What is the immune system?

The immune system is the body’s natural guardian against disease. It consists of a complex network of specialized cells that has evolved to defend the body against attacks by ‘foreign’ invaders such as bacteria, viruses, fungi and parasites; it can also fight cancers. The battle between these various insults and our immune system is continuous. Without an immune system, we would very quickly succumb to disease.

The immune system, which equals in complexity the intricacies of the brain and nervous system, displays several remarkable characteristics. It can distinguish between ‘self’ and ‘nonself.’ It is able to remember previous experiences and react accordingly; thus, once you have had chicken pox, your immune system will prevent you from getting it again. The immune system displays both enormous diversity and extraordinary specificity; not only is it able to recognize many millions of distinctive nonself molecules, it can produce molecules and cells to match up with and counteract each one of them. And it has at its command a sophisticated array of weapons.

The success of this system in defending the body relies on an incredibly elaborate and dynamic regulatory-communications network. Millions and millions of cells, organized into sets and subsets, pass information back and forth like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produces an immune response that is prompt, appropriate, effective and self-limiting.

At the heart of the immune system is the ability to distinguish between self and nonself. Virtually every cell in our body carries distinctive molecules that identify it as self. The body’s immune defenses do not normally attack tissues that carry a self marker. Rather, immune cells and other cells coexist peaceably in a state known as self-tolerance. But when immune defenders encounter cells or organisms carrying molecules bearing a ‘foreign’ signature, the immune troops move quickly to eliminate the intruders. These foreign molecules are known as antigens.

In abnormal situations, the immune system can wrongly identify self as nonself and execute a misdirected immune attack. The result can be an autoimmune disease such as type 1 diabetes, multiple sclerosis or rheumatoid arthritis.

Generating an immune response

What does it mean when we say our body has generated an immune response? This question can be answered by looking at the chain of events that occurs when we encounter a microbe. First, the microbe attempting to enter the body must find a chink in the body’s external protection: the skin and the mucous membranes that line the body’s portals pose a physical barrier that must be breached.

Next, invaders must elude a series of nonspecific defenses — various kinds of immune cells and substances equipped to tackle infectious agents without regard for their antigenic peculiarities. Many potential infections are cut short when microbes are intercepted by patrolling scavenger cells (such as a macrophage) or disabled by immune system chemicals (such as cytokines).

The nonspecific components of the immune system can eventually rally support from the tremendous arsenal of cells that are capable of recognizing highly specific antigenic structures. These cells are known as T cells and B cells and they comprise the two arms of the immune system, known generally as cell-mediated immunity and humoral immunity, respectively.

T and B cells are of enormous variety — in fact, many tens of millions of unique versions of each exist. These cells are equipped with receptors that enable them to detect the entire universe of antigens associated with pathogens or cancers. The T and B cells comprise the ‘specific’ components of the immune system.

T cells that have never engaged in battle are not able to directly attack target cells bearing a foreign signature. Rather, they first must be trained by specialized immune cells called dendritic cells. These cells use threadlike tentacles to enmesh antigens, which are subsequently digested, processed and presented to T cells along with an activation signal that readies the T cell for battle. At this point, T cells can interact directly with their targets, attacking cells that have been commandeered by viruses and certain bacteria or altered due to malignancy.

B cells work chiefly by secreting soluble substances called antibodies into the body's fluids. Antibodies typically interact with circulating antigens such as bacteria and toxic molecules, but are unable to penetrate living cells. Once an antibody has contacted an antigen, other immune cells and factors are recruited to eliminate the threat.

When we say that the body has generated an effective immune response, we are referring to the totality of immune activities — both nonspecific and specific immune responses that typically come into play to control infections and cancers.