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.
Structure of the skin. It is part of the general defence system and is the first line of defence against pathogens.
Inflammation at the site of infection

Mucous membranes

  • Traps foreign particles.
  • Lining of respiratory tracts – mucous traps debris and microorganisms and cilia move it 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.
A phagocyte (yellow) engulfing bacteria (orange)


  • 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 which causes blood vessels to dilate (redness) and attracts white blood cells.
  • Lysozyme (which kills bacteria) is present in tears.

Specific Defence System

Specific Defence System refers to the immune system (it reacts to individual pathogens). Organs that are part of the immune system include:

  • 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. Immunity to specific antigens usually lasts for a long time (10 to 20 years).


  • Develop into macrophages which engulf tagged (antibody attached to antigen) invaders and untagged invaders.
Three monocytes pictured amongst many red blood cells.
Monocytes can mature into macrophages (pictured).
  • 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.
A macrophage engulfing and digesting pathogens


  • 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 lymphocytes which attach to invader. Other lymphocytes recognise the antibody that is attached to invader (i.e. the invader has been tagged for destruction) and phagocytose (engulf and digest) it.

Two basic types of induced (acquired) immunity:

  1. Active immunity
  2. Passive immunity

Active Immunity

  • There are two types of active immunity:
    1. Natural active immunity: is where an individual suffers from infection, develops symptoms and produces antibodies against the pathogen (primary response). Natural active immunity is usually long-lasting because after infection has been eliminated, the immune system produces memory lymphocytes (see below) that are capable of responding to the same antigen many years after initial infection (see graph below). If the individual is infected again with the same pathogen/antigen, then there is a secondary immune response which is faster and larger (more antibodies are produced) than the primary response.
Primary and secondary immune responses
  1. Artificial active immunity: is where an individual receives a vaccination (see below). The individual receives a weakened dose of a pathogen and the immune system reacts against it producing antibodies and memory lymphocytes. The individual does not (generally) suffer symptoms.
A vaccination dose being prepared for injection

Passive Immunity

  • There are two types of passive immunity:
    1. Natural passive immunity: is where an individual receives 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
    2. Artificial passive immunity: is where an individual receives antibodies (made in a different organism) by injection (immunisation – see below) to fight off a potentially life-threatening disease (e.g. tetanus or rabies).
  • Neither natural nor artificial passive immunity involves the production of memory lymphocytes and is therefore only effective for short time.
Breast-feeding is a type of natural passive immunity

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/white blood cells):

These are specialised cells that recognise particular types of antigen and respond to them in a variety of ways. There are two main groupings of lymphocytes:

  1. B lymphocytes: produced and mature in bone marrow and then migrate to lymphoid tissue – such as lymph nodes, tonsils and spleen. They produce antibodies.
  2. T lymphocytes: produced in bone marrow but mature in thymus gland and then migrate to lymphoid tissue in same way as B lymphocytes. They have a variety of functions (see below).

B lymphocytes (or also called B cells):

  • Each B lymphocyte carries receptors for only one specific antigen.
  • Each B lymphocyte produces only one type of antibody in response to that specific antigen.
  • Once a B lymphocyte has been activated by presence of antigen it multiplies itself to produce a clonal population, called plasma B cells. Some plasma B cells become memory B cells that remain in lymph nodes for a long time and can respond to the same antigen in the future (long-term immunity).
Antigen-antibody specific interactions
  • Once a B lymphocyte has been activated by presence of antigen it multiplies itself to produce a clonal population, called plasma B cells. Some plasma B cells become memory B cells that remain in lymph nodes for a long time and can respond to the same antigen in the future (long-term immunity).
Interactions between macrophages and B cells

T lymphocytes (or also called T cells):

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

  1. Helper T-cells
  2. Killer T-cells
  3. Suppressor T-cells
  4. Memory T-cells
  1. 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)

Stimulation of B cells and killer T cells by helper T cells

2. 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)

Killer T cell causing an infected cells to burst

3. Suppressor T-cells

Responsible for maintaining the immune response at a manageable level, prevent it getting out of control (type of negative feedback mechanism) and/or stopping an immune reaction. Suppressor T-cells cause the killer T-cells and excess B-cells to die at the end of the immune reaction.

4. Memory T-cells

Like memory B-cells they survive a long time in the lymph nodes and can respond to a specific pathogen in the future. Memory T-cells stimulate memory B-cells to start producing antibodies when the encounter the same pathogen again and they stimulate killer T-cells to multiply/become active.