NEET MDS Lessons
General Microbiology
Immunoglobulin (Ig)
Immunoglobulins are glycoprotein molecules that are produced by plasma cells in response to an immunogen and which function as antibodies. The immunoglobulins derive their name from the finding that they migrate with globular proteins when antibody-containing serum is placed in an electrical field
FUNCTION
1. Immunoglobulins bind specifically to one or a few closely related antigens. Each immunoglobulin actually binds to a specific antigenic determinant. Antigen binding by antibodies is the primary function of antibodies and can result in protection of the host.
2. The significant biological effects are a consequence of secondary "effector functions" of antibodies.Phagocytic cells, lymphocytes, platelets, mast cells, and basophils have receptors that bind immunoglobulins. This binding can activate the cells to perform some function. Some immunoglobulins also bind to receptors on placental trophoblasts, which results in transfer of the immunoglobulin across the placenta. As a result, the transferred maternal antibodies provide immunity to the fetus and newborn.
STRUCTURE OF IMMUNOGLOBULINS
The basic structure of the immunoglobulins is illustrated in figure 2. Although different immunoglobulins can differ structurally, they all are built from the same basic units.
A. Heavy and Light Chains
All immunoglobulins have a four chain structure as their basic unit. They are composed of two identical light chains (23kD) and two identical heavy chains (50-70kD)
B. Disulfide bonds
1. Inter-chain disulfide bonds - The heavy and light chains and the two heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions The number of inter-chain disulfide bonds varies among different immunoglobulin molecules.
2. Intra-chain disulfide binds - Within each of the polypeptide chains there are also intra-chain disulfide bonds.
C. Variable (V) and Constant (C) Regions
When the amino acid sequences of many different heavy chains and light chains were compared, it became clear that both the heavy and light chain could be divided into two regions based on variability in the amino acid sequences. These are the:
1. Light Chain - VL (110 amino acids) and CL (110 amino acids)
2. Heavy Chain - VH (110 amino acids) and CH (330-440 amino acids)\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)h the arms of the antibody molecule forms a Y. It is called the hinge region because there is some flexibility in the molecule at this point.
E. Domains
Three dimensional images of the immunoglobulin molecule show that it is not straight as depicted in figure 2A. Rather, it is folded into globular regions each of which contains an intra-chain disulfide bond (figure 2B-D). These regions are called domains.
1. Light Chain Domains - VL and CL
2. Heavy Chain Domains - VH, CH1 - CH3 (or CH4)
F. Oligosaccharides
Carbohydrates are attached to the CH2 domain in most immunoglobulins. However, in some cases carbohydrates may also be attached at other locations.
IMMUNOGLOBULIN FRAGMENTS: STRUCTURE/FUNCTION RELATIONSHIPS
Immunoglobulin fragments produced by proteolytic digestion –
A. Fab
Digestion with papain breaks the immunoglobulin molecule in the hinge region before the H-H inter-chain disulfide bond Figure 6. This results in the formation of two identical fragments that contain the light chain and the VH and CH1 domains of the heavy chain.
Antigen binding – These fragments are called the Fab fragments because they contained the antigen binding sites of the antibody. Each Fab fragment is monovalent whereas the original molecule was divalent. The combining site of the antibody is created by both VH and VL.
B. Fc
Digestion with papain also produces a fragment that contains the remainder of the two heavy chains each containing a CH2 and CH3 domain. This fragment was called Fc because it was easily crystallized.
Effector functions – The effector functions of immunoglobulins are mediated by this part of the molecule. Different functions are mediated by the different domains in this fragment .
Treatment of immunoglobulins with pepsin results in cleavage of the heavy chain after the H-H inter-chain disulfide bonds resulting in a fragment that contains both antigen binding sites . This fragment is called F(ab’)2because it is divalent. The Fc region of the molecule is digested into small peptides by pepsin. The F(ab’)2binds antigen but it does not mediate the effector functions of antibodies.
