Immune System : Cytokines, Bacteria, Viruses and Parasites

by National Institute of Health

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Cytokines

Components of the immune system communicate with one another by exchanging chemical messengers called cytokines. These proteins are secreted by cells and act on other cells to coordinate an appropriate immune response. Cytokines include a diverse assortment of interleukins, interferons, and growth factors.

Some cytokines are chemical switches that turn certain immune cell types on and off.

One cytokine, interleukin 2 (IL-2), triggers the immune system to produce T cells. IL-2's immunity-boosting properties have traditionally made it a promising treatment for several illnesses. Clinical studies are ongoing to test its benefits in other diseases such as cancer, hepatitis C, and HIV infection and AIDS. Other cytokines also are being studied for their potential clinical benefit.

Other cytokines chemically attract specific cell types. These so-called chemokines are released by cells at a site of injury or infection and call other immune cells to the region to help repair the damage or fight off the invader. Chemokines often play a key role in inflammation and are a promising target for new drugs to help regulate immune responses.

Complement

The complement system is made up of about 25 proteins that work together to "complement" the action of antibodies in destroying bacteria. Complement also helps to rid the body of antibody-coated antigens (antigen-antibody complexes). Complement proteins, which cause blood vessels to become dilated and then leaky, contribute to the redness, warmth, swelling, pain, and loss of function that characterize an inflammatory response.

Complement proteins circulate in the blood in an inactive form. When the first protein in the complement series is activated - typically by antibody that has locked onto an antigen - it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder inserted into - and puncturing a hole in - the cell's wall. With fluids and molecules flowing in and out, the cell swells and bursts. Other components of the complement system make bacteria more susceptible to phagocytosis or beckon other cells to the area.

Mounting an Immune Response

Infections are the most common cause of human disease. They range from the common cold to debilitating conditions like chronic hepatitis to life-threatening diseases such as AIDS. Disease-causing microbes (pathogens) attempting to get into the body must first move past the body's external armor, usually the skin or cells lining the body's internal passageways.

The skin provides an imposing barrier to invading microbes. It is generally penetrable only through cuts or tiny abrasions. The digestive and respiratory tracts - both portals of entry for a number of microbes - also have their own levels of protection. Microbes entering the nose often cause the nasal surfaces to secrete more protective mucus, and attempts to enter the nose or lungs can trigger a sneeze or cough reflex to force microbial invaders out of the respiratory passageways. The stomach contains a strong acid that destroys many pathogens that are swallowed with food.

If microbes survive the body's front-line defenses, they still have to find a way through the walls of the digestive, respiratory, or urogenital passageways to the underlying cells. These passageways are lined with tightly packed epithelial cells covered in a layer of mucus, effectively blocking the transport of many organisms. Mucosal surfaces also secrete a special class of antibody called IgA, which in many cases is the first type of antibody to encounter an invading microbe. Underneath the epithelial layer a number of cells, including macrophages, B cells, and T cells, lie in wait for any germ that might bypass the barriers at the surface.

Next, invaders must escape a series of general defenses, which are ready to attack, without regard for specific antigen markers. These include patrolling phagocytes, NK cells, and complement.

Microbes that cross the general barriers then confront specific weapons tailored just for them. Specific weapons, which include both antibodies and T cells, are equipped with singular receptor structures that allow them to recognize and interact with their designated targets.

Bacteria, Viruses, and Parasites

The most common disease-causing microbes are bacteria, viruses, and parasites. Each uses a different tactic to infect a person, and, therefore, each is thwarted by a different part of the immune system.

Most bacteria live in the spaces between cells and are readily attacked by antibodies. When antibodies attach to a bacterium, they send signals to complement proteins and phagocytic cells to destroy the bound microbes. Some bacteria are eaten directly by phagocytes, which signal to certain T cells to join the attack.

All viruses, plus a few types of bacteria and parasites, must enter cells to survive, requiring a different approach. Infected cells use their MHC molecules to put pieces of the invading microbes on the cell's surface, flagging down cytotoxic T lymphocytes to destroy the infected cell. Antibodies also can assist in the immune response, attaching to and clearing viruses before they have a chance to enter the cell.

Parasites live either inside or outside cells. Intracellular parasites such as the organism that causes malaria can trigger T-cell responses. Extracellular parasites are often much larger than bacteria or viruses and require a much broader immune attack. Parasitic infections often trigger an inflammatory response when eosinophils, basophils, and other specialized granular cells rush to the scene and release their stores of toxic chemicals in an attempt to destroy the invader. Antibodies also play a role in this attack, attracting the granular cells to the site of infection.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.