We don’t often think about it, but our bodies constantly are under attack by millions of unseen enemies — viruses, bacteria, fungi, parasites and other disease-causing pathogens that can invade our bodies and cause disease. Fortunately, most of us have a huge army with an effective arsenal on constant alert that is ready to attack these intruders. That army is the immune system.
Immunology is the study of the immune system, which is composed of a complex network of cells, tissues and organs that employs an elaborate and sensitive communications network. A variety of white blood cells (leukocytes) and antibodies continuously are on the look-out for pathogens, ready to spring into action and alert the rest of the troops by sending out chemical messages through the bloodstream. To work effectively, most immune cells need the cooperation of their comrades. Sometimes immune cells communicate by direct physical contact, other times they do it by releasing chemical messengers.
By understanding what all these cells and molecules do and how they communicate with each other, immunologists can find ways to help the immune system to do its job better (for example, by developing new vaccines). They also can work out ways of stopping the immune system when it is makes a mistake and reacts to an inappropriate target, as happens in allergy (a hyperreaction to pollen or other allergens) and in autoimmune diseases (when the immune system reacts against your own body).
Our bodies protect us from infections by using three basic strategies:
- By maintaining nonspecific barriers that prevent bacteria and viruses from getting a foothold. These include skin, mucus secretions in the respiratory and gastrointestinal tracts, saliva, tears and stomach acid. Within the intestines, the beneficial bacteria that reside there and help digest certain types of food (called the normal flora) compete with pathogenic bacteria for food and space and reduce the chances that disease-causing pathogens will multiply and cause disease.
- By employing our innate immune system, which detects invaders that circumvent the first-line barriers. The cells and other mechanisms involved in innate immunity recognize and respond to pathogens in a nonspecific way and do not confer any long-lasting or protective immunity. They primarily are phagocytic (“eating”) cells, which engulf and digest a pathogen or particle. Macrophages, polymorphonuclear cells, dendritic cells and mast cells all are types of phagocytic leukocytes. A nonphagocytic member of the innate immune system (the part of the immune system equipped with specialized cells that detect and often eliminate a bodily invader before it is able to reproduce) is the natural killer cell (NK). The NK does not attack a pathogen directly, but instead destroys infected cells. NK cells also attack tumor cells.
- By activating a specific adaptive immune response against pathogens if they sneak past the barriers and the phagocytes and begin to multiply. The basis of adaptive immunity lies in the remarkable ability of our lymphocytes to distinguish between the body’s own cells (self) and the unwanted invaders (nonself) because of distinctive self markers on our cells. When our immune defenders encounter organisms or cells with markers that say “foreign,” they quickly launch an attack. It is through the adaptive immune response that the immune system gains the ability specifically to recognize a pathogen and to mount an even stronger and quicker attack each time the pathogen is encountered. In humans, the primary self markers are known as HLA.
Anything that can trigger an immune response is called an antigen. An antigen can be a microbe such as a virus, or even a part of a microbe. Tissues or cells from another person (except an identical twin) also carry nonself markers and act as antigens. This explains why tissue transplants may be rejected.
The two main types of lymphocytes — B cells and T cells — are the key players in our immune system. B cells work chiefly by secreting substances called antibodies into the body’s fluids. Antibodies ambush antigens circulating in the bloodstream. They are powerless, however, to penetrate cells. The job of attacking target cells — either cells that have been infected by viruses or cells that have been distorted by cancer — is left to T cells or other immune cells.
Unlike B cells, T cells do not recognize free-floating antigens. Rather, their surfaces contain specialized antibodylike receptors that see fragments of antigens on the surfaces of infected or cancerous cells. T cells contribute to immune defenses in two major ways: some direct and regulate immune responses; others directly attack infected or cancerous cells.
Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. Some stimulate nearby B cells to produce antibodies, others call in phagocytes and still others activate other T cells.
Killer T cells, also called cytotoxic T lymphocytes or CTLs, perform a different function. These cells directly attack other cells carrying certain foreign or abnormal molecules on their surfaces by attaching to them and injecting a lethal burst of chemicals that they safely carry within little membrane “bags” called granules. CTLs are especially useful for attacking viruses that are growing inside infected cells. CTLs recognize small fragments of these viruses peeking out from the cell membrane and launch an attack to kill the cell.
Some of the B cells and T cells involved in the attack on a foreign invader go on to become long-lived memory cells. At a second encounter with the invader, they can reproduce to mount a faster and stronger immune response. This phenomenon is the scientific basis for vaccination.
The various cells of the immune system produce and secrete a number of chemicals that signal other cells that an invader is present and help activate an immune response. These include histamine (which dilates blood vessels, causes the characteristic signs of inflammation and recruits neutrophils and macrophages), interferons (which are released when viruses and tumor cells are encountered) and interleukins (which are vital to the functions of the immune system).
Subdisciplines of Immunology
This involves the study of the genetics (inheritance) of the immune response — for example, the study of the blood groups Rh and ABO or the HLA system important to kidney and other transplants. The field also examines the genetic control of the individual’s ability to respond to an antigen.
This is the study of diseases caused by the immune system and diseases of the immune system from a medical perspective. Most of these diseases fall into three categories: immunodeficiency, in which parts of the immune system fail to provide an adequate response; autoimmunity, in which the immune system attacks its own cells; and hypersensitivity, in which the immune system responds inappropriately to harmless compounds (allergies and asthma) or responds too intensely. AIDS (acquired immunodeficiency syndrome) is a prime example of a disease that affects the immune system. It is characterized by a lack of T helper cells and macrophages, which are destroyed by HIV (human immunodeficiency virus).
Clinical immunologists also study ways to prevent the rejection of organ transplants and ways to boost the immune system to fight off cancer.
This primarily involves the study of the molecules secreted by cells that control the movement and activity of immune cells. Such molecules include substances that attract cells to a site of infection and initiate the inflammation process.
This involves the study of T cells.
This concentrates on the study of B cells and the antibodies they produce.