INNATE (NON-SPECIFIC) IMMUNITY

The elements of the innate (non-specific) immune system include anatomical barriers, secretory molecules and cellular components. 

Among the mechanical anatomical barriers are the skin and internal epithelial layers, the movement of the intestines and the oscillation of broncho-pulmonary cilia. 

Associated with these protective surfaces are chemical and biological agents.

A. Anatomical barriers to infections

1. Mechanical factors

The epithelial surfaces form a physical barrier that is very impermeable to most infectious agents. Thus, the skin acts as our first line of defense against invading organisms. The desquamation of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces. 

2. Chemical factors

Fatty acids in sweat inhibit the growth of bacteria. Lysozyme and phospholipase found in tears, saliva and nasal secretions can breakdown the cell wall of bacteria and destabilize bacterial membranes. The low pH of sweat and gastric secretions prevents growth of bacteria. Defensins (low molecular weight proteins) found in the lung and gastrointestinal tract have antimicrobial activity. Surfactants in the lung act as opsonins (substances that promote phagocytosis of particles by phagocytic cells). 

3. Biological factors

The normal flora of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces.

B. Humoral barriers to infection

Humoral factors play an important role in inflammation, which is characterized by edema and the recruitment of phagocytic cells. These humoral factors are found in serum or they are formed at the site of infection.

1. Complement system – The complement system is the major humoral non-specific defense mechanism (see complement chapter). Once activated complement can lead to increased vascular permeability, recruitment of phagocytic cells, and lysis and opsonization of bacteria. 

2. Coagulation system – Depending on the severity of the tissue injury, the coagulation system may or may not be activated. Some products of the coagulation system can contribute to the non-specific defenses because of their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells. In addition, some of the products of the coagulation system are directly antimicrobial. For example, beta-lysin, a protein produced by platelets during coagulation can lyse many Gram positive bacteria by acting as a cationic detergent.

3. Lactoferrin and transferrin – By binding iron, an essential nutrient for bacteria, these proteins limit bacterial growth.

4. Interferons – Interferons are proteins that can limit virus replication in cells.

5. Lysozyme – Lysozyme breaks down the cell wall of bacteria. 

6. Interleukin -1 – Il-1 induces fever and the production of acute phase proteins, some of which are antimicrobial because they can opsonize bacteria.

C. Cellular barriers to infection

Part of the inflammatory response is the recruitment of polymorphonuclear eosinophiles and macrophages to sites of infection. These cells are the main line of defense in the non-specific immune system.

1. Neutrophils – Polymorphonuclear cells  are recruited to the site of infection where they phagocytose invading organisms and kill them intracellularly. In addition, PMNs contribute to collateral tissue damage that occurs during inflammation.

2. Macrophages – Tissue macrophages  and newly recruited monocytes , which differentiate into macrophages, also function in phagocytosis and intracellular killing of microorganisms. In addition, macrophages are capable of extracellular killing of infected or altered self target cells. Furthermore, macrophages contribute to tissue repair and act as antigen-presenting cells, which are required for the induction of specific immune responses.

3. Natural killer (NK) and lymphokine activated killer (LAK) cells – NK and LAK cells can nonspecifically kill virus infected and tumor cells. These cells are not part of the inflammatory response but they are important in nonspecific immunity to viral infections and tumor surveillance. 

4. Eosinophils – Eosinophils  have proteins in granules that are effective in killing certain parasites.

CROSS INFECTION AND STERLIZATION IN DENTISTRY

Cross infection is defined as the transmission of infectious agents amongst patients and staff with in hospital environment.

Routes of Infection 
Two routes are important : transdermal  and respiratory. 

 In transdermal route microorganisms enter the tissues of the recipient by means of injection through intact skin or mucosa (usually due to an accident involving a sharp instrument) or via defects in the skin e.g. recent cuts and abrasions.
 
Microorganisms causing cross infection in dentistry

Transmitted through skin 

Bacteria : Treponema pallidum, Staphylococcus aureus

Viruses :Hepatitis virus, HIV ,Herpes simplex virus, Mumps, Measles , Epstein-Barr virus

Fungi: Dermatomycoses, Candidiasis, 

Transmitted through aerosols

Bordetella pertussis, Myco.tuberculosis, Streptococcus pyogenes, Influenza virus
Rhinovirus,  Rubella 
 

ANTIGEN-ANTIBODY REACTIONS

Affinity of the antigen-antibody reaction refers to the intensity of the attraction between antigen and antibody molecule.
Antigen-antibody reactions

Reaction test            Modified test

Precipitation  -> Immunoelectrophoresis, Immunoprecipitation
Agglutination -> Latex agglutination, Indirect, Haemagglutination , Coagglutination ,Coombs test

Neutralization  -> Measurement of LD, Plaque assays

Complement fixation  -> Conglutination

Immunofluorescence ->  Indirect immunofiuorescence, Immunoofluoremetric Assay

Enzyme immunoassay -> Enzyme linked, Immunosorbent assay

Radioimmunoassay -> Immunoradiometric assay

Avidity is the strength of the bond after the formation of antigen-antibody complex.

Sensitivity refers to the ability of the test to detect even very minute quantities of antigen or antibody. A test shall be called as highly sensitive if false negative results are absent or minimal.

Specificity refers to the ability of the test to detect reactions between homologous antigens and antibodies only, and with no other. In a highly specific test, false positive reactions will be minimal or absent.

