2 | Defense Systems |
Functions and Disturbances of the Immune System
Our immune system plays the major role in preventing and overcoming infections. In the absence of a functioning immune system, not even the most effective antibiotics can fight infections. The defense system can be affected by various disorders, such as congenital immunoglobulin deficiency, partial weakness of the immune system (cellular as well as humoral), iatrogenic immunosuppression, or acquired immunodeficiency syndrome (AIDS). Other influences that reduce the activity of the immune system include malnutrition, stress, and aging.
The human body can coexist quite well with a multitude of different microorganisms. Skin and intestinal tract are colonized with 1012–1016 bacteria. An infection can only occur when the equilibrium between the defense system and microorganisms is disturbed, for example, by the invasion of pathogens into otherwise sterile regions, by the uptake of especially pathogenic germs from the outside, or by weakening of the defense systems.
The number of invading pathogens and their virulence also play a major role in initiating an infection.
Table 2.1 Defense systems
1. In general: skin mucosa secretions normal flora 2. Immune system: a) nonspecific: complement system interferons lysozymes, etc. leukocytes (microphages) lymphocytes (macrophages) natural killer cells (T lymphocytes) b) specific: immunoglobulins (antibodies) IgM, IgA, IgG specific macrophages (T cells) |
General Defense Mechanisms
Intact skin and mucosae normally represent an effective mechanical barrier against invading pathogens (Table 2.1).
The cornified skin is much more resistant against microorganisms than the stratified or single-layered mucosae. Hence, mucosae represent the main site of entrance for most microorganisms.
However, mucosae have their own protection through the secretion of mucus (e.g., the cervical mucosa) and through various specific defense mechanisms (IgA antibodies) or nonspecific defense mechanisms (e.g., lysozymes).
Another protective mechanism is the colonization of the vagina with lactic acid-producing lactobacilli.
Nonspecific Humoral Defense Systems
Complement system. This complex enzyme system is able to dissolve invading pathogens through a series of cascading reactions of individual components. Many interactions take place between the individual defense systems.
Activation of the complement system is promoted by specific antibodies binding to the invading microorganism. Phagocytosis by macrophages is also increased.
Other nonspecific humoral defense systems include:
the properdin system, which activates the alternative pathway of the complement system
lysozymes, which dissolve certain bacterial cell walls
the various interferons, which exhibit antiviral and antiproliferative activities and also have immunoregulatory functions (cell membrane effects, cell differentiation, increased cytotoxicity).
Apart from these systems, there are also numerous substances (mediators) that are released by macrophages or T lymphocytes upon stimulation.
Nonspecific Cellular Defense System
This includes polymorphonuclear leukocytes (neutrophil and eosinophil granulocytes, also called microphages) and mononuclear phagocytes (macrophages).
Polymorphonuclear leukocytes are the first cells to reach the site of infection. They have limited resistance, die early, and cause pus formation.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
