Every minute of every day wars rage within our bodies. The combatants are too tiny to see, yet they employ tactics that can vanquish the much larger cells they swarm upon. We have evolved legions of defenders, specialized cells that silently fights the unseen enemy. These warriors sometimes mistake harmless invaders for deadly foes, thereby mounting an allergic reaction or are caught unprepared, and we get sick. Occasionally some of our own cells become rebellious, producing cancer cells, but for every successful penetration of our defenses, thousands of attempts are repelled. We sleep securely, trusting the invisible vigilantes of our immune system but unfortunately, a nightmare crept into our peaceful sleep – AIDS, a deadly scourge that relentlessly disarms the immune system.
Of all the body’s enemies, bacteria, protozoa, fungi and viruses, virus is both the simplest and the most devious. A virus is a protein-coated bundle of genes containing instructions for making identical copies of itself. A virus in its own self is not alive, but when a it slips inside one of our cells, that bundle of genetic information works like our cell’s DNA, issuing its own instructions. The cell becomes a virus factory, producing new, identical viruses. Eventually they may rupture the cell, killing it. The viral clones then fan out to invade nearby cells declaring war on the immune system.
Not to worry, of the innumerable cells that make up the body, one in every hundred is there to defend. 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 defense.
The first defenders are 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. We can watch phagocytes at work when our skin is injured. Skin is our first defense 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.
A special kind of phagocyte called a macrophage plucks 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 specialized class of lymphocytes, the T cells.
The T cells recognizes the flag by shape, the antigens on the surface of the virus fits exactly into these T cells’ receptors. The T cells are formed in the thymus – a pale gray gland that sits behind the breastbone, above the heart. Somehow, as the T cells mature in the thymus, one learns to recognize the antigens of, say, the hepatitis virus, another to identify a strain of flu antigens, a third to detect rhinovirus 14, and so on.
Nature can create antigens in hundreds of millions of different shapes. Remarkably, we have T cells trained to recognize them even artificial antigens created in the lab.
The T cells – helper T’s scouts the body for antigens. They carry no weapons, rather they send urgent chemical signals to a small squadron of allies—the killer T cells. The message: Multiply fast!. Like all T cells, killer T’s are trained to recognize 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 fight. 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, with many of them concentrating in the lymph nodes. These small bean-shaped capsules are scattered along the intricate branchings 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 a small weaponry, 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 invader cells locking on to their 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 their operation each of the B cells can churn out thousands of antibodies a second.
As the immune defenses gather, 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. But a large contingent will lead long lives. Before the invasion, only a few sentries were trained to spot the invader’s antigen, now a virtual army of so-called memory cells can. Any other new invader virus may catch the immune system off-guard but should a known invader virus attack again, they’ll stand no chance, they will be defeated. The body is immuned.
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. 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 mobilize 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 then run free.