Specific Cellular Defence

Range of white blood cells (WBCs) circulate monitoring for damage, pathogens or cancerous cells

In response to damage or infection some WBCs produce cytokines which lead to increased WBCs in the area.

These WBCs include Phagocytes, Lymphocytes (Natural Killer Cells, T Cells and B Cells)

The body has a huge number of different lymphocytes each with a different membrane receptor that is specific to for one particular antigen

Stages in clonal selection;◦ An antigen binds to its specific receptor on a

lymphocyte ◦ The lymphocyte undergoes repeated division,

resulting in several clones of identical lymphocytes

T- Lymphocytes (T-Cells) Produced in the bone marrow Mature in Thymus Gland Can be distinguished from other

lymphocytes by their cell surface receptors (TCRs)

Allow them to detect specific antigens that enter the body

These proteins allow T Cells to distinguish between self and non self

Cytotoxic T-Cells (Killer T-Cells)(TC Cells) Destroy cells which the immune system

regards as foreign (infected cells and tumour cells)

Recognise antigens on foreign cells, bind to the cells and destroy by apoptosis

Majority of TC Cells then undergo apoptosis themselves

A few remain as memory cells

Killer T-cells: Can attach themselves to cells with foreign antigens on their surface and destroy them

1. Virus attaches to and enters host cell

2. Viral DNA causes host cell to express viral anitgens on its surface

3. Killer T-cell attaches to viral

antigen on infected cell

4. Killer T-cell induces death of

infected cell

Do not destroy infected cells Secrete cytokines that activate B

lymphocytes and TC Cells

When a tissue has been infected by a particular pathogen, phagocytes are attracted to the pathogens by chemicals and engulf them in phagocytosis

Some phagocytes then move part of the pathogen to their own surface

They are now antigen presenting cells Can activate the production of a clone of T

Cells that move to the site of infection, directed by cytokines.

specific defencesT lymphocytes• One group of T lymphocytes destroy infected cells

by inducing apoptosis. • Another group of T lymphocytes secrete cytokines

that activate B lymphocytes and phagocytes. • When pathogens infect tissue, some phagocytes

capture the pathogen and display fragments of its antigens on their surface.

• These antigen presenting cells activate the production of a clone of T lymphocytes that move to the site of infection under the direction of cytokines.

Summary slide

B-Lymphocytes (B-cells) Produced and mature in bone marrow Activated by APCs and T cells produce a

clone of B-cells Secrete antibodies into lymph and blood

systems Clonal Selection Theory Each antibody fits an antigen like a key fits

a lock

Antibodies bind to the antigen to form an antigen-antibody complex which can;

Inactivate the pathogen or toxin Make it more susceptible to phagocytosis Stimulate a response by T cells resulting in

cell lysis (cell destruction)

Some T and B cells produced by clonal selection survive long term as memory cells

They remain in the body and the same antigen is encountered in a secondary exposure a new clone of lymphocytes are produced giving a response which is◦ Quicker◦ Greater

Immunological memory is specific for a particular antigen and is long lived

When the body reacts to a small trace of a harmless foreign substance

The antigen involved is called an allergen Animal hair Pollen Moulds Dust mites

Some common diseases caused by allergic reactions are;

Hay fever Anaphylactic shock Allergic asthma

Animation: The Immune Response

Specific defencesImmune surveillance•A range of white blood cells constantly circulate monitoring the tissues. •If tissues become damaged or invaded, cells release cytokines which increase blood flow resulting in specific white blood cells accumulating at the site of infection or tissue damage. •Recognition of self and non self through specific surface proteins (antigens) •Lymphocytes respond specifically to antigens on foreign cells, cells infected by pathogens and toxins released by pathogens.

specific defencesB lymphocytes• Each B lymphocyte clone produces a specific

antibody molecule that will recognise a specific antigen surface molecule on a pathogen or a toxin.

• Antigen-antibody complexes may inactivate a pathogen or toxin or render it more susceptible to phagocytosis.

• In other cases the antigen-antibody complex stimulates a response which results in cell lysis.

• B lymphocytes activated by antigen presenting cells and T lymphocytes produce a clone of B lymphocytes that secrete antibodies into the lymph and blood where they make their way to the infected area.

Summary slide

Specific defencesImmunological memory cells• Some T and B lymphocytes produced in

response to antigens by clonal selection survive long term as memory cells.

• A secondary exposure to the same antigen rapidly gives rise to a new clone of lymphocytes producing a rapid and greater immunological response.

•

Failures of immune system

•Failure in regulation of the immune system leads to a T lymphocyte immune response to self cells (auto immune disease).

•Allergy is a hypersensitive B lymphocyte response to an antigen that is normally harmless.