Sentinels That Protect Your Health

“MY DEAR MADAM,” said the doctor as he examined the results of blood tests, “your immunologic defenses are quite low.” Veronica had not been feeling well for some time. Repeated bouts of bronchitis had debilitated her, and recently she had also had an ear infection and an irritating sinus condition.

What are the immunologic defenses, and why are they so important? How do they work?

Protected From Attack

The immune system involves an intricate network of molecules and specialized cells that work in close cooperation to fight infection. We rely upon our immune system to defend us from attack by foreign invaders, such as bacteria or viruses.

To illustrate, we could compare the body to an ancient city. A typical city may have been located on high ground so that any hostile armies could be seen far away. And the city was protected by an array of walls and gates, manned by guards and sentinels. With such defenses, the city remained a safe place of dwelling. If we compare our body to such a city, we can better understand what is required to defend it from attack.

For our bodies, the first line of defense against invasion by germs is made up of the skin and the mucous membranes (for example, those that line the nose and the throat). Our skin acts as an important physical barrier. Many of the billions of germs that we have on our skin surfaces are eliminated when they are shed with the outer layers of skin.

The mucous membranes are not as tough as skin and are more vulnerable. However, they contain many natural substances that fight germs. One such substance, called lysozyme, is found in tears, saliva, and sweat.  While the mere acidity of sweat is sufficient to impede the growth of many germs, lysozyme kills them by destroying their cell walls. For that reason, an animal can help heal its wounds simply by licking them.

Primary Sentinels—White Blood Cells

Let us imagine that bacteria capable of causing disease manage to penetrate our “city” through a wound or by contagion. An army of cells immediately goes into action, with but one purpose—elimination of the invading germ and consequent recovery from illness. The cells that fight to defend the body are called leukocytes, or white blood cells. Three important types of white blood cells in this stage of the struggle are monocytes, neutrophils, and lymphocytes.

When monocytes “hear” chemical signals indicating inflammation in a certain zone, they leave the bloodstream and penetrate the stricken tissue, where they become macrophages, that is, “big eaters.” There they devour all that is foreign to the organism. In addition, they secrete important substances called cytokines, which prepare the body to fight the infection. Among their functions, the cytokines provoke fever. Fever is a useful phenomenon in that it is a sign that defensive mechanisms have gone into action. It can accelerate the healing process and also function as a useful diagnostic indicator.

Next, neutrophils “hear” the chemical signal from the inflamed zone and dash to help the macrophages. They too engulf, or swallow, bacteria. When these neutrophils die, they are expelled from the body as pus. Thus, the formation of pus is another type of defense. In this case, the Latin expression used by doctors for centuries would apply: pus bonum et laudabile. This means “good and praiseworthy pus.” Its formation helps to stem infection. After digesting the germs, our friends the macrophages “present,” or display, fragments of the germ to the lymphocytes to warn them of the invader.

The lymphocytes make up a superspecialized elite in the fight against infection. They produce substances called antibodies, which bind specifically to a particular germ fragment. There are two principal teams of lymphocytes with differing abilities. First are the B cells, which release the antibodies that they produce into the bloodstream. The B cells have been called the armed corps of the immune response, and they shoot their arrows, the antibodies, with extreme precision. These antibodies will “seek” the germ they recognize and will strike a vital site on the germ. The other principal team of lymphocytes, the T cells, keep the antibodies that they recognize anchored to their surface. They use them to strike the enemy—engaging in hand-to-hand combat, so to speak.

The story becomes even more complex. A subgroup of T cells, called helper T cells, help their companions, the B cells, to secrete large quantities of antibodies. Before the attack, the helper T cells communicate with one another. Recent research has shown that by means of chemical signals, these cells “talk” excitedly among themselves, exchanging information on the foreign agent, in what has been called vibrant conversation.

Help is lent by another important group, the natural killer cells. These do not produce antibodies, but they are ready to kill cells that  have become “foreign” because of being infected. So natural killer cells too contribute to guarding the integrity of the body.

Finally, by virtue of their immunologic memory, lymphocytes are capable of remembering the characteristics of a germ, as though they had a record of it on file. So if ever that type of germ should reappear, these lymphocytes already have specific antibodies to destroy it immediately.

The macrophages, cells that activate the immune response, also help complete the job by staying around to assist in quenching inflammation. They free the affected area of all the dead cells, cell fragments, or debris left on the “battlefield” after the fight, restoring quiet and order to the “city.”

When Defenses Are Low

The foregoing is but a basic outline of how the immunologic defenses are thought to work. But defenses can be low for several reasons: There can be primary defects innate to the immune system and secondary defects acquired in the course of one’s life because of contracting diseases.

One of the most serious of these diseases is AIDS, the fearsome pandemic that exploded during the 1980’s. It is caused by the human immunodeficiency virus (HIV), which can strike at the heart of the immune system, progressively destroying a particular class of lymphocytes. An extremely important part of the individual’s defenses is thus incapacitated. After that, infections recur and are never completely eradicated. In fact, they get worse, and the body is left without the means for defending itself. It is like a city in ruins, without walls, that can be conquered by anyone.

Fortunately, not all immunologic deficits are of such a serious nature. Veronica, mentioned at the outset, had a minor defect in the production of one kind of antibody that is usually present in the mucous membranes, particularly along the airways. That explained the repeated and persistent infections she had been suffering.

Veronica got better. After listening to her doctor’s explanation, she decided that she would scrupulously follow the therapy he prescribed. When she got over her sinusitis, she agreed to a course of injections that would stimulate the production of antibodies. * She also stopped smoking and managed to get more rest. Shortly thereafter, her health improved considerably.

Yes, we have been designed to enjoy life in good health. When we reflect on the amazing complexity of the immune system and other intricate mechanisms of the human body, we are moved to admiration and gratitude for the wisdom of our Creator. (Psalm 139:14; Revelation 15:3) And while at this time because of human imperfection we do not always enjoy good health, God’s inspired Word assures us that in the new world soon to come, humans will be restored to perfection of mind and body, so that “no resident will say: ‘I am sick.’”—Isaiah 33:24.

[Footnote]

[Box on page 13]

LINES OF DEFENSE:

• SKIN AND MUCOUS MEMBRANES

• LEUKOCYTES, OR WHITE BLOOD CELLS

Monocytes penetrate stricken tissue and devour invading bacteria

Neutrophils help swallow bacteria and are expelled from the body as pus

Lymphocytes have immunologic memory; if the same type of germ reappears, antibodies will destroy it immediately

• B cells release antibodies like well-aimed arrows; these “seek” germs and attack them

• T cells help produce antibodies, which engage in “hand-to-hand” combat with germs

—Helper T cells help B cells secrete large quantities of antibodies

—Natural killer cells kill infected cells directly without producing antibodies

[Picture on page 15]

White blood cells attack bacteria

[Credit Line]

Lennart Nilsson