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antibodies

However, upon entry of a foreign antigen, the lymphoid tissue macrophages phagocytize the antigen and then present it to the adjacent B lymphocytes. In addition, the antigen is also presented to T cells at the same time, and activated "helper" T cells then also contribute to the activation of the B lymphocytes.

The Nature of the Antibodies

The antibodies are gamma globulins called immunoglobulins, and they have molecular weights between approximately 150,000 and 900,000. Usually they constitute about 20 per cent of all the plasma proteins.All the immunoglobulins are composed of combinations of light and heavy polypeptide chains; most are a combination of two light and two heavy chains. Some of the immunoglobulins, though, have combinations of as many as ten heavy and ten light chains,which gives rise to the much larger molecular weight immunoglobulins. Yet in all immunoglobulins, each heavy chain is paralleled by a light chain at one of its ends, thus forming a heavy-light pair, and there are always at least two such pairs in each immunoglobulin molecule.

Mechanisms of Action of Antibodies 

Antibodies act mainly in two different ways to protect the body against invading agents:

(1) by direct attack on the invader and (2) by activation of the complement system that then destroys the invader.

Direct Action of Antibodies on Invading Agents

Because of the bivalent nature of the antibodies and the multiple antigen sites on most invading agents, the antibodies can inactivate the invading agent in one of several ways, as follows:

1.    Agglutination 

2.    Precipitation

3.    Neutralization

4.    Lysis

The Complement System for Antibody Action

 Complement" is a collective term to describe a system of about 20 different proteins, many of which are enzyme precursors. All these are present normally among the plasma proteins and also among the plasma proteins that leak out of the capillaries into the tissue spaces. The enzyme precursors are normally inactive, but they can be activated in two separate ways:

1.The Classical Pathway

The classical pathway is activated by an antigen-antibody reaction. That is, when an antibody binds with an antigen, a specific reactive site on the "constant" portion of the antibody becomes uncovered, or activated, and this in turn binds directly with the C1 molecule of the complement system, setting into motion a "cascade" of sequential reactions.

2. The Alternate Pathway

complement system sometimes is activated without the intermediation of an antigen-antibody reaction. This occurs especially in response to large polysaccharide molecules in the cell membranes of some invading microorganisms.

Cellular (Cell-Mediated) Immune Response

Like B cells, immunocompetent T cells are activated to form a clone by binding with a "recognized" antigen. However, unlike B cells, the T cells are not able to bind with free antigens. Instead, the antigens must be "presented" by macrophages, and a double recognition must occur. The macrophages engulf the antigens, process them internally, and then finally display parts of the processed antigens on their external surface in combination with one of their own (self) proteins.

Apparently, antigen presentation is a major role of macrophages and is essential for activation and clonal selection of the T cells. Without macrophage "presenters," the immune response is severely impaired. Cytokine chemicals (monokines, particularly interleukin 1) released by macrophages also play important roles in the immune response.

The different classes of T cell clones are:

I- Cytotoxic (killer) T cells, cells that specialize in killing virus-infected, cancer, or foreign graft cells. One way they accomplish this is by binding to them and inserting a toxic chemical (perform or others) into the foreign cell s plasma membrane.

cells that specialize in killing virus-infected, cancer, or foreign graft cells. One way they accomplish this is by binding to them and inserting a toxic chemical or others) into the foreign cell s plasma membrane.

II- Helper T cells are the T cells that act as the "directors" or "managers" of the immune system. Once activated, they circulate through the body, recruiting other cells to fight the invaders. For example, helper T cells interact directly with B cells (that have already attached to antigens), prodding them into more rapid division (clone production) and then signaling for antibody formation to begin. They also release a variety of cytokine chemicals called lymphokines that act indirectly to rid the body of antigens by (1) stimulating cytotoxic T cells and B cells to grow and divide; (2) attracting other types of protective white blood cells, such as neutrophils, into the area; and(3) enhancing the ability of macrophages to engulf and destroy microorganisms.

are the T cells that act as the "directors" or "managers" of the immune system. Once activated, they circulate through the body, recruiting other cells to fight the invaders. For example, helper T cells interact directly with B cells (that have already attached to antigens), prodding them into more rapid division (clone production) and then signaling for antibody formation to begin. They also release a variety of cytokine chemicals called that act indirectly to rid the body of antigens by (1) stimulating cytotoxic T cells and B cells to grow and divide; (2) attracting other types of protective white blood cells, such as neutrophils, into the area; and(3) enhancing the ability of macrophages to engulf and destroy microorganisms.

