antibody production
  • This mechanism of endocytosis is called phagocytosis ('cell-eating')

Overview of Phagocytosis by a Leucocyte

The third line of defence are the specific defences, coordinated by a type of leucocyte called lymphocytes

These can recognise and respond specifically to different types of micro-organism and have memory (can respond more effectively upon reinfection)

Specific Immunity

6.3.5  Distinguish between antigens and antibodies

Antigen:  A substance that the body recognises as foreign and that can evoke an immune response

Antibody:  A protein produced by certain white blood cells (B lymphocytes, plasma cells) in response to an antigen

  • Antibodies are made up of 4 polypeptide chains (2 light and 2 heavy chains) joined together by disulphide bonds to form a Y-shaped molecule
  • The ends of the arms are where the antigens bind and these areas are called the variable regions, as these will differ between antibodies
  • Each type of antibody will recognise a unique antigenic fragment, making this interaction specific (like enzyme-substrate interactions) 

Structure of a Generalised Antibody

6.3.6  Explain antibody production

  • B lymphocytes (B cells) are antibody-producing cells that develop in the bone marrow to produce a highly specific antibody that recognises one type of antigen
  • When wandering macrophages encounter a pathogen, they digest it and present the antigenic fragments on their surface to helper T lymphocytes    (TH cells) 
  • These cells activate the appropriate B cell which divides and differentiates into short-lived plasma cells that produce massive quantities of antibody (~2,000 molecules per second for ~4 - 5 days)
  • A small proportion of B cell clones develop into memory cells, which may survive for years providing long-term immunity

6.3.7  Outline the effect of HIV on the immune system

  • The human immunodeficiency virus (HIV) is a retrovirus that infects helper T lymphcytes (TH cells)
  • Reverse transciptase allows viral DNA to be produced from its RNA code, which is integrated into the host cells genome
  • After a number of years of inactivity (during which infected TH cells have continually reproduced), the virus becomes active and begins to spread, destroying the TH cells in the process (known as the lysogenic cycle)
  • This results in lower immunity as antibody production is compromised - the individual is now susceptible to opportunistic infections

Timeline of HIV Infection

6.3.8  Discuss the cause, transmission and social implications of AIDS


  • Acquired Immunodeficieny Syndrome (AIDS) is a collection of symptoms and infections caused by the destruction of the immune system by HIV
  • While HIV infection results in a lowering in immunity over a number of years, AIDS describes the final stages when observable symptoms develop


  • HIV is transmitted through the exchange of bodily fluids (including unprotected sex, blood transfusions, breast feeding, child birth, etc.)
  • The risk of exposure to HIV through sexual contact can be reduced by using latex protection (condoms)
  • A minority of people are immune to HIV infection (they do not have the CD4+ T cell receptor that HIV needs to infect the cell)

Social Implications

  • People with HIV may be stigmatised and discriminated against, potentially leading to unemployment and poverty
  • Majority of people who die from AIDS are at a productive age, which may cripple a country's workforce and economic growth
  • It can result in an increased number of orphans, taxing a country's welfare resources
  • Poverty may increase transmission of AIDS (due to poor education and high cost of treatments), creating a moral obligation for assistance from wealthier countries