Cellular Defence Mechanisms -Cellular defence mechanisms in animals
Two of the body’s main defence mechanisms against disease are brought about by the white blood cells which recognise and respond to the presence of invading foreign particles.
Immune Response Immunity is an organism’s ability to resist infectious disease and can be nonspecific or specific.
Phagocytosis Phagocytosis (“cell-eating�) is the process by which foreign bodies are engulfed and destroyed. Cells capable of phagocytosis are called phagocytes A phagocyte contains many specialised organelles called lysosomes which contain digestive enzymes
Phagocytosis is an example of non-specific immune response since it provides general protection against a wide range of microorganisms
Specific immune response – antibody production An antigen is a complex molecule, such as a protein, which is recognised as an alien by the body’s lymphocytes. The antigen’s presence stimulates lymphocytes to produce special protein molecules called antibodies
An antibody is a Y-shaped molecule with each arm bearing a receptor (binding) site which is specific t a particular antigen. The human body possesses thousands of different types of lymphocyte each capable of responding to one specific antigen and producing the appropriate antibody.
A viral particle, for example, is surrounded by a protein coat which provides many sites which act as antigens. When an antibody recognises a specific antigen, the two combine at the receptor site and the antigen is rendered harmless.
Immunological memory When a person is infected by a disease-causing micro-organism, antibodies are produced – this is known as the primary response. Due to a “lag� in the first appearance of antibodies following exposure to the antigens of the micro-organism, the primary response is often insufficient to prevent the person becoming ill.
Antibody concentration
If the person survives and is reexposed to the same antigen some time later, a secondary response is elicited. This time the disease is usually prevented because:
• Antibody production is much more
rapid • Concentration of antibodies reaches a higher level • The higher concentration of antibodies is maintained for longer
Antibody concentration
The secondary response is produced by memory cells. These lymphocytes, specific to the antigen from first exposure produce clones of antibodyforming lymphocytes to fight off the disease-causing microorganism on any subsequent exposure to its antigens.
Types of Specific Immunity Active Immunity The organism produces its own antibodies in one of two ways:
• Naturally acquired immunity
– as described above the person suffers a disease then continues to be able to make antibodies against it i.e. is immune to further infection e.g. chicken pox
• Artificially acquired immunity (immunisation) – the person receives a small dose of vaccine containing antigens which have been treated so as to promote antibody production but not cause disease e.g. polio
Passive immunity Instead of making antibodies, they are passed into a person’s body.
• This occurs naturally when antibodies are passed from mother to foetus via the placenta and suckling.
• This can also occur artificially when antibodies made by one mammal (e.g. horse) are extracted and injected into another (e.g. human) The effect of both types of passive immunity are generally short-lived
Rejection of transplanted tissues When organs are transplanted, the recipient’s immune system regard the donor’s tissue as many foreign antigens and attempt to destroy them – this is called tissue rejection. Tissue rejection is reduced by choosing a donor that is genetically similar to the recipient (by tissue typing)
Immunosuppresor drugs are also used. Unfortunately these drugs inhibit the recipient’s general immune system and they can be more susceptible to serious diseases such as pneumonia.