III- Suppressor Tcells, releases chemicals that suppress the activity of both T and B cells. Suppressor T cells are vital for winding clown and finally stopping the immune response after an antigen has been successfully inactivated or destroyed. This helps prevent uncontrolled or unnecessary immune system activity

releases chemicals that suppress the activity of both T and B cells. Suppressor T cells are vital for winding clown and finally stopping the immune response after an antigen has been successfully inactivated or destroyed. This helps prevent uncontrolled or unnecessary immune system activity

VI- memory cells; most of the T cells enlisted to fight in a particular immune response are dead within a few days. However, a few members of each clone are long-lived memory cells that remain behind to provide the immunological memory for each antigen encountered and enable the body to respond quickly to its subsequent invasions.

; most of the T cells enlisted to fight in a particular immune response are dead within a few days. However, a few members of each clone are long-lived memory cellsthat remain behind to provide the immunological memory for each antigen encountered and enable the body to respond quickly to its subsequent invasions.

cells that specialize in killing virus-infected, cancer, or foreign graft cells. One way they accomplish this is by binding to them and inserting a toxic chemical or others) into the foreign cell s plasma membrane. are the T cells that act as the "directors" or "managers" of the immune system. Once activated, they circulate through the body, recruiting other cells to fight the invaders. For example, helper T cells interact directly with B cells (that have already attached to antigens), prodding them into more rapid division (clone production) and then signaling for antibody formation to begin. They also release a variety of cytokine chemicals called that act indirectly to rid the body of antigens by (1) stimulating cytotoxic T cells and B cells to grow and divide; (2) attracting other types of protective white blood cells, such as neutrophils, into the area; and(3) enhancing the ability of macrophages to engulf and destroy microorganisms. releases chemicals that suppress the activity of both T and B cells. Suppressor T cells are vital for winding clown and finally stopping the immune response after an antigen has been successfully inactivated or destroyed. This helps prevent uncontrolled or unnecessary immune system activity ; most of the T cells enlisted to fight in a particular immune response are dead within a few days. However, a few members of each clone are long-lived memory cellsthat remain behind to provide the immunological memory for each antigen encountered and enable the body to respond quickly to its subsequent invasions.

Vaccination

process of vaccination has been used for many years to cause acquired immunity against specific diseases. A person can be vaccinated by injecting dead organisms that are no longer capable of causing disease but which still have their chemical antigens. This type of vaccination is used to protect against typhoid fever, whooping cough, diphtheria, and many other types of bacterial diseases.

Also, immunity can be achieved against toxins that have been treated with chemicals so that their toxic nature has been destroyed even though their antigens for causing immunity are still intact. This procedure is used in vaccinating against tetanus and other similar toxic diseases. And, finally a person can be vaccinated by infection with live organisms that have been "attenuated." That is, these organisms either have been grown in special culture media or have been passed through a series of animals until they have mutated enough that they will not cause disease but do still carry the specific antigens. This procedure is used to protect against poliomyelitis, yellow fever, measles, smallpox, and many other viral diseases.

Passive Immunity

All the acquired immunity has been active immunity. That is, the person s body develops either antibodies or activated lymphocytes in response to invasion of the body by a foreign antigen. However, temporary immunity can be achieved in a person without injecting any antigen whatsoever. This is done by infusing antibodies, activated T cells, or both from someone else or from some other animal that has been actively immunized against the antigen. The antibodies will last for 2 to 3 weeks, and during that time the person is protected against the invading disease. Activated T cells will last for a few weeks if transfused from another person, and for a few hours to a few days if transfused from an animal. The transfusion of antibodies or lymphocytes to confer immunity is called passive immunity.

Allergy

One of the important side effects of immunity is the development, under some conditions, of allergy. There are several different types of allergy, some of which can occur in any person, and others that occur only in persons who have a specific allergic tendency

An Allergy that Occurs in Normal People; Delayed-reaction Allergy

This type of allergy frequently causes skin eruptions in response to certain drugs or

chemicals, particularly some cosmetics and household chemicals, to which one s skin is

often exposed. Delayed-reaction allergy is caused by activated T cells and not by

antibodies.

Allergies in the "Allergic" Person

Some persons have an "allergic" tendency. Their allergies are called atopic allergies

because they are caused by a non ordinary response of the immune system. The allergic

tendency is genetically passed on from parent to child, and it is characterized by the

presence of large quantities of IgE antibodies.

Autoimmune Diseases

When the immune system loses its ability to distinguish self-antigen while stile recognize and attack foreign antigens, the body produces antibodies (auto antibodies) and sensitized cells that attack and damage its own tissues. Most common immune diseases are: type I diabetes mellitus which destroys pancreatic beta cells, and Rheumatoid arthritis which systematically destroys joints.

Immunodeficiency

The most important acquired immunodeficiency is acquired immune deficiency syndrome [AIDS] which is caused by a virus transmitted in blood, semen, vaginal secretions, and saliva. The virus named HIV (human immunodeficiency virus), specifically targets and destroys helper T cells, resulting in depression of cell-mediated immunity.