📖 General Microbiology
NITRIC OXIDE-DEPENDENT KILLING
General MicrobiologyNITRIC OXIDE-DEPENDENT KILLING
Binding of bacteria to macrophages, particularly binding via Toll-like receptors, results in the production of TNF-alpha, which acts in an autocrine manner to induce the expression of the inducible nitric oxide synthetase gene (i-nos ) resulting in the production of nitric oxide (NO) . If the cell is also exposed to interferon gamma (IFN-gamma) additional nitric oxide will be produced (figure 12). Nitric oxide released by the cell is toxic and can kill microorganism in the vicinity of the macrophage.
STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND SUBCLASSES
General MicrobiologySTRUCTURE 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.
ANTIGENS
General MicrobiologyANTIGENS
Immunogen
A substance that induces a specific immune response.
Antigen (Ag)
A substance that reacts with the products of a specific immune response.
Hapten
A substance that is non-immunogenic but which can react with the products of a specific immune response. Haptens are small molecules which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule. Free haptens, however, can react with products of the immune response after such products have been elicited. Haptens have the property of antigenicity but not immunogenicity.
Epitope or Antigenic Determinant
That portion of an antigen that combines with the products of a specific immune response.
Antibody (Ab)
A specific protein which is produced in response to an immunogen and which reacts with an antigen.
FACTORS INFLUENCING IMMUNOGENICITY
- Larger the molecule the more immunogenic it is likely to be.
- More complex the substance is chemically the more immunogenic it will be.
- Particulate antigens are more immunogenic than soluble ones and denatured antigens more immunogenic than the native form.
- Antigens that are easily phagocytosed are generally more immunogenic. This is because for most antigens (T-dependant antigens, see below) the development of an immune response requires that the antigen be phagocytosed, processed and presented to helper T cells by an antigen presenting cell (APC).
- Some substances are immunogenic in one species but not in another. Similarly, some substances are immunogenic in one individual but not in others (i.e. responders and non-responders). The species or individuals may lack or have altered genes that code for the receptors for antigen on B cells and T cells or they may not have the appropriate genes needed for the APC to present antigen to the helper T cells.
Method of Administration
1. Dose
The dose of administration of an immunogen can influence its immunogenicity. There is a dose of antigen above or below which the immune response will not be optimal.
2. Route
Generally the subcutaneous route is better than the intravenous or intragastric routes. The route of antigen administration can also alter the nature of the response
3. Adjuvants
Substances that can enhance the immune response to an immunogen are called adjuvants. The use of adjuvants, however, is often hampered by undesirable side effects such as fever and inflammation.
TYPES OF ANTIGENS
T-independent Antigens
T-independent antigens are antigens which can directly stimulate the B cells to produce antibody without the requirement for T cell help In general, polysaccharides are T-independent antigens. The responses to these antigens differ from the responses to other antigens.
Properties of T-independent antigens
1. Polymeric structure
These antigens are characterized by the same antigenic determinant .
2. Polyclonal activation of B cells
Many of these antigens can activate B cell clones specific for other antigens (polyclonal activation). T-independent antigens can be subdivided into Type 1 and Type 2 based on their ability to polyclonally activate B cells. Type 1 T-independent antigens are polyclonal activators while Type 2 are not.
3. Resistance to degradation
T-independent antigens are generally more resistant to degradation and thus they persist for longer periods of time and continue to stimulate the immune system.
T-dependent Antigens
T-dependent antigens are those that do not directly stimulate the production of antibody without the help of T cells. Proteins are T-dependent antigens. Structurally these antigens are characterized by a few copies of many different antigenic determinants as illustrated in the Figure 2.
HAPTEN-CARRIER CONJUGATES
Hapten-carrier conjugates are immunogenic molecules to which haptens have been covalently attached. The immunogenic molecule is called the carrier.
Structure
Structurally these conjugates are characterized by having native antigenic determinants of the carrier as well as new determinants created by the hapten (haptenic determinants). The actual determinant created by the hapten consists of the hapten and a few of the adjacent residues, although the antibody produced to the determinant will also react with free hapten. In such conjugates the type of carrier determines whether the response will be T-independent or T-dependent.
SUPERANTIGENS
When the immune system encounters a conventional T-dependent antigen, only a small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen and become activated (monoclonal/oligoclonal response). However, there are some antigens which polyclonally activate a large fraction of the T cells (up to 25%). These antigens are called superantigens .
Examples of superantigens include: Staphylococcal enterotoxins (food poisoning), Staphylococcal toxic shock toxin (toxic shock syndrome), Staphylococcal exfoliating toxins (scalded skin syndrome) and Streptococcal pyrogenic exotoxins (shock).
Method of Sterilization for common items
General MicrobiologyMethod of Sterilization for common items
Autoclaving : Animal cages, Sugar tubes, Lab. Coats, Cotton , Filters, Instruments Culture media, Rubber, Gloves , Stopper, Tubing, Slides, Syringe and Wax needles , Test tubes, Enamel metal trays ,Wire baskets, Wood, Tongue depressor, Applicator, Endodontic instruments, Orthodontic pliers , Orthodontic kits, Saliva ejector, Handpieces Cavitron heads, Steel burs, Steel tumbler, Hand instruments
Hot air oven
Beakers, Flasks, Petri dish, Slides, Syringes, Test tubes, Glycerine, Needles ,Oil, Paper Saliva ejector, Matrix Band
Ethylene oxide
Fabric, Bedding, Blanket, Clothing, Matteresses, Pillows, Disposable instruments , Instruments, Blades, Knives, Scalpels, Scissors ,Talcum powder, Books, Cups, plates , Plastics., Flask, Petridish, Tubes, Tubing, Rubber , catheters, Drains, Gloves ,Special items - Bronchoscope, Cystoscope, Heart lung machine
Glutaraldehyde
Orthodontic kits, Orthodontic pliers , Steel burrs, 3 in 1 syringe tips ,Cystoscope ,Endoscope
Filtration
Antibiotics, Serum, Vaccines