What is a low white blood cell count ?

***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.”