microbiology branch

Dentistry College - first class
Medical biology - Lec. 8


Is a branch of biology concerned with study of immune response to a foreign challenge and how these responses are used to resist an infection.
Immunity: refers to a 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 are playing 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- Spleen (a fist – size organ located in the abdominal cavity).
7- Adenoid (two glands located at the back of the nasal passage).























Fig. distribution of lymphoid organs through human body

8- Lymphatic vessels are a network of channels throughout the body that carries lymphocytes to the lymphoid tissue in the abdominal cavity (Fig.).

















Host resistance
To establish an infection, pathogenic microorganisms must first overcome many surface barriers such as physicochemical which directly killing or inhibit attachment of M.O to host, so any M.O can penetrate these barriers encounter two levels of resistance:
1- Non- specific resistant (innate immunity)

a-Non – specific innate defenses
The body has a number of natural 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 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 has numbers of chemical substances 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 beta lysine .

b-Non – specific cellular defense
The body has range of different cell types that involved in non – specific cellular defense a fallow:

a- Neutrophile ( 60-70% of white blood cell 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 and immediate response of body to tissue injury or wound infection caused by a pathogen. There are gross features were described as the cardinal signs of acute inflammation include redness, warmth, pain, swelling and altered function. During the acute inflammation, the pathogen was eliminated by a series of events as the fallowing:
1- Increase in blood flow and capillary dilation bring into the area more antimicrobial factors and leukocytes that destroy the pathogens.
2- Rise in temperature stimulate the inflammation response and may inhibit microbial growth.
3- A fibrin clot often forms may limit the spread of the invaders to remain localized.
4- Phagocytes collected in an inflamed area to engulf the pathogens and to stimulate bone marrow to release neutrophile and increase the rate of granulocyte production.

2-Specific immune response
Specific immune response or acquired immunity that resist particular foreign agents and improve on repeated exposure to same agents. There are four characteristics distinguish specific immunity of 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 noticeable disease.
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: specific immune system responds only to non self antigens and not attack the body itself.

Below there are two branches of specific immunity are recognized:

1-Humoral (antibody – mediated ) immunity
It is antibody (Ab.) 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 that processing in bone marrow and fetal liver. There are five classes of Abs. each are 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 (fig.).
5-IgM (10% of total Abs.) the predominant early Abs. that activate in an initial attack of antigen because high number of antigen binding sites (5 sites).





























Fig. examples to sites of binding antibodies with antigens

Antigen (Ag.): it is a foreign substance that stimulates 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 higher organisms.


2- Cell – mediated immunity
It is involved T- lymphocyte cell, which 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 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.



The cells responsible for specific and non- specific immunity are leukocytes (meaning white cell) they processed in stem cells and migrate to other body sites undergo further development, some become residents within tissues while others circulate in body fluids. Below there are different types of leukocyte:



















1-Lymphoid cell: Lymphocyte are major cells of specific immune system, lymphocyte can be divided into three populations T, B and natural killer (NK) cells.







2- Mononuclear cells
There are two types of mononuclear: monocyte and macrophages are highly phagocyte were engulfed and 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 .












Basophile Acidophile neutrophile






Human saliva contains a number of physical and chemical agents that protect oral tissues against noxious compounds, 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 , phagocytes in addition to activate the neutrophils. In this mixture of defense factors, many show additive or even synergistic interactions against oral pathogens.





















Fig. Host defenses associated with the tooth surfaces

Fig. Host defenses associated with tooth surfaces