What are the roles of leucocytes in Phagocytosis and secretion of antibodies?

We are all surrounded by bacteria, viruses, fungi and other organisms that are capable of invading our bodies and causing disease. We are able to overcome these pathogens (disease-causing organisms) because we have an immune system. This is a complex system that allows us to develop immunity - resistance to infections.

The non-specific immune response

Inflammation is triggered by damaged cells. Ruptured cells and some white cells release alarm chemicals such as histamine and kinins. These substances dilate blood vessels and increased blood flow leads to the classic signs of inflammation.

Complement is a collection of several proteins found in the blood plasma. It is called this because it complements the action of the bodies specific immune mechanisms. It makes two important contributions to the non-specific immune system.

) Aids Phagocytosis

2) The proteins work together to kill the bacteria directly.

The whole complement system is activated by lipid-sugar molecules on the surface of some bacteria. When one of the complement proteins binds to this structure a cascade, similar to that seen in the blood clotting process is set off.

Specific immunity

The specific immune system protects the body from 'invasion' from microorganisms and parasites and also makes sure that the bodies defences do not turn on its own tissues. The specific immune response is made up from two different systems that cooperate closely:

) Hormonal immunity - also called antibody-medicated immunity, involves only chemicals, called antibodies, attack bacteria and viruses before they get inside body cells. They also react with toxins and other soluble foreign proteins. Antibodies are produced by white cells called B-Cells.

2) Cell-mediated immunity as the name suggests, involves cells, which attack foreign organisms directly. Activated T-Cells kill some micro-organisms, but they mostly attack infected body cells. Cell mediated immunity is used by the body to deal with multicellular parasites, fungi, cancer cells.

B-Cells are produced in the bone marrow and are distributed throughout the body in the lymph nodes. B-cells respond to the foreign antigens of a pathogen by producing specific antibodies. Antibodies are complex proteins that are released into the blood and carried to the site of infection. B-cells do not fight pathogens directly. Antibodies interact with the antigen and render it harmless.

Reference: Markus Barbor - 2000 - Biology

When attacked by disease-causing organisms, the body retaliates by waging biochemical's warfare. The most important weapons in the body's armoury are antibodies: specialised protein molecules that bind specifically to antigens, their target molecule. B cells of the immune system can manufacture antigen bodies with an almost infinite variety of shapes and sizes to fit any antigen.

Antibodies are proteins known as immunoglobulins (Ig) they are synthesised in response to the presence of foreign substances. Each antibody molecule has:

* Two identical heavy chains of amino acids (H) of 50 000 to 60 000 relative molecular mass.

* Two identical light chains of amino acids (L) of 23 000 relative molecular mass.

In each antibody, there are constant regions where the amino acid sequence is the same of very similar and variable regions where the amino acid sequence varies and is unique to different molecules. The variable regions form a specific three-dimensional structure, called antigen-binding site. The antigen-binding site is specific to one type of antigen (Any foreign substance, such as a virus, bacterium, or protein, that elicits an immune response by stimulating the production of antibodies).

B cells have Ig molecules on their surfaces, which act as receptors. When a B cell detects complementary antigen, it is activated and undergoes a large number of mitotic divisions, producing plasma cells and memory cells. The plasma cells synthesise large quantities of antibodies, specific to the particular antigen, which triggered the response. These antibodies are released into the blood plasma and tissue fluid, where they neutralise toxins or speed up Phagocytosis by adhering to the surfaces of the invading organism. The memory cells produced have IG receptor molecules identical to the original activated B cell and they will recognise the antigen, which stimulated their production. If there is exposure to the same antigen at a later date, these memory B cells are activated and there is a rapid production of the specific antibodies, referred to as the secondary response. Memory cells may remain in the body for many years, the production of antibodies by B cells is referred to as the humoral immune response.

Reference: John Adds - 1999 - Respiration and coordination

T cells undergo maturation in the thymus gland and then pass to the lymph nodes and spleen. T cells can also recognise antigens by means of specific receptors and, once activated by a specific antigen, they undergo mitotic divisions to produce clones of T cells, with different functions.

Name

Function

Killer T cells

Can attach themselves to invading cells and destroy them; they also attract macrophages and help to activate Phagocytosis.

Helper T cells

Help the humoral response by involvement in the maturation of B cells and by triggering the production of antibodies by the B cells.

Memory T cells

Work in the same way as the memory B cells, ensuring a rapid response on subsequent exposure to the same non-self antigens.

Suppressor T cells

Regulate the immune system response by suppressing the activity of the killer T cells.

The response made by T cells is referred to as the cell mediated immune response. Killer T cells cause cells infected with the virus particles to lyse. T cells in the thymus gland, sometimes referred to as thymocytes manufacture and release molecules called lymphokines, which include interferon. Interferon prevents the replication of the virus.

Bibliography

NAS Respiration and Coordination - John Adds

Collins Biology - Marcus Barbor

Collins Advanced Biology - Michael Kent

Biology - Immune Response October 2003

Page 1 of 5