The cell cycle
1) Labile cells (GI tract, blood cells)
- Described as parenchymal cells that are normally found in the G0 phase that can be stimulated to enter the G1
- Undergo continuous replication, and the interval between two consecutive mitoses is designated as the cell cycle
- After division, the cells enter a gap phase (G1), in which they pursue their own specialized activities
• If they continue in the cycle, after passing the restriction point (R), they are committed to a new round of division
• The G1 phase is followed by a period of nuclear DNA synthesis (S) in which all chromosomes are replicated
• The S phase is followed by a short gap phase (G2) and then by mitosis
• After each cycle, one daughter cell will become committed to differentiation, and the other will continue cycling
2) Stable cells (Hepatocytes, Kidney)
- After mitosis, the cells take up their specialized functions (G0).
- They do not re-enter the cycle unless stimulated by the loss of other cells
3) Permanent cells (neurons)
- Become terminally differentiated after mitosis and cannot re-enter the cell cycle
- Which cells do not have the ability to differentiate -> Cardiac myocytes
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.
DISINFECTION AND STERILIZATION
• Sterilization is the best destruction or com removal_of all forms of micro organisms.
• Disinfection is the destruction of many microorganisms but usually the b spores.
• Antisepsis is the destruction or inhibition of microorganisms in living tissues thereby limiting or preventing the harmful effect of infection.
• Astatic Agent would only inhibit the growth of microorganisms (bacteriostatic, fungistatic, sporostatic).
• Acidal agent would kill the microorganism (bactericidal. virucidal, fungicidal)
• Sterilants are the chemicals which under controlled conditions can kill sporinQ bacteria.
Immunology:
The branch of life science which deals with immune reaction is known as immunology.
Components of Immune System:
The immune system consists of a network of diverse organs and tissue which vary structurally as well as functionally from each other. These organs remain spreaded throughout the body. Basically, immune system is a complex network of lymphoid organs, tissues and cells.
These lymphoid organs can be categorized under three types depending upon their functional aspects:
i. Primary lymphoid organ.
ii. Secondary lymphoid organ.
iii.Tertiary lymphoid organ.
White blood cells or leukocytes are the basic cell types which help to give rise to different types of cells which participate in the development of immune response . WBC are classified into granulocytes and agranulocytes depending on the presence or absence of granules in the cytoplasm.
Agranular leukocytes are of two types, viz., lymphocytes and monocytes. Lymphocytes play pivotal role in producing defensive molecules of immune system. Out of all leukocytes, only lymphocytes possess the quality of diversity, specificity, memory and self-non self recognition as various important aspects of immune response.
Other cell types remain as accessory one; help to activate lymphocytes, to generate various immune effector cells, to increase the rate of antigen clearance
All cells of the immune system have their origin in the bone marrow
myeloid (neutrophils, basophils, eosinpophils, macrophages and dendritic cells)
lymphoid (B lymphocyte, T lymphocyte and Natural Killer) cells .
The myeloid progenitor (stem) cell in the bone marrow gives rise to erythrocytes, platelets, neutrophils, monocytes/macrophages and dendritic cells whereas the lymphoid progenitor (stem) cell gives rise to the NK, T cells and B cells.
For T cell development the precursor T cells must migrate to the thymus where they undergo differentiation into two distinct types of T cells, the CD4+ T helper cell and the CD8+ pre-cytotoxic T cell.
Two types of T helper cells are produced in the thymus the TH1 cells, which help the CD8+ pre-cytotoxic cells to differentiate into cytotoxic T cells, and TH2 cells, which help B cells, differentiate into plasma cells, which secrete antibodies.
Function of the immune system is self/non-self discrimination.
This ability to distinguish between self and non-self is necessary to protect the organism from invading pathogens and to eliminate modified or altered cells (e.g. malignant cells).
Since pathogens may replicate intracellularly (viruses and some bacteria and parasites) or extracellularly (most bacteria, fungi and parasites), different components of the immune system have evolved to protect against these different types of pathogens.
Method 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
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.