The skin is an active element of the immune system. It is, in fact, the first line of defense against foreign invaders. More than just a passive protective covering, it has cells that warn the immune system of invading microorganisms. Billions of friendly bacteria live on the skin—in some places nearly 20 million per square inch. Certain ones produce fatty acids that hinder growth of harmful kinds of bacteria and fungi.

Joining the skin as a part of the body’s protective covering are the membranes that line the internal surfaces of the body. These membranes secrete mucus that traps microbes. Saliva, nasal secretions, and tears contain microbe-killing substances. Hair-like cilia in the air passages leading to the lungs push mucus and debris into the throat, where they can be eliminated by sneezing and coughing. If any invaders reach the stomach, they are either killed by the acids there, broken down by digestive enzymes, or trapped in the mucus that lines the stomach and the intestines. Eventually, they are evacuated along with other body waste.

However, some alien organisms are able to breach these outer defenses and enter the bloodstream and body tissues or fluids. They have invaded the territory of the big guns of the immune system—the white blood cells, two trillion strong. Born in the bone marrow—about a million every second—they emerge to mature and form three distinct divisions: phagocytes and two kinds of lymphocytes, namely, T cells (three major kinds—helper, suppressor, and killer cells) and B cells.

Now, the immune system may have a trillions-strong army, but each soldier can fight only one class of invader. During a disease millions of germs can be generated, and every one of those germs will have the same kind of antigen. But different diseases, even different varieties of the same disease, have different antigens. Before the T cells and the B cells can attack these invaders, they must have receptors that can bind to their particular antigens. Hence, among the T cells and the B cells, there must be many different receptors, receptors specific for the antigens of each and every different disease—but each individual T cell and B cell has receptors that are specific for only one disease antigen.

The immune system is designed to recognize foreign invaders. To do so it generates on the order of 10 ¹¹ (100,000,000,000) different kinds of immunological receptors so that no matter what the shape or form of the foreign invader there will be some complementary receptor to recognize it and effect its elimination. Thus, there are groups of T cells and B cells that, among them, can match every disease antigen that enters our body—just as a key fits a lock.

Like locks with their keyholes, millions of germs with their antigens invade your body and circulate through your bloodstream and lymph system. Like millions of keys, your immune cells with their receptors also circulate there and fit onto the matching antigens of the germs.

Each category of lymphocytes has its special role to play in the fight against infection. The helper T cells (one of the three major T cells) are crucial. They are the ones that orchestrate the various reactions of the immune system, directing the battle strategy. Triggered by the presence of enemy antigens, the helper T cells by chemical signals (proteins called lymphokines) rally the troops of the immune system and increase their ranks by the millions.

Phagocytes are the scavengers of the immune system. Their name means ‘eating cells.” Phagocytes are of two kinds, neutrophils and macrophages. The bone marrow pours out some one hundred billion neutrophils a day. They live only a few days, but during an infection, their numbers skyrocket, increasing fivefold. Each neutrophil may engulf and destroy up to 25 bacteria and then die, but replacements come in a steady stream. Macrophages, on the other hand, may destroy a hundred invaders before they expire. They are bigger, tougher, and live longer than the neutrophils. They respond in only one way both to invaders and to trash—they eat them.

When the macrophage ingests an enemy microorganism, however, it does more than just eat it. Like virtually all body cells, on its surface it carries the MHC molecules that identify it as self. But when the macrophage eats a germ, the MHC molecule draws out and displays a fragment of this enemy antigen in one of the grooves on its surface. This strip of antigen then acts as a red flag to the immune system, sounding the alarm that a foreign organism is on the loose inside of us. By sounding this alarm, the macrophage is calling for reinforcements, more macrophages, millions of them! And this is where the helper T cell comes in. Billions of them are milling around in the body, but the macrophage must recruit a specific kind. It needs one with the kind of receptor that will fit onto the particular antigen that the macrophage is displaying.

Once this kind of helper T cell arrives and connects to the enemy antigen, macrophage and helper T cell exchange chemical signals. These hormone like chemicals, or lymphokines, are extraordinary proteins that come with a bewildering array of functions to regulate and boost the immune system’s response to disease germs. The result is that both macrophage and helper T cell begin reproducing themselves prodigiously. This means more macrophages to eat more of the invading germs and more of the right kind of helper T cells to latch onto the antigens those macrophages will display. Thus the ranks of the immune forces explode, and hordes of these particular disease germs are destroyed.