PATHOGENS AND DEFENCE AGAINST INFECTIOUS DISEASE By: Stephanie, Emily, Cem, and Julie
Pathogen Pathogen: an organism or virus that causes a disease. Examples: bacteria, fungi, protozoa, virus Disease Cause Pathogen Tuberculosis Bacterial infection Mycobacterium tuberculosis AIDS Viral infection Human immunodeficiency virus (HIV) Malaria Protozoan infection Plasmodium sp.
Methods by which pathogens are transmitted Airborne/inhalation Direct contact Cuts Ingestion of contaminated food or water Sexual intercourse Insects Blood tranfusions
Antibiotics Microorganisms produce antibiotics to inhibit growth of their competitors. Antibiotics interfere with the metabolic processes of bacteria, such as: DNA replication, transcription, translation, ribosome function, and cell wall formation.
Why are antibiotics effective against bacteria but not viruses? The role of an antibiotic is to decrease the growth or kill bacteria. How? They block metabolic pathways of bacteria (inhibiting cell wall formation and protein synthesis) resulting in the death of the bacteria. Viruses are not alive and use the organism s host cells to replicate. They are therefore not attacked by antibiotics. Viruses use the chemical processes inside host cells instead of metabolism of their own. Since antibiotics interfere with metabolic functions, they are not effective against viruses. To kill a virus by antibiotics required the killing of the human cell.
Structure of Antigens
Skin and mucous membranes in defence against pathogens Skin and mucous membranes form a barrier that prevents pathogens from entering the body. In addition, other layers of skin are tough enough to form another physical barrier Dry, keratinised layers of skin discourage pathogen growth Mucus has antiseptic properties because it contains the enzyme lysozyme which kills bacteria. It also forms a physical barrier, as pathogens and other harmful particles are trapped, swallowed, or expelled. Sebaceous glands (hair follicles) secrete a chemical called sebum, which maintains skin moisture and slightly lowers skin ph. Lower skin ph inhibits the growth of bacteria and fungi.
The Body s Defenses Natural populations of harmless microbes inhibit growth of most pathogenic microbes Stick mucus and cilla (tiny hairs) trap microorganisms 1 st Layer of Defense: Skin forms physical barrier to entry of pathogens. Tears, mucus, and saliva wash bacteria away. Mucus membranes and secretions 2 nd Layer of Defense: Many mechanisms inhibit or destroy pathogens. White blood cells are involved in most responses. Eosinophils Basophils Neutrophils, macrophages Produce toxic proteins against parasites, some phagocytosis Release heparin and histamine which promote inflammation Engulf and destroy foreign material (eg. Bacteria) 3 rd Layer of Defense: After pathogen is identified, lymphocytes (white blood cells) respond. T-lymphocytes differentiate in bone marrow T Specialized lymphocytes B B-lymphocytes differentiate in the thymus
Major histocompatibility complex/ complement-mhc
Cytotoxic T-cells-Class I MHC
MHC
Helper T cell-mhc Class II
Phagocytic leucocytes ingest pathogens in blood and body tissues 1. Have the ability to easily move through the walls of blood capillaries and sites where there is an infection. 2. A phagocyte comes in contact with pathogen cells and does not recognize the glycoprotein structure in its cell wall. 3. Pathogen is digested through endocytosis. Plasma membrane forms around pathogen. 5. Many phagocytes form pus. 4. Pathogen vacuole binds with lysosome containing digestive enzymes that break down pathogens.
Antigens and Antibodies Antigens Antibodies - Any chemical or macromolecules that can stimulate an immune response by lymphocytes (when micro-organisms get past barriers of the skin) - Proteins (secreted by plasma cells) that bind to specific antigens and mark it for elimination. - Produced by lymphocytes (white blood cells)
Clonal Selection (antibody production) After a pathogen is engulfed by a phagocyte, antigens from the pathogen are put onto the surface of the phagocyte, bound to a membrane protein called MHC protein. On the surface of the T-lymphocyte, a receptor binds to the antigen and become activated. The T-lymphocyte binds to a specific B-lymphocyte which stimulates that B-lymphocyte to divide and form a clone. Plasma B-lymphocyte secretes antibodies specific to the pathogen. Clone b-cells remain after infection, and are involved in the producing a faster response in case the pathogen infects the body again.
Binding of antigens and antibodies Makes pathogen more recognizable to phagocytes; they are engulfed more regularly. Prevents viruses from docking to host cells so that they cannot be taken up by host cells. Agglutination (or sticking together) of pathogens so they are prevented from entering cells and are easier for phagocytes to ingest.