The Body Immune System

There are many different types of pathogenic microorganisms in our environment. When these organisms gain entry into our bodies, they can cause disease. The body's immune system works to eliminate these pathogens and minimize any damage they may cause.

Intact skin provides an effective barrier to penetration and represents one of the body's most important first lines of defense. When this defense fails a sophisticated immune system is called into action.

In order to launch an immune response the pathogen or foreign material must first be recognized as "nonself". Only then can an immune reaction be mounted. Human leukocyte antigens (HLA antigens) are proteins found on the surface of nearly all body cells. These proteins help the body recognize a cell or tissue as "self" rather than as foreign. Each person (with the exception of identical twins) has different HLA antigens.

There are two basic types of immune responses. These are known as the innate response and the specific or adaptive response. The primary difference between these two types of responses is that immune mechanisms are active against a wide range of pathogens while the adaptive mechanisms are active against one specific invader.

Innate immunity is provided by phagocytic cells that ingest foreign matter and digest it. Neutrophils or polymorphonucleocytes (PMNs) are the most frequently encountered short-lived phagocytic cells in the blood. They engulf foreign matter destroy it and then die.

Mononuclear phagocytes live longer. These cells arise in the bone marrow and become monocytes when they enter the bloodstream. Eventually these cells migrate out into tissues where they develop into macrophages. As already noted, Type A synoviocytes resemble tissue macrophages in both form and function. They ingest foreign particles and destroy them. They are also very effective at "presenting" foreign matter to other immune system cells.

Adaptive or specific immunity is provided by a different type of white blood cell known as a lymphocyte. These cells have receptors on their surface that are capable of binding to specific foreign protens (i.e. antigens) found on the surface of foreign cells. B-lymphocytes are genetically programmed to recognize one and only one antigen. When a B-lymphocyte encounters its specific antigen it rapidly differentiates into a plasma cell which manufactures antibodies.

Another cell type that participates in the specific immune response is the T-lymphocytes and T-lymphocyte. There are several different types of T-lymphocytes and each performs a different function. T-helper (TH) cells interact with B-lymphocytes to help them divide and differentiate into plasma cells. They also interact with mononuclear phagocytes to help them destroy intracellular pathogens. T-cytotoxic (TC) cells pay an important role in killing host cells that are infected with viruses.

Many of the effects of T-lymphocytes are accomplished through the release of chemical messengers called cytokines. These include interferons, interleukins colony stimulating factors and other cytokines such as tumor necrosis factor.

Another important immune system component is complement. The complement system is a group of about 20 serum proteins that help control inflammation. Complement proteins interact with each other and with other elements of the immune system to facilitate phagocytosis and attract phagocytic cells to sites of infection. They also increase blood flow in inflamed areas and increase the permeability of capillaries to large plasma proteins. Complement can "punch a hole" in the cell membrane of a foreign invader, and can also interact with other immune system cells to enhance the release of inflammatory substances.

The different components of the immune system work together to generate a well-orchestrated attack on foreign invaders. Unfortunately, this system sometimes fails and actually produces disease.

As we have noted, the various immune system components normally react to the presence of foreign proteins (Antigens) These same cells generally ignore the body's own proteins because they are recognized as "self". Occasionally, an error occurs that causes immune system components to react to the body's own proteins. This is the basis for autoimmune disease.