Immune System Health Article

Definition

The immune system is composed of cells, organs, tissues, and molecules that protect the body from disease. The term "immunity" comes from the Latin word immunitas.

Physical barriers

Anatomic barriers provide protection against invading bacterial and viral pathogens. The skin is composed primarily of keratin, which cannot be digested by most microorganisms. The skin is usually dry with a high salt concentration due to sweat. These conditions are not favorable for bacterial growth. Sweat and sebaceous skin secretions also contain substances that kill bacteria. Some types of bacteria inhabit the skin surface and do not cause disease under normal conditions (microflora). These bacteria may produce substances that kill other more pathogenic bacteria. The microflora may also consume nutrients required by pathogens. This gives rise to a competitive relationship that limits the growth of the pathogens. If the skin is broken, due to injuries or burns, harmful bacteria may enter and give rise to infection. The cilia of the lungs protect this organ from inhaled pathogens, transporting secretions to the throat so that they can be swallowed and destroyed by stomach acid. Secretions in the nose, saliva, and components of tears also contain substances that protect against bacteria and viruses.

Cells of the immune system

LYMPHOCYTES. There are two major types of lymphocytes, the T-cells and B-cells, which comprise 20–50% of the white blood cells in normal adult human circulation. T-cells mature and differentiate in the thymus gland and assist in cellular immune responses. These cells are responsible for the recognition of antigens (materials that give rise to an immune response, such as components of pathogenic bacteria). There are three major types of T-cells that are classified according to their function: cytotoxic T cells (Tc) that kill abnormal cells, helper T cells (Th) that enhance an immune response, and suppressor T-cells (Ts) that diminish the immune response. The B-cells mature in the bone marrow and recognize antigens with the help of T-cells. Upon activation, these cells give rise to plasma cells, which produce antibodies (immunoglobulins). Antibodies bind with toxic pathogen proteins or antigens and interact with other cells to remove the invader from the system. Plasma cells are found in the lymph nodes, spleen and bone marrow. B-cells also give rise to memory cells that remain alive for long periods of time and assist in a more effective immune response upon the next exposure to the same antigen.

The natural killer cells (NK) are a third type of lymphocyte and comprise approximately 3% of normal blood circulation. These large cells are responsible for the killing of some tumors and virus-infected cells. Additionally, some cells can be induced to kill their targets in a non-specific manner under the appropriate conditions. These cells are called lymphokine activated killer (LAK) cells.

GRANULOCYTES. The granulocytes or polymorphonuclear leukocytes (PMNs) are a group of cells that display a characteristic staining of granules in blood smears, hence their name. These cells have a short life span in the blood (about two or three days), and make up the majority of the white blood cells under normal conditions. They are usually found in greater numbers during an immune response to injury or infection. The neutrophils are a very important type of granulocyte and demonstrate phagocytosis (ingestion of particles by cells, such as particles of bacteria, with ultimate destruction by lysosomal enzymes). These cells are critical in the development of the immune response to pathogens and can migrate from the blood to the tissues during infection by a process known as chemotaxis (the movement of cells in response to and external chemical stimulation). They comprise approximately 40–75% of the blood. The eosinophils are mainly involved in an immune response to parasitic infection and also play a role in the allergic response, and comprise only 1–6% of the blood. The basophils, normally present in low numbers in the circulation (less than 1% of the blood), are thought to play a role in the inflammation and damage to tissue associated with allergic reactions.

MONOCYTES, MACROPHAGES, AND MAST CELLS. Monocytes are a type of cell that circulates in the bloodstream, comprising 2–10% of the blood. Upon migration into the tissues, these cells differentiate into macrophages that are capable of ingesting microorganisms by phagocytosis and have a critical role in the host defense to pathogens. They also produce substances called monokines that are a type of secreted protein (cytokine) that affects the actions of other cells.

Mast cells are distributed in the connective tissues, especially in the skin and mucosal surfaces of the respiratory, gastrointestinal, and urogenital tracts as well as the eye. These cells are also involved in the allergic response.

PLATELETS. Platelets are cell fragments in the blood that are involved in blood clotting and inflammation.

DENDRITIC CELLS. Dendritic cells are potent stimulators of immune responses. These cells play an important role in the increased immune response upon a second exposure to an antigen. Dendritic cells are distributed throughout the body, especially in the T-cell areas of lymphoid organs. In the lymphoid tissue, dendritic cells are involved in the stimulation of T-cell responses.