NORMAL MICROBIAL FLORA 

A. Properties. Normal microbial flora describes the population of microorganisms that usually reside in the body. The microbiological flora can be defined as either 
1) Resident flora - A relatively fixed population that will repopulate if disturbed, 

2) Transient flora - that are derived from the local environment. These microbes usually reside in the body without invasion and can
even prevent infection by more pathogenic organisms, a phenomenon known as bacterial interference. 
The flora have commensal functions such as vitamin K synthesis. However, they may cause invasive disease in immunocompromised hosts or if displaced from their normal area. 

B. Location. Microbial flora differ in composition depending on their anatomical locations and microenvironments. The distribution of normal microbial flora.

Immunofluorescence

This is precipitation or complement fixation tests. The technique can detect proteins at concentrations of around 1 µg protein per ml body fluid. Major disadvantage with this technique is frequent occurrence of nonspecific fluorescence in the tissues and other material.
The fluorescent dyes commonly used are fluorescein isothocyanate (FITC). These dyes exhibit fluorescence by absorbing UV light between 290 and 495 nm and emitting longer wavelength coloured light of 525 nm which gives shining appearance (fluorescence) to protein labelled with dye. Blue green (apple green) fluorescence is seen with FITC and orange red with rhodamine.

Enzyme Immunoassays

These are commonly called as enzyme linked immunosorbent assays or EL1SA. It is a simple and versatile technique which is as sensitive as radioimmunoassays. It is now the
technique for the detection of antigens, antibodies, hormones, toxins and viruses.

Identification of organisms by immunofluorescence

Type of agent         Examples

Bacterial            Neisseria gonorrhoeae, H. influenzae ,Strept pyogenes, Treponema pallidum
Viral                  Herpesvirus, Rabiesvirus, Epstein-Barr virus
Mycotic             Candida albicans

Enzymatic activity results in a colour change which can be assessed visibly or quantified in a simple spectrophotometer.

STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND SUBCLASSES

A.  IgG

1. Structure

 All IgG’s are monomers (7S immunoglobulin). The subclasses differ in the number of disulfide bonds and length of the hinge region.

2. Properties

IgG is the most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.

a) IgG is the major Ig in serum – 75% of serum Ig is IgG

b) IgG is the major Ig in extra vascular spaces

c) Placental transfer – IgG is the only class of Ig that crosses the placenta. Transfer is mediated by a receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally well; IgG2 does not cross well.

d) Fixes complement – Not all subclasses fix equally well; IgG4 does not fix complement

e) Binding to cells – Macrophages, monocytes and neutrophils and some lymphocytes have Fc receptors for the Fc region of IgG.  A consequence of binding to the Fc receptors on such cells  is that the cells can now internalize the antigen better. The antibody prepares the antigen for killing by the phagocytic cells. The term opsonin is used to describe substances that enhance phagocytosis. (Coating of the surface of pathogen by antibody is called opsonization).IgG is a good opsonin. Binding of IgG to Fc receptors on other types of cells results in the activation of other functions.

IgM

1. Structure
 IgM normally exists as a pentamer (19S immunoglobulin) but it can also exist as a monomer. In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10. IgM has an extra domain on the mu chain (CH4) and it has another protein covalently bound via a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer.

2. Properties

a) IgM is the third most common serum Ig.

b) IgM is the first Ig to be made by the fetus and the first Ig to be made by a virgin B cells when it is stimulated by antigen.

c) As a consequence of its pentameric structure, IgM is a good complement fixing Ig. Thus, IgM antibodies are very efficient in leading to the lysis of microorganisms.

d) As a consequence of its structure, IgM is also a good agglutinating Ig . Thus, IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.

e) IgM binds to some cells via Fc receptors.

f) B cell surface Ig 

Surface IgM exists as a monomer and lacks J chain but it has an extra 20 amino acids at the C-terminus to anchor it into the membrane . Cell surface IgM functions as a receptor for antigen on B cells.

IgA

1. Structure

Serum IgA is a monomer but IgA found in secretions is a dimer as presented in Figure 10. When IgA exits as a dimer, a J chain is associated with it.

When IgA is found in secretions is also has another protein associated with it called the secretory piece or T piece; sIgA is sometimes referred to as 11S immunoglobulin. Unlike the remainder of the IgA which is made in the plasma cell, the secretory piece is made in epithelial cells and is added to the IgA as it passes into the secretions . The secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.

2. Properties

a) IgA is the 2nd most common serum Ig.

b) IgA is the major class of Ig in secretions – tears, saliva, colostrum, mucus. Since it is found in secretions secretory IgA is important in local (mucosal) immunity.

c) Normally IgA does not fix complement, unless aggregated.

d) IgA can binding to some cells – PMN’s and some lymphocytes.

IgD

1. Structure

 IgD exists only as a monomer.

2. Properties

a) IgD is found in low levels in serum; its role in serum  is uncertain.

b) IgD is primarily found on B cell surfaces where it functions as a receptor for antigen.

c) IgD does not bind complement.

E. IgE

1. Structure

IgE exists as a monomer and has an extra domain in the constant region.

2. Properties

a) IgE is the least common serum Ig since it binds very tightly to Fc receptors on basophils and mast cells even before interacting with antigen.

b) Involved in allergic reactions – As a consequence of its binding to basophils and mast cells, IgE is involved in allergic reactions. Binding of the allergen to the IgE on the cells results in the release of various pharmacological mediators that result in allergic symptoms.

c) IgE also plays a role in parasitic helminth diseases. Since serum IgE levels rise in parasitic diseases, measuring IgE levels is helpful in diagnosing parasitic infections. Eosinophils have Fc receptors for IgE and binding of eosinophils to IgE-coated helminths results in killing of the parasite.

d) IgE does not fix complement.