After completing this lesson, you will be able to:
· describe the basic structure of the immune system of the human body;
· outline the activities of the immune system
· define immune deficiency; and
· identify the role of HIV in human immune deficiency;
Every minute of every day wars rage within our bodies. The combatants are too tiny to see. Some, like the infamous virus that causes AIDS, or acquired immune deficiency syndrome, are so small that 200 million would fit on the tip of a needle. Yet they employ tactics that can kill much larger cells they swarm upon.
Usually we never even notice the battles in the incessant wars within us. We have evolved legions of defenders, specialised cells that silently rout the unseen enemy. Sometimes these warriors mistake harmless invaders, such as pollen, for deadly foes, and they mount an allergic reaction. Sometimes our defenders are caught unprepared, and we develop a cold, the flu, or worse. Occasionally some of our own cells begin the mutinous proliferation of cancer and manage to evade the surveillance of our body's defence forces. But for every successful penetration of our defences, thousands of attempts are repelled. We sleep securely, trusting the invisible vigilantes of our immune system.
The immune system of the human body is made up of group of cells, molecules, and organs that act together to defend the body against foreign invaders that may cause disease. The health of the body is dependent on the immune system’s ability to recognise and then repel or destroy these invaders.
In humans the immune system consists of about a trillion (1012) cells called lymphocytes and about 100 million trillion (1020) molecules called antibodies that are produced and secreted by the lymphocytes. The special capability of the immune system is pattern recognition and its assignment is to patrol the body and guard its identity.
White blood cells are the mainstay of the immune system. Some white blood cells, known as macrophages, play a function in innate immunity by surrounding, ingesting, and destroying invading bacteria and other foreign organisms. Lymphocytes are specialised white blood cells whose function is to identify and destroy invading antigens. All lymphocytes begin as “stem cells” in the bone marrow, the soft tissue that fills most bone cavities, but they mature in two different places. Some lymphocytes mature in the bone marrow and are called B lymphocytes. B lymphocytes, or B cells, make antibodies, which circulate through the blood and other body fluids, binding to antigens and helping to destroy them. Other lymphocytes, called T lymphocytes, or T cells, mature in the thymus, a small glandular organ located behind the breastbone. Mature lymphocytes constantly travel through the blood to the lymphoid organs and then back to the blood again. This recirculation ensures that the body is continuously monitored for invading substances.
Immune
Deficiency
Deficiencies in immune function may be either inherited or acquired. Inherited immune deficiencies usually reflect the failure of a gene important to the generation or function of immune system components. DiGeorge syndrome is an inherited immune disorder in which a person has no thymus and, therefore, cannot produce mature T lymphocytes. People with this disorder can mount only limited humoral immune responses, and their cell-mediated immune responses are severely limited. The most extreme example of a hereditary immune deficiency is severe combined immunodeficiency (SCID). Individuals with this disease completely lack both T and B lymphocytes and thus have no adaptive immune responses. People with SCID must live in a completely sterile environment, or else they will quickly die from infections.
Acquired immune deficiencies can be caused by infections and also other agents. For example, radiation therapy and some kinds of drugs used in treating disease reduce lymphocyte production, resulting in damaged immune function. People undergoing such therapies must be carefully monitored for lowered immune function and susceptibility to infections. Environmental and lifestyle factors, such as poor nutrition or stress, can also affect the immune system’s general status.
An infectious agent resulting in fatal immune deficiency is the human immunodeficiency virus (HIV). This virus causes acquired immunodeficiency syndrome (AIDS) by infecting and eventually destroying helper T cells. Because helper T cells regulate all immune responses, their loss results in an inability to make adaptive immune responses. This complete lack of immune function makes individuals with AIDS highly susceptible to all infectious agents.
Activity
of the Immune System
Of the one hundred trillion cells that make up a human body, one in every hundred is there to defend us. They are the white blood cells that are born in the bone marrow. When they emerge, they form three distinct regiments of warriors—the phagocytes and two kinds of lymphocytes, the T cells and B cells. Each has its own strategies of defence. The first defenders to arrive would be the phagocytes—the scavengers of the system. Phagocytes constantly scour the territories of our bodies, alert to anything that seems out of place. What they find, they engulf and consume.
