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.

CELLS ORGANELLES

Cell parts:

Mitochondrion – double MB structure responsible for cellular metabolism – powerhouse of the cell

Nucleus – controls synthetic activities and stores genetic information

Ribosome – site of mRNA attachment and amino acid assembly, protein synthesis

Endoplasmic reticulum – functions in intracellular transportation

Gogli apparatus/complex – composed of membranous sacs – involved in production of large CHO molecules & lysosomes

Lysosome – organelle contains hydrolytic enzymes necessary for intracellular digestion

Membrane bag containing digestive enzymes

Cellular food digestion – lysosome MB fuses w/ MB of food vacuole & squirts the enzymes inside. Digested food diffuses through the vacuole MB to enter the cell to be used for energy or growth. Lysosome MB keeps the cell iself from being digested 

-Involved mostly in cells that like to phagocytose
-Involved in autolytic and digestive processes
-Formed when the Golgi complex packages up an especially large vesicle of digestive enzyme proteins

Phagosome 
– vesicle that forms around a particle (bacterial or other) w/in the phagocyte that engulfed it
- Then separates from the cell membrane bag & fuses w/ lysozome to receive contents
- This coupling forms phagolysosomes in which digestion of the engulfed particle occurs

Microbodies:
- Contain catalase
- Bounded by a single membrane bag
-  Compartments specialized for specific metabolic pathways
-  Similar in function to lysosomes, but are smaller & isolate metabolic reactions involving H2O2

-  Two general families:
·        Peroxisomes: transfer H2 to O2, producing H2O2 – generally not found in plants
·        Glyoxysomes: common in fat-storing tissues of the germinating seeds of plants
¨      Contain enzymes that convert fats to sugar to make the energy stored in the oils of the seed available

 Inclusions

– transitory, non-living metabolic byproducts found in the cytoplasm of the cell
- May appear as fat droplets, CHO accumulations, or engulfed foreign matter.

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.

MICROBIAL VIRULENCE FACTORS 

Microbial virulence factors are gene products required for a microbial pathogen to establish itself in the host. These gene products are located on the bacterial chromosome, or on mobile genetic elements, such as plasmids or transposons.

Primary pathogens express virulence factors that allow them to cause disease in the normal  host.

Opportunistic pathogens are environmental organisms or normal flora that lack the means to overcome normal host defense mechanisms. They cause disease only when the normal host defenses are breached or deficient. 

Virulence factors can be divided into several categories.

Skin - Propionibacterium acnes, Staphlococcus epidermis , diptheroids; transient colonization by Staphlococcus
aureus

Oral cavity - Viridans Streptococci, Branhamella species, Prevotella melaninogenicus, Actinomyces species, Peptostreptococcus species, other anaerobes

Nasopharynx Oral organisms; transient colonization by S. pneumoniae, Haemophilus species, N. meningitidis  

Stomach Rapidly becomes sterile 

Small intestine Scant

Colon - Bacteroides species, Clostridium species, Fusobacterium species, E. coli, Proteus species, Pseudomonas aeruginosa, Enterococcus species, other bacteria and yeasts 

Vagina - Childbearing years:Lactobacillus species, yeasts, Streptococcus species 

Prepuberty / Postmenopause: colonic and skin flora 

A. Enzyme production can be of several types depending on the needs of the organism, its requirements for survival, and the local environment.
 
1. Hyaluronidase breaks down hyaluronic acid to aid in the digestion of tissue. 
2. Protease digests proteins to enhance the spread of infections. 
3. Coagulase allows coagulation of fibrinogen to clot plasma. 
4. Collagenase breaks down collagen (connective tissues). 

B. Toxins 

1. Exotoxins are heat-labile proteins with specific enzymatic activities produced by many Gram-positive and Gram-negative organisms. Exotoxins are released extracellularly and are often the sole cause of disease. 
a. Some toxins have several domains with discrete biological functions that confer maximal toxicity. An example is A-B exotoxin, where the B subunit binds to host tissue cell glycoproteins and the A subunit enzymatically attacks a susceptible target.
b. Many toxins are ADP-ribosylating toxins

2. Endotoxin is the heat-stable lipopolysaccharide moiety found in the outer membrane of Gram-negative organisms. when released by cell lysls, the lipid A portion of lipopolysaccharide can induce septic shock characterized by fever, acidosis, hypotension, complement consumption, and disseminated intravascular coagulation (DIC).  

C. Surface components 

may protect the organism from immune responses such as phagocytosis or aid in tissue invasion. For example, the polysaccharide capsules of H. influenzae type b and the acidic polysaccharide capsule of Streptococcus pneumoniae interfere with phagocytosis. Other surface proteins, such as adhesins or filamentous appendages (fimbriae, pili), are involved in adherence of invading microorganisms to cells of the host. 

Classification:

Neutrophiles (pH = 7.0)
- P. aeruginosaqo
- Clostridium sporogenes
- Proteus species

Acidophiles (pH < 7.0)
- Thiobacillus thiooxidans
- Sulfollobus acidocaldaarius
- Bacillus acidocaldarius

Alkaliphiles (pH > 7.0)
- Nitrobacter species
- Streptococcus pneumoniae

Types of microscopy used in bacteriology

Light microscopy
Phase contrast microscopy
Fluorescence microscopy
Darkfield microscopy
Transmission electron microscopy
Scanning electron microscopy

Fluorescent microscopy in which ultraviolet rays are used to examine cells after treatment with fluorescent days.

Phase contrast microscope enhances the refractive index differences of the cell components. This microscopy can be used to reveal details of the internal structures as well as capsules, endospores and motility

Electron microscope The resolving power is more than 200 times that of light microscope.