The immune system forms the body's defence against a foreign substance, whether a micro-organism (bacteria, fungi, viruses, protozoa, and parasites), a potentially toxic material (foreign protein, carbohydrate, or nucleic acid), or an abnormal cell (one invaded by a virus or having become malignant).

It attacks the foreign substance and maintains a memory of the invader so a second exposure will provoke a greater, faster response. Immunity refers to the ability of an individual who has recovered from a disease to remain well after a second exposure to the same disease.

FUNCTIONS OF THE IMMUNE SYSTEM

Under normal circumstances the immune system responds to foreign organisms by the production of antibodies and the stimulation of specialised cells, which destroy the organisms or neutralise their toxic products.

When the immune system involved in this function becomes too active, however, the result may be undesirable features, such as hypersensitivity or allergic reactions.

On the other hand, when the immune response is not working properly (immunodeficient), as in acquired immune deficiency syndrome (see AIDS), the individual may become more susceptible to repeated infections.

A major function of the immune system is surveillance of the cells of the body to ensure that they are not abnormal. Malignant cells, cells infected with viruses, or cells from another animal even of the same species but not an identical twin have protein markers on their outer membrane (antigens) that act as a signal to the immune system to destroy them. In some cases, for reasons that are not clear, normal cells may be wrongly identified as abnormal. The immune system may develop antibodies to these cells as well as activating cells of the immune system, producing autoimmune diseases.

The immune system also has the ability to recognise and eliminate abnormal cells that frequently arise within the body. These mutant, or cancer, cells, may occur spontaneously or be induced by certain viruses or chemicals (mutagens). An immune system that is functioning properly can usually recognise and dispose of such cancer cells by means of a process called immune surveillance. The malfunction of this process may result in the development of cancer.

note: An antigen is a substance that, when introduced into an organism, induces an immune response consisting of the production of a circulating antibody. They represent the ‘ID’ of the cell.

CELLS IN THE IMMUNE RESPONSE

The immune system has two general responses:

-Immune cells are activated to destroy a harmful cell by cell-to-cell interaction (cell- mediated immune response)

-Immune cells are activated to produce large protein molecules called antibodies to bind to and render harmless bacteria, yeast, some viruses, and even toxins. (humoral immune response)

In many cases, both occur.

1- Cell-mediated immune response

A particular type of white cell -T cells kill foreign cells directly and also have important regulatory roles in the immune system. T cells destroy cells transformed by cancer and they attack cells transplanted from other animals.T cells recognise a pathogen by binding a receptor protein on the intruder’s antigen.

2- Humoral immune response

To activate immune cells, large cells called macrophages eat and partially digest the invading material, placing pieces of it, called the antigen, on the surface of the macrophage.

A specific B cell, formed in the bone marrow, will bind on the antigen and then multiply to form a clone. A clone then begins to make antibody after the conversion of some clonal cells to plasma cells, while other clonal cells form the immune memory for this antigen by remaining indefinitely in an alert state, ready to multiply again should the antigen be present in the future (by differentiating into memory cells). Antibodies formed by this mechanism control not only bacteria but also viruses, fungi, yeast, parasites, protozoa, and many toxic chemicals.

ANTIBODIES

structure

The molecules responsible for recognising antigenic determinants on foreign molecules or on cell surfaces are called antibodies. Antibodies are members of a related group of gamma globulin molecules known as immunoglobulins (lg).

A typical immunoglobulin is made of four protein chains joined together in two pairs. One pair member is much smaller and is called the light (L) chain. The much larger member of each pair is the heavy (H) chain.

Each chain is made of segments; the light chain has two and the typical heavy chain has four. One segment of a chain has an amino acid structure that varies depending on with which antigen it reacts. All other segments are constant for each class of immunoglobulins.

A single antigen molecule binds to both a heavy and light chain. Five classes of immunoglobulins exist, based on structural differences: they are called IgG, IgM, IgA, IgD, and IgE, respectively.

Antibodies in defence

1- Agglutination:

The simplest and most prevalent means by which the immune system defends the body against bacteria and viruses is by the combination of a specific antibody with the antigenic determinants located on the surface of invading organisms. An aggregate of cells, called an agglutination, is formed by antibodies bound by one of their two combining sites to one cell, and to another cell by their other site. These aggregates are then engulfed and digested by the body's wandering scavenger cells, the macrophages. Antibodies also bind to toxic molecules, called toxins, given off by microorganisms, forming large, insoluble aggregates (precipitates) that are also removed by macrophages.

2- lysis:

The antibody combines with the pathogen and destroys it.

3-other uses

Antibodies also cover up the attachment sites of viruses and thereby prevent their ability to infect cells. Precipitin and agglutination reactions are used as diagnostic tools for identifying and quantifying the antibodies of infectious organisms in blood samples and other body fluids.

T CELLS & B CELLS RELATIONSHIP

Half of all T cells control and regulate the action of B cells rather than directly disposing of intruders.Helper T cells signal B cells to spring into action.They communicate to other molecules by secreting molecules that alter the activity of specific targets.

Helper cells do not control the type of response but rather the degree of response (i.e. the number of B cells to be commited)

TRANSPLANTS AND THE IMMUNE SYSTEM

Organs

The development of the transplantation of organs and tissues from person to another has led to much work being done on the immune system and its suppression.However closely the tissues of a donor organ (e.g. a kidney) are matched to the tissue of the patient who is to receive the transplant, a perfect match is not possible unless the donor is an identical twin. This means that the immune system of the recipient will set out to destroy or reject the donor organ.

The problem is how to prevent the recipient from rejecting the transplanted organ without reducing the ability of the immune system to an extent that the patient dies from a succession of infections which the body cannot fight.Rejection isprevented by a cocktail of immunosuppressant drugs (drugs that prevent the immune system to spring into action) which to get the balance right..Transplant patients have to take these drugs for the rest of their lives.

Blood types

People can be classified into one of several blood groups depending on the presence or abscence of certain antigens (agglutinogens) on the red blood cells. Antibodies (agglutinins) may also be present in the plasma. Transfusion from a person of one group into someone of another group may be fatal.

Our red cells have one, both or neither of two agglutinogens called A and B. Our plasma has one, both or neither of two agglutinins called Anti-A and Anti-B

Cells with A agglutinogen belong to group A, those with B belong to group B, those with both belong to AB and those with neither belong to O.

Rh factor

The Rh factor is an antigen whose name is derived from the rhesus monkey, on whose red blood cells it was first discovered. Later found in humans, the Rh factor, along with other blood antigens, must be taken into account in blood transfusions. Blood from an Rh-positive donor will cause an Rh-negative recipient to produce antibodies against the Rh factor. The antibodies will cause a hemolytic transfusion reaction (damage to red blood cells) if the recipient again receives Rh-positive blood. The hemolytic reaction destroys the donated cells.