The immune system of a multicellular organism has several functions. It acts primarily as a defense against foreign pathogens (such as viruses, bacteria, parasites), and some poisons. It also functions in the return of extracellular fluid to the blood, and the formation of white blood cells.
There are several variations of immune systems throughout species, and sometimes more than one immune system within the same organism (for example, the human brain has its own immune system that is separate from the "normal" one).
Recognizing self and non-self: the problem of immunity
The Latin term immunis means exempt, referring to protection against foreign agents. The recognition of what is foreign is found in all life. In self-pollinating plants, a pollen grain landing on the stigma of a flower will send a pollen tubule down the style to the ovary for fertilization. A pollen grain from a genetically distinct plant will not germinate or the pollen tubule, once formed, will disintegrate in the style. In cross-pollinating species, self-marked pollen grains disintegrate, while nonself grains germinate and fertilize.
We may conceive of an arrangement where the cells of self are marked, so that they are not attacked by its own defense mechanism. But not all foreign cells may be destroyed since some must be assimilated for nourishment. Therefore, the immune system must have the capacity to detect self and some nonself. But since self needs to assimilate some nonself for its survival, it cannot mark itself. It is easier to mark potentially dangerous selves. But if only certain nonselves are marked, how does the body prepare to defend itself from selves not seen? The defense system must have the capacity to transform itself to deal with future dangers. It must also have the capacity to change, since the self itself evolves with time. An additional challenge to understanding is the mechanism by which sexually reproducing organisms prevent the growing embryo from being destroyed by the immune system of the 'mother'. It is believed that this is achieved by the specialized tissue such as the placentum in placental mammals. New theories attempt to solve some of these paradoxes. One such is the 'danger theory' proposed by Polly Matzinger which suggests that cellular apoptosis signals and directs the immune mechanism.
Structure of the immune system
Many organisms have an immune system. This immune system consists of an innate immunity which began in early eukaryotes such as the amoeba and generally consists of a set of mostly hard-wired responses to pathogens and does not change during the lifetime of the organism. Adaptive immunity in which the response to pathogens changes during the lifetime of an individual, appeared somewhat abruptly in evolutionary time with the appearance of cartilaginous (jawed) fish. Organisms that possess an adaptive immunity also possess an innate immunity and many of the mechanisms between the systems are common, so it not always possible to draw a hard and fast boundary between the individual components involved in each, despite the clear difference in operation. Humans have both an innate and an adaptive immune system.
Innate immune system
The adaptive immune system functions over extended time frames and may take days or weeks after an initial infection to have an effect. However, most organisms are under constant assault from pathogens, which must be kept in check by the fast-acting innate immune system. Innate immunity does not recognize specific pathogens, but rather fights general classes of pathogens using less specific defenses. Plants and many animals do not posses an adaptive immune system and instead rely on innate immunity.
The current understanding of innate immunity is very limited. Recent studies of innate immunity have made use of model organisms that lack adaptive immunity such as the plant Arabidopsis thaliana, the fly Drosophila melanogaster, and the worm Caenorhabditis elegans.
The first defense includes barriers to infection such as skin and mucus coating of the gut and airways. Pathogens which penetrate these barriers encounter anti-microbial molecules that restrict the infection. Pathogens may also be recognized by molecules that distinguish generally between classes of pathogens such as Gram-negative or Gram-positive bacteria. These pathogen-recognition molecules activate the expression of defensive molecules which target the infection.
The second-line defense includes phagocytic cells, which includes macrophages and neutrophil granulocytes (Polymorphonuclear leukocytes, PMN) that can envulf foreign substances. Macrophages mature continuously from circulating monocytes.
Phagocytosis involves chemotaxis, where phagocytic cells are attracted to microorganisms by means of chemotactic chemicals like microbial products, complements, damaged cells and white blood cell fragments; chemotaxis is followed by adhesion, where phagocytes are sticked to microorganisms. Adhesion is enhanced by opsonization, where proteins like opsonins are coated on the surface of the bacterium. This is followed by ingestion, where phagocytes extends their projections, forming pseudopods that engulf the organism. Finally the bacterium would be digested by the enzymes in the lysosome.
In addition, anti-microbial proteins may be activated if a pathogen pass through the barrier offered by skin. There are several class of antimicrobial proteins, e.g. acute phase proteins (C-reactive proteins, for example, binds to the C-protein of S. pneumoniae - enhances phagocytosis and activates complement) and complement.
The complement system is a very complex group of serum proteins which is activated in a cascade fashion, and there are three pathways of activation - classical, alternative and lectin. Only alternative pathway is of significance here though - because classical pathway is activated by the adaptive immune system, the antigen-antibody complex - alternative pathway activates C1,C4,C2,C3,C5 and finally C6 to C9, which forms the membrane attack complex. Lectin pathway activates C2, C3, C4, and some C1 homologue calcium-dependent lectin family proteins.
Complement binding will result in cytolysis, chemotaxis, opsonization and inflammation. Last but not least, Interferon α and β are important for resistance to viral infection.
Adaptive immune system
The adaptive immune system, which is much better understood than the innate immune system, is based on immune cells called leukocytes (or white blood cells) that are produced by stem cells in the bone marrow. The immune system can be divided into two parts. Many species, including mammals, have the following type:
· The humoral immune system, which acts against bacteria and viruses in the body liquids (such as blood). Its primary means of action are immunoglobulins, also called antibodies, which are produced by B cells (B means they develop in the bone marrow).
· The cellular immune system, which takes care of other cells that are infected by viruses. This is done by T cells, also called T lymphocytes (T means they develop in the thymus). There are two major types of T cells:
· Cytotoxic T cells (TC cells) recognize infected cells by using T-cell receptors to probe the surface of other cells. If they recognize an infected cell, they signal the cell to become apoptotic ("commit suicide"), thus killing that cell and any viruses it is in the process of creating.
· Helper T cells (TH cells) interact with macrophages (which ingest dangerous material), and also produce cytokines (interleukins) that induce the proliferation of B and T cells.
Disorders of the human immune system
There are a number of different autoimmune disorders, such as lupus erythematosus, multiple sclerosis, psoriasis, and rheumatoid arthritis. In these the self-recognition ability of the immune system fails and it attacks a part of the patient's own body.
By contrast AIDS, the "Acquired Immune Deficiency Syndrome" is an infectious disease, transmitted by HIV, which causes degeneration of the body's immune system.
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