Phagocytes are not choosy. They will eat anything suspicious that they find in the bloodstream, tissues, or lymphatic system. In the lungs, for instance, they consume particles of dust and other pollutants that enter with each breath. They can cleanse lungs that have been blackened with the contaminants of cigarette smoke, provided the smoking stops. Too much cigarette smoking, over too long a time, destroys phagocytes faster than they can be replenished. Environmental pollutants like silica and asbestos also overwhelm them.
We can watch phagocytes at work when our skin is injured. The skin is our first defence line—until a cut allows bacteria and other microorganisms to invade. Immediately cells near the wound release substances that stimulate nearby blood vessels to dilate, causing swelling and reddening around the cut. Phagocytes flow in through the distended blood vessels, devouring the invaders. In time the body weaves threads of fibrin across the wound to restore the skin's barrier.
There is a special kind of phagocyte called a macrophage. As the macrophage engulfs a stray virus, it plucks a special piece, an antigen, from the invader. It displays that small piece on its own cell surface like a captured banner of war. That flag plays a critical role in the immune system's response: It alerts a highly specialised class of lymphocytes, the T cells. All our lives a small contingent of those lymphocytes has circulated through our bodies, waiting for this particular virus. They recognise it, as the virus identified its victim among the cells, by shape. The antigens on the surface of the virus fit exactly into these T cells' receptors.
How did that particular group of T cells know the shape of the antigen? Their training takes place in the thymus, a mysterious pale grey gland that sits behind the breastbone, above the heart. (The “T” in T cell stands for thymus-derived.) This unsung little gland swells in size from birth to puberty and then begins to shrink. Somehow, as the T cells mature in the thymus, one learns to recognise the antigens of, say, the hepatitis virus, another to identify a strain of flu antigens, a third to detect rhinovirus 14, and so on.
Most T cells die in the thymus, We do not know why. A guess is that the thymus is selecting only the best T cells, those with the sharpest powers of recognition. And what a staggering task the thymus confronts. Nature can create antigens in hundreds of millions of different shapes. The thymus must turn out a group of T cells that recognises each one. Remarkably, we have T cells trained to recognise even artificial antigens created in the lab—antigens the body has never encountered in its millions of years of evolution.
The thymus pumps out T cells by the tens of millions. Even though only a few of them may recognize any one antigen, the collective scouting force is vast enough to identify the almost infinite variety of antigens nature produces.
So diligent are our T cells that even desirable cells transplanted from one person to another are quickly recognised as foreign and destroyed. The process, called rejection, can defeat a lifesaving heart or kidney transplant unless surgeons use drugs to keep the immune system at bay.
The T cells that first detect antigens, known as helper T's, carry no weapons. Rather they send urgent chemical signals to a small squadron of allies in the body—the killer T cells. The message: Multiply fast!
Like all T cells, killer T's are trained to recognise one specific enemy. When alerted by the helper T's, the squadron reproduces into an army. The killer T's are lethal. They can trigger a chemical process that punctures the cell membranes of bacteria or destroys infected cells before viruses inside have time to multiply.
Besides summoning the killer T's, helper T cells call more phagocytes into the battle. They also rush toward the spleen and the lymph nodes. There they will alert the last major regiment of the immune system, the B cells.
B cells migrate after their birth in the bone marrow, with many of them concentrating in our lymph nodes. These small bean-shaped capsules are scattered along the intricate branching of the lymph system. We are aware of them only during certain infections, when they become swollen and sometimes painful to the touch. Our lymph nodes are small munitions factories, staffed by the B cells. Their product: the chemical weapons called antibodies.
