Cellular Defence Cellular Defence MechanismsMechanisms

-Cellular defence -Cellular defence mechanisms in mechanisms in

animalsanimals

Two of the body’s main defence Two of the body’s main defence mechanisms against disease mechanisms against disease are brought about by the are brought about by the white white blood cells blood cells which recognise which recognise and respond to the presence of and respond to the presence of invading foreign particles.invading foreign particles.

Immune ResponseImmune Response

ImmunityImmunity is an organism’s is an organism’s ability to resist infectious ability to resist infectious disease and can be disease and can be non- non-specificspecific or or specificspecific..

PhagocytosisPhagocytosis

PhagocytosisPhagocytosis (“cell-eating”) is (“cell-eating”) is the process by which foreign the process by which foreign bodies are bodies are engulfedengulfed and and destroyed.destroyed.

Cells capable of phagocytosis Cells capable of phagocytosis are called are called phagocytesphagocytes

A phagocyte contains many A phagocyte contains many specialised organelles called specialised organelles called lysosomeslysosomes which contain which contain digestive enzymesdigestive enzymes

PhagocytosisPhagocytosis is an example of is an example of non-specific immune non-specific immune responseresponse since it provides since it provides general protection against a general protection against a wide range of microorganismswide range of microorganisms

Specific immune response Specific immune response – antibody production– antibody production

An An antigen antigen is a complex is a complex molecule, such as a protein, molecule, such as a protein, which is recognised as an which is recognised as an alienalien by the body’s by the body’s lymphocytes.lymphocytes.

The antigen’s presence The antigen’s presence stimulates lymphocytes to stimulates lymphocytes to produce special protein produce special protein molecules called molecules called antibodiesantibodies

An antibody is a Y-shaped An antibody is a Y-shaped molecule with each arm molecule with each arm bearing a bearing a receptor (binding) receptor (binding) sitesite which is specific t a which is specific t a particular antigen.particular antigen.

The human body possesses The human body possesses thousands of different types of thousands of different types of lymphocyte each capable of lymphocyte each capable of responding to one specific responding to one specific antigen and producing the antigen and producing the appropriate antibody.appropriate antibody.

A viral particle, for example, is A viral particle, for example, is surrounded by a protein coat surrounded by a protein coat which provides many sites which provides many sites which act as antigens.which act as antigens.

When an antibody recognises a When an antibody recognises a specific antigen, the two specific antigen, the two combine at the receptor site combine at the receptor site and the antigen is rendered and the antigen is rendered harmless.harmless.

Immunological memoryImmunological memory

When a person is infected by a When a person is infected by a disease-causing micro-organism, disease-causing micro-organism, antibodies are produced – this is antibodies are produced – this is known as the known as the primary responseprimary response..

Due to a “lag” in the first appearance Due to a “lag” in the first appearance of antibodies following exposure to of antibodies following exposure to the antigens of the micro-organism, the antigens of the micro-organism, the primary response is often the primary response is often insufficient to prevent the person insufficient to prevent the person becoming ill.becoming ill.

Antibody

concentration

If the person survives and is re-If the person survives and is re-exposed to the same antigen some exposed to the same antigen some time later, a time later, a secondary responsesecondary response is elicited.is elicited.

This time the disease is usually This time the disease is usually prevented because:prevented because:

• Antibody production is much more Antibody production is much more rapidrapid

• Concentration of antibodies Concentration of antibodies reaches a higher levelreaches a higher level

• The higher concentration of The higher concentration of antibodies is maintained for longerantibodies is maintained for longer

Antibody

concentration

The secondary response is The secondary response is produced by produced by memory cellsmemory cells..

These lymphocytes, specific to These lymphocytes, specific to the antigen from first exposure the antigen from first exposure produce produce clonesclones of antibody- of antibody-forming lymphocytes to fight off forming lymphocytes to fight off the disease-causing micro-the disease-causing micro-organism on any subsequent organism on any subsequent exposure to its antigens.exposure to its antigens.

Types of Specific Types of Specific ImmunityImmunity

Active ImmunityActive Immunity

The organism produces its own The organism produces its own antibodies in one of two ways:antibodies in one of two ways:

• Naturally acquired immunityNaturally acquired immunity – as described above the – as described above the person suffers a disease then person suffers a disease then continues to be able to make continues to be able to make antibodies against it i.e. is antibodies against it i.e. is immune to further infection e.g. immune to further infection e.g. chicken poxchicken pox

• Artificially acquired Artificially acquired immunity (immunisation)immunity (immunisation) – – the person receives a small the person receives a small dose of dose of vaccinevaccine containing containing antigens which have been antigens which have been treated so as to promote treated so as to promote antibody production but not antibody production but not cause disease e.g. poliocause disease e.g. polio

Passive immunityPassive immunity

Instead of making antibodies, Instead of making antibodies, they are passed into a person’s they are passed into a person’s body.body.

• This occurs This occurs naturallynaturally when when antibodies are passed from antibodies are passed from mother to foetus via the mother to foetus via the placenta and suckling.placenta and suckling.

• This can also occur This can also occur artificiallyartificially when antibodies made by one when antibodies made by one mammal (e.g. horse) are mammal (e.g. horse) are extracted and injected into extracted and injected into another (e.g. human)another (e.g. human)

The effect of both types of The effect of both types of passive immunity are generally passive immunity are generally short-livedshort-lived

Rejection of Rejection of transplanted tissuestransplanted tissues

When organs are transplanted, When organs are transplanted, the recipient’s immune system the recipient’s immune system regard the donor’s tissue as regard the donor’s tissue as many foreign antigens and many foreign antigens and attempt to destroy them – this attempt to destroy them – this is called is called tissue rejection tissue rejection..

Tissue rejection is reduced by Tissue rejection is reduced by choosing a donor that is choosing a donor that is genetically similar to the genetically similar to the recipient (by recipient (by tissue typing tissue typing))

Immunosuppresor drugsImmunosuppresor drugs are are also used.also used.

Unfortunately these drugs inhibit Unfortunately these drugs inhibit the recipient’s general immune the recipient’s general immune system and they can be more system and they can be more susceptible to serious diseases susceptible to serious diseases such as pneumonia.such as pneumonia.