An Overview of the Immune System

An Overview of the Immune System

1. An Overview of the Immune System Atifa Ambreen M. Phil, Microbiology Government College University, Faisalabad

2. Functions of the Immune System Protect from pathogens & foreign molecules : parasites, bacteria Destroy abnormal (cancerous) cells Apoptosis , scavenge dead, dying body cells

3. Lines of Defense Physical and Chemical Barriers ( Innate Immunity )- first line of defense Nonspecific Resistance ( Innate Immunity )- second line of defense Specific Resistance ( Acquired Immunity )- third line of defense

4. Physical Barriers The skin has thick layer of dead cells in the epidermis which provides a physical barrier. Periodic shedding of the epidermis removes microbes. The mucous membranes produce mucus that trap microbes. Hair within the nose filters air containing microbes, dust, pollutants Cilia lines the upper respiratory tract traps and propels inhaled debris to throat Urine flushes microbes out of the urethra Defecation and vomiting expel micro-organisms.

5. Chemical Barriers Lysozyme, an enzyme produced in tears, perspiration, and saliva can break down cell walls and thus acts as an antibiotic (kills bacteria) Gastric juice in the stomach destroys bacteria and most toxins because the gastric juice is highly acidic (pH 2-3 ) Saliva dilutes the number of microorganisms and washes the teeth and mouth Acidity on skin inhibit bacterial growth Sebum (unsaturated fatty acids) provides a protective film on the skin and inhibits growth Hyaluronic acid is a gelatinous substance that slows the spread of noxious agents

6. Nonspecific Resistance (Innate Immunity) The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. Macrophages leave the bloodstream and enter body tissues to patrol for pathogens. When the macrophage encounters a microbe, this is what happens : The microbe attaches to the phagocyte. The phagocyte's plasma membrane extends and surrounds the microbe and takes the microbe into the cell in a vesicle. The vesicle merges with a lysosome , which contains digestive enzymes. The digestive enzymes begin to break down the microbe. The phagocyte uses any nutrients it can and leaves the rest as indigestible material and antigenic fragments within the vesicle.

7. Nonspecific Resistance (Innate Immunity) The phagocyte makes protein markers, and they enter the vesicle. The indigestible material is removed by exocytosis . The antigenic fragments bind to the protein marker and are displayed on the plasma membrane surface. The macrophage then secretes interleukin-1 which activates the T cells to secrete interleukin 2 , as described below under specific resistance . Inflammation is a localized tissue response that occurs when your tissues are damaged and in response to other stimuli. Inflammation brings more white blood cells to the site where the microbes have invaded. The inflammatory response produces swelling, redness, heat, pain. Fever inhibits bacterial growth and increases the rate of tissue repair during an infection.

8. Specific Resistance (Acquired Immunity) The third line of defense is specific resistance. This system relies on antigens, which are specific substances found in foreign microbes. Most antigens are proteins that serve as the stimulus to produce an immune response. The term "antigen" comes from ANTIbody GENerating substances.

9. Immune Response Steps in an immune response : When an antigen is detected by a macrophage (as describe above under phagocytosis ), this causes the T-cells to become activated. The activation of T-cells by a specific antigen is called cell-mediated immunity. The body contains millions of different T-cells, each able to respond to one specific antigen . The T-cells secrete interleukin 2. Interleukin 2 causes the proliferation of certain cytotoxic T cells and B cells . From here, the immune response follows 2 paths: one path uses cytotoxic T cells and the other uses B cells.

10. Types of T Cells Cytotoxic T cells: These cells secrete cytotoxin which triggers destruction of the pathogen's DNA or perforin which is a protein that creates holes in the pathogens plasma membrane. The holes cause the pathogen to lyse (rupture). Helper T cells: These cells secrete interleukin 2 (I-2) which stimulates cell division of T cells and B cells. In other words, these cells recruit even more cells to help fight the pathogen. Memory T cells: These cells remain dormant after the initial exposure to an antigen. If the same antigen presents itself again, even if it is years later, the memory cells are stimulated to convert themselves into cytotoxic T cells and help fight the pathogen.

11. Cytotoxic T Cell Pathway • The cytotoxic T cells are capable of recognizing antigens on the surface of infected body cells . The cytotoxic T cells bind to the infected cells and secrete cytotoxins that induce apoptosis (cell suicide) in the infected cell and perforins that cause perforations in the infected cells . Both of these mechanisms destroys the pathogen in the infected body cell.

12. T Cell Pathway T-cells can either directly destroy the microbes or use chemical secretions to destroy them. At the same time, T cells stimulate B cells to divide, forming plasma cells that are able to produce antibodies and memory B cells. If the same antigen enters the body later, the memory B cells divide to make more plasma cells and memory cells that can protect against future attacks by the same antigen. When the T cells activate (stimulate) the B cells to divide into plasma cells, this is called antibody-mediated immunity.