By sticking to the surface of unwelcome cells, antibody molecules slow them down, making them easier targets—as well as more attractive ones—for phagocytes. Antibodies can also kill. Locking on to the enemy's antigens, which they precisely mirror in shape, the antibodies collect substances in the bloodstream called complement. When this complement comes together in the right sequence, it detonates like a bomb, blasting through the invader's cell membrane. At the peak of operation, each B cell can churn out thousands of antibodies a second. As the immune defences gather, the tide of battle turns. Normally within a week or so the invader is in retreat. Then the third member of the T-cell family takes over—the suppressor T, the peacemaker.
Suppressor T's release substances that turn off B cells. They order killer cells to stop the fight. Suppressor T's even command helper T's to cease and desist. The battle is won. In the aftermath phagocytes range over the area, cleaning up the litter of dead cells and spent substances. Tissue damage is repaired. The threat is over—but not forgotten. Most of the T and B cells recruited for battle die off within days of an infection.
There is one simple reason why the AIDS virus is so deadly. It kills the one lymphocyte most critical to the immune response: the helper T cell. Like Greeks hidden inside the Trojan horse, the AIDS virus enters the body concealed inside a helper T cell from an infected host. Almost always it arrives as a passenger in blood or semen. In the invaded victim, helper T's immediately detect the foreign T cell. But as the two T's meet, the virus slips through the cell membrane into the defending cell. Before the defending T cell can mobilise the troops, the virus disables it.
Some researchers believe the AIDS virus also may change the surface of helper T cells in such a way that they fuse together. That strategy makes it even easier for the virus to pass from cell to cell undetected.
Once inside an inactive T cell, the virus may lie dormant for months, even years. Then, perhaps when another, unrelated infection triggers the invaded T cells to divide, the AIDS virus also begins to multiply. One by one, its clones emerge to infect nearby T cells. Slowly but inexorably the body loses the very sentinels that should be alerting the rest of the immune system. Phagocytes and killer cells receive no call to arms. B cells are not alerted to produce antibodies. The enemy can run free!
By the late 1960s, it had become clear that stem cells give rise to two broad lineages of lymphocytes (as well as the other blood cells). One consists of the B cells, which originate in the bone marrow and produce antibodies that bind to foreign proteins and mark them for attack by other cells. They act against extracellular pathogens such as bacteria. The other, the T cells, arises in the thymus. T cells handle such intracellular pathogens as viruses in addition to such intracellular parasites as tuberculosis. T cells also secrete molecules known as lymphokines, which direct the activity of B cells, other T cells and other parts of the immune system.
Once formed, cells of both types migrate to the spleen, lymph nodes and intestinal lymphoid tissues. There they can encounter antigen, the molecular signature of microbial or viral invaders, and be called into action. Lymphocytes continuously circulate through the body's vascular and lymphatic systems, stopping periodically in the lymphoid organs as they patrol for foreign antigens.
Use the drama method to teach this lesson. Pupils should be assigned roles as invading germs and white blood cells. Let pupils act their roles based on a script developed from the basic content of this lesson.
In this lesson, we learned that
· The immune system of the human body is made up of group of cells, molecules, and organs that act together to defend the body against foreign invaders that may cause disease.
· The system consists of about a trillion (1012) cells called lymphocytes and about 100 million trillion (1020) molecules called antibodies that are produced and secreted by the lymphocytes.
· White blood cells also called lymphocytes are the mainstay of the immune system.
· Some lymphocytes mature in the bone marrow and are called B lymphocytes. Other lymphocytes, called T lymphocytes, or T cells, mature in the thymus, a small glandular organ located behind the breastbone.
· Deficiencies in immune function may be either inherited or acquired. Inherited immune deficiencies usually reflect the failure of a gene important to the generation or function of immune system components. Acquired immune deficiencies can be caused by infections and also other agents.
· An infectious agent resulting in fatal immune deficiency is the human immunodeficiency virus (HIV). This virus causes acquired immunodeficiency syndrome (AIDS) by infecting and eventually destroying helper T cells. Because helper T cells regulate all immune responses, their loss results in an inability to make adaptive immune responses. This complete lack of immune function makes individuals with AIDS highly susceptible to all infectious agents.