immune system
immunology : the science that is concerned with immune response to a foreign challenge and how these responses are used to resist an infection , it includes the distinction between self and non self of immune response.
immunity : refers to the general ability of the host to resist a particular infection or disease.
immune system
it is a complex network of specialized cells , organs and tissues evolved to defend the body against attacks by foreign invaders .the organs involved the immune system are called lymphoid organs (fig.) play a role in production and activate lymphocytes they are:-
1- bone morrow.
2- thymus (two lobes that join in front of the trachea).
3- lymph nodes (small organs shaped like beans located throughout the body) .
4- tonsils (two oval masses in the back of throat)
5- payer s patches (lymphoid tissue in the small intestine).
6- lymphatic vessels (a network of channels throughout the body that carries lymphocytes to the lymphoid tissue in the abdominal cavity).
7- spleen (a fist – size organ located in the abdominal cavity).
8- adenoid (two glands located at the back of the nasal passage).
host resistance
the interaction between a host and microorganism is a dynamic process in which each protagonist acts to maximize it?s survival , in some instances , after a m.o enters or contacts a host a positive relationship occurs , while in other m.o produces deletingrious effects on the host , so , there are many factors influenced on host – microbe relationship either directly such as nutrition , physiology , age and genetics or indirectly such as personal hygiene , socioeconomic status etc. as a result of influenced on the immunity , for example when the host is either very young or very old , sensitivity to infection increases .
to establish an infection , the pathogenic m.o must first overcome many surface barriers ,such as physicochemical barriers that are either directly antimicrobial or inhibit attachment of m.o to the host . any m.o can penetrates these barriers encounter two levels of resistance:
1- non- specific resistant (innate immunity)
2- specific resistant (acquired immunity)
a-non – specific innate defenses
the body has a number of built in defense mechanisms which provide protection against a wide range of pathogens , it is includes general mechanisms inherited as part of innate structure and act as a first line of defenses called non - specific innate defenses such as:
1- physical barriers
a- skin b- tears in eyes c- waxes in ears d- urine in bladder e- cilia and mucous in respiratory system f- mucous membrane and peristalsis in gastrointestinal tract g- saliva in mouth
2-chemical barriers
mammalian host have numbers of chemical substances to combat the continuous on sloughs of m.o , some of this chemicals like : gastric juice , salivary glycoprotein , lysozyme , oleic acid on the skin in addition to blood , lymph and other body fluids contain defensive chemicals such as bacteriocin and betalysine .
b-non – specific cellular defense
in addition to non- specific innate defense , the body also has a range of different cell types that involved in non – specific cellular defense as fallow :
a- neutrophile ( 60-70% of wbc count)
b- natural killer cell (5-8% of wbc count)
c- macrophage (5% of wbc count)
d - eosinophile (1.5% of wbc count)
inflammation
inflammation is an important non – specific defense reaction to tissue injury or wound caused by a pathogen . acute inflammation is the immediate response of body to injury or cell death , the gross features were described over many years ago and are still known as the cardinal signs of inflammation , these signs include redness , warmth , pain , swelling and altered function .
during the acute inflammation , the offending pathogen is neutralized and eliminated by a series of events , the most important of which are the fallowing :
1- the increase in blood flow and capillary dilation bring into the area more antimicrobial factors and leukocytes that destroy the pathogens .
2- the rise in temperature stimulate the inflammation response and may inhibit microbial growth .
3- a fibrin clot often forms and may limit the spread of the invaders so that they remain localized .
4- phagocytes collected an inflamed area and phagocytes the pathogen in addition to chemicals stimulate the bone marrow to release neutrophile and increase the rate of granulocyte production .
2-specific immune response
also known as acquired immunity, resist a particular foreign agents, moreover, specific immune response improve on repeated exposure to the same agents . four characteristic distinguish specific immunity from non- specific immunity :
1- specificity : immunity is directed against a particular pathogen or foreign substance .
2- memory : when exposed to the same pathogen or substance the body reacts so quickly that there is no notice able pathogenesis .
3- diversity : the system is able to generate an enormous diversity of molecules such as antibodies that recognize billions of different antigens .
4- discrimination between self and non self
the specific immune system always responds only to non self antigens thus, does not attack human itself .
below there are two branches of specific immunity are recognized :
1-humoral (antibody – mediated ) immunity
it is antibody ( abs.) dependent and relies upon body fluids especially the lymph and blood plasma to spread abs. around the body . the abs. are produced by b- lymphocyte and it is thought that processing of b-cell occurs in bone marrow and fetal liver . there are five classes of abs. each is called immunoglobulin as fallow:
1- iga (15% of total abs. count) found in mucous secretion of respiratory and digestive tract , vagina and in colostrums the substance is produced before the milk after delivery will protect the baby for about six months .
2- igd (less than 1%) appears to have a role in activating and suppressing lymphocyte activity .
3- ige (less than 1%) it is mediator in allergic response , activate histamine secreting cells and play role in parasitic infection .
4- igg (75% of total count) stimulate phagocyte cells , activate the complement system and neutralized toxins . it is the only abs. that can cross the placenta and confer immunity on the fetus.
5- igm (10% of total abs.) this is the predominant early abs. that activate in an initial attack of ag. because of it is high number of antigen binding sites have 5 sites ( fig.) ,this is important in an initial activation of b –cells macrophage and complement system .
2- cell – mediated immunity
it is involved t- lymphocyte cell, that is originally derive from stem cells of the bone marrow and then passes to the thymus gland in the chest , where they multiply. cell – mediated is based on the action of specific kinds of cell t- lymphocyte that directly attack cells infected with viruses or parasites , transplanted cells or organs and cancer cells .t- lymphocyte can lyses these cells or release chemicals like cytokines that enhance specific immunity and non- specific defenses such as phagocytes and inflammation.
antigen ( ag.) : it is a foreign substance such as protein , polysaccharide and polypeptide that stimulate an immune response and reacts with the products of that response.
antibody ( abs.) : are a group of glycoprotein present in the blood , tissue , fluid and mucous membrane of vertebrates .
the cells responsible for both specific and non- specific immunity are leukocytes (meaning white cell) from the stem cells which migrate to the other body sites undergo further development , some become residents within tissues while others circulate in body fluids . in figure below there are different types of leukocyte :
1-lymphoid cells
lymphocyte are major cells of specific immune system , lymphocyte can be divided into three population t, b and natural killer (nk) cells .
2- mononuclear cells
there are two types of mononuclear : monocyte & macrophages are highly phagocytic were engulfed & destroyed pathogenic m.o.
3- granulocytes
have irregular- shaped nuclei with 2-5 lobes and the cytoplasm matrix has granules contain reactive substances which kill m.o. there are three types of granules : neutrophil , acidophile , basophile .
oral immunity
human saliva contains a number of physical and chemical agents that protect oral tissues against noxious compounds, in particular those produced by various microorganisms. among such protective factors, the flushing effect of saliva flow is the most important one, not only because it so effectively removes exogenous and endogenous microorganisms and their products into the gut but also because a steady supply of saliva continuous presence of both non-immune and immune factors in mouth , for example non – specific factors are mucin , lysozyme , lactoferrin etc., the main functions are physical removal of organisms in addition to make on cell lyses , antibacterial and antifungal while the specific factors are siga , igg , igm , complement and polymorphs that make on prevent adhesion of m.o , bactericidal , phagocytosis in addition to activate the neutrophils as seen in figure below . in this mixture of defense factors, many show additive or even synergistic interactions against oral pathogens.