Chapter 34: The human defence system

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Chapter 34: The Human Defence System
General Defence System

  • Skin
  • Mucous membranes
  • Phagocytosis
  • Fever
  • Defence chemicals


  • Sweat and sebum secretions contain chemicals that kill bacteria and fungi
  • Blood clotting prevents entry of microorganisms if the skin is compromised by a wound

Mucous membranes

  • Traps foreign material
  • Lining of respiratory tracts
  • Mucous traps debris and microorganisms and cilia move the mucous and debris up to the pharynx where it is swallowed
  • Lining of digestive tract – HCl in stomach kills all microorganisms
  • Lining of reproductive tracts – low pH in vagina kills microorganisms


  • Phagocytes are a type of white blood cell – they move and feed like Amoeba
  • Recognise foreign material and engulf it
  • Only takes one-hundredth of one second to engulf one bacterium
  • Each phagocyte can engulf over 100 bacteria
  • Attracted to and accumulate in extremely large numbers at an infection site


  • Chemicals released by defence cells cause the hypothalamus to raise the body’s temperature
  • Increased body temperature interferes with enzymes in bacteria and viruses which prevents the reproduction of these microorganisms

Defence chemicals

  • Virus-infected cells release interferon that acts as a warning chemical to other cells making them more resistant to proteins entering cells
  • Liver secretes complement proteins that help the immune system in ridding the body of the foreign invader
  • Irritation (caused by infection or foreign material) causes cells to release histamine – causes blood vessels to dilate (redness) and attracts WBCs
  • Lysozyme (which kills bacteria) is present in tears
Specific Defence System

Specific Defence System refers to the immune system
Organs specific to the immune system are:

  • Spleen
  • Thymus
  • Lymph nodes

Blood and lymph contain white blood cells called monocytes and lymphocytes (produced in bone marrow)
All microorganisms have antigens on their surfaces that make it foreign to the body

  • Antigens are foreign molecules capable of eliciting an antibody response
  • An antibody is a protein produced by lymphocytes in response to an antigen

Antigens are found in bacterial cell walls, viral coats, foreign cells, and on cancerous cells
Antigen immunity usually lasts for a long time (>10 years)



  • Develop into macrophages which engulf tagged (antibody attached to antigen) invaders and untagged invaders
  • Macrophage that have engulfed tagged pathogens display the antigen belonging to the pathogen on their surface stimulating other cells to respond to the antigen and kill the invader


  • Involved in induced immunity: (acquired immunity) production of antibodies in response to the presence of specific antigens on pathogens
  • Antigens may be displayed on cells that have been infected with a virus
  • Lymphocytes specifically recognise foreign bodies and set up an immune reaction where a response to the invader is carried out
  • Antibodies are produced by lypmhocytes which attach to invader. Other lymphocytes recognise the antibody that is attached to invader (i.e. the invader has been tagged for destruction) and phagocytise it.

Two basic types of induced (acquired) immunity:

  1. Active induced immunity
  2. Passive induced immunity

Active Induced Immunity

  • Involves production of antibodies in response to antigen (primary response).
  • Long-lasting because after infection has been dealt with the immune system produces memory lymphocytes that are capable of responding to the same antigen many years after initial infection (see graph below).
  • If the person is infected again with the same antigen, then there is a secondary immune response which is faster and larger than the primary response.

Passive Induced Immunity

  • Involves the supply of antibodies from an external source – e.g. breast milk supplies antibodies to infant and in serious life-threatening disease antibodies can be injected into patient to fight disease such as rabies or tetanus
  • Does not involve production of memory cells and thus is only effective for short time

Immunisation and Vaccination

  • Immunisation is protection against pathogens or toxins by vaccination or by injection of antibodies or antidotes
  • Vaccination is the administration usually by injection of a non-disease-causing dose of a pathogen or part of a pathogen (e.g. the antigen of the pathogen or its toxin) which elicits the production of antibodies and importantly memory lymphocytes

Advanced Study of Lymphocytes

Lymphocytes (leucocytes/WBCs):
Specialised cells that recognise particular types of antigen and respond to them in a variety of ways
Two types:

  1. B-cells: produced and mature in bone marrow and then migrate to lymphoid tissue – such as lymph nodes, tonsils, spleen, intestine
  2. T-cells: produced in bone marrow but mature in thymus gland and then migrate to lymphoid tissue in same way as B-cells


  • Each B-cell carries receptors for only one specific antigen
  • Each B-cell produces only one type of antibody in response to that specific antigen
  • Once a B-cell has been activated by presence of antigen it multiplies itself to produce a clonal population. Some B-cells of the clonal population become memory cells that are able to respond to the same antigen in the future

T cells multiply rapidly when activated by a specific antigen – the daughter cells differentiate (change) into three major types of immune cell:

  • Helper T-cells
  • Killer T-cells
  • Suppressor T-cells
  • Memory T-cells

Helper T-cells
Helper T-cells enlarge during an immune response and secrete chemicals, such as interferon that stimulate B-cells to increase production of antibodies
Helper T-cells also stimulate killer T-cells and accelerate the action of phagocytes (monocytes)


Killer T-cells
Recognise cancer cells and cells that have been infected with virus and act by placing proteins called perforins in their membranes – perforins cause the infected cell to die by a process called apoptosis (programmed cell death)

Suppressor T-cells
Responsible for maintaining the immune response at a manageable level – prevent it getting out of control – negative feedback mechanism!
Suppressor T-cells cause the killer T-cells and excess B-cells to die by apoptosis at the end of the immune reaction

Memory T-cells
Like memory B-cells they survive a long time and can respond to a specific invader in the future
Memory T-cells stimulate memory B-cells to start producing antibodies and they stimulate killer T-cells