Immune System; Immunology; Cell Populations

Immunology Cell Groups

Immune cells are found circulating in the blood stream or patrolling in the intercellular spaces of all body tissues. Immune networks are also distributed in discrete organs of the lymphatic system which includes lymph nodes, tonsils, liver and spleen. Bone marrow is the major manufacturing area of immune cells.

The thymus is the organ which originates one of two populations of immune cells, T-lymphocytes, at least in early life. In later life the thymus continues to exert regulatory influences on the immune system. Immunity means that immune cells remember the identity of an antigen challenge and initiate a successful defensive response. Immunizing injections contain antigens which belong to the infecting organisms. The first response to the injected antigen is the activation of antigen-specific lymphocytes who proliferate into clones of antigen-specific cells. These immune clones can later identify and attack the infecting organism. Several exposures to the vaccine (antigen) boost immune memory to an effective level of vigilance against the infecting organism.

We are really a community of cells in prodigious array. Lymphocytes are important immune players and to get an idea of the size of immune populations, think of a young tadpole as containing about one million lymphocytes. Human immune networks contain about 10 trillion cells.

Some of our cells stay in place and do more or less predictable things. Immune cells tend to wander around and, like bees, forage in our various body parts looking for items of interest. An appropriate image of immune networks would be the foraging and swarming of ants, each moving about, with different job descriptions in the colony and a meta order achieved by the collective behavior of many individuals. The overall activity of the hive or colony decides how the society or system looks and acts. They have the property of getting excited, recruiting their peers and attacking interlopers.

The Cells of Immune Networks

Immune cells circulating in the blood are collectively referred to as white blood cells. White blood cells (WBC) are routinely counted and recognized on stained blood smears, one the most common medical lab tests. WBC groups include lymphocytes, neutrophils, eosinophils, basophils, and monocytes. These cells are made in the bone marrow and the lymphatic tissues (lymph nodes, thymus, spleen, and gut-associated lymphatic tissue).

Some cells identify the characteristic molecular shape of an antigen and respond by proliferation. A virus, for example, will have protein identifiers stuck to its capsule which mark it, much like an ID badge. Several cell populations will emerge when the virus is present to identify and combat it. One cell group manufactures antibody, a protein specific to one antigen. Molecules in the food supply, especially proteins, act as antigens. Food additives and contaminants may increase the probability of allergic reactions to food. Immune cells do not know the difference between viral, bacterial or food antigens. We expect and regularly observe similar immune reactions to different antigen sources.

A diverse population of cells present antigens to other immune cells, initiating immune responses. These antigen-presenting cells are found, for example, on the surface of blood vessels especially in the liver and spleen. These cells act as monitors and filters of the blood stream. In tissues these cells are called macrophages.

Lymphocytes are the cells which recognize antigens presented by macrophages, proliferate after contact with antigens, attack antigen-labelled cells, and manufacture antibodies. These cells belong to many different functional groups which interact in a complex way, resembling a large military organization. Some T-Lymphocytes (Killer cells or K-cells) act directly to attack cells identified by specific antigen and are responsible for cell-mediated delayed immune-injury.

Other T-lymphocytes act as controllers of the antibody-producing cells, the B-Lymphocytes. Controller T-cells fall into groups which remember how to respond (memory cells) and other groups which enhance immune response (helper cells). Helper cells secrete cytokines - activation and growth factors such as interleukins which stimulate other cells to act and proliferate. B-lymphocytes are transformed to make antibodies to specific antigens.

B-lymphocytes leave the bloodstream and enter tissues where they transform into antibody producers, called plasma cells. Other immune cells directly attack foreign cells or infecting organisms, removing them from the body space. Improperly labeled cells, such as tissue transplanted into a body, will be rejected and destroyed. Transplant surgery is made possible by matching donor and recipient as closely as possible for the same cell labels, known as histocompatibility antigens.

Imagine (but do not do) an experiment: make a small incision in the skin of your arm and place a few fibers of meat (eg.scraping from the surface of a steak) in the incision; tape the incision closed and cover with a protective dressing; observe at daily intervals. The meat fibers include muscle cell components from an animal whose tissue is foreign to yours. Many of these cell components will act as antigens. Immune cells will swarm around the foreign cells and, within 48 hours, local inflammation will set in. The small wound will swell and fester. The typical signs of inflammation, redness, swelling, pain, heat will persist for many days, leaving a scarred lump behind as a permanent reminder of the event. This experiment would illustrate the transplant rejection reaction. Similar zones of inflammation can be set up in your tissues by meat-fiber antigens arriving via an internal route.