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sagar90

If I could have been a spectator at the viral invasion that set off my sniffling and sneezing, what would I have seen?

I would have taken comfort, first off, in knowing that of the one hundred trillion cells that make up my body, one in every hundred is there to defend me. 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 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. 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.

In my battle with the cold virus I see a troop of patrolling phagocytes happen upon remnants of a cell burst open by the fast-replicating rhinovirus 14. With gusto they gobble up the wreckage, consuming viruses in the process. But my phagocytes cannot destroy the foes fast enough to keep them from infecting nearby cells.

Now I observe a special kind of phagocyte called a macrophage. As the macrophage engulfs a stray rhinovirus, 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 specialized class of lymphocytes, the T cells. All my life a small contingent of those lymphocytes has circulated through my body, waiting for this particular cold virus. They recognize it, as the virus identified its victim among my cells, by shape. The antigens on the surface of the virus?the peaks Rossmann pointed out?fit exactly into these T cells' receptors.

How did that particular group of T cells know the shape of the rhinovirus 14 antigen? Their training takes place in the thymus, a mysterious pale gray 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 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.

?Most T cells die in the thymus,? Mark Davis of Stanford University told ?We don't know why. One 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 recognizes each one. Remarkably, we have T cells trained to recognize 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 recognized 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 my body?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 fray. They also rush toward the spleen and the lymph nodes. There they will alert the last major regiment of my 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 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 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 of my B cells can churn out thousands of antibodies a second.

As my immune defenses gather, the tide of my battle with rhinovirus 14 turns. 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.

I am a bio student and its my duty to tell u this and u should also know this.

if u dont understand anything of it thn nudge me.

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