Lecture 23 Immune System

Introduction A human or animal must defend itself against multitude of different pathogens including viruses, bacteria, fungi, and protozoan. Prevent disruption to homeostasis. There are plenty of effector mechanisms capable of defending the body against such agents. They can be soluble molecules or cells.

Definitions When an organism is threatened by microorganisms, viruses, or cancer cells, the immune response acts to provide protection. Normally, the immune system does not mount a response against self. This lack of an immune response is called tolerance. In some cases, the immune system does mount an immune response against self. If an error is made, and an immune response is made against self, tolerance to self is lost. This condition is called autoimmunity (from Greek, "self- immunity"). Examples of autoimmune diseases in humans are: asthma, lupus, and arthritis.

Immunity The body's capability to repel foreign substances and cells. The nonspecific (innate) responses are the first line of defense - block the entry and spread of disease-causing agents. Highly specific (adaptive) responses are the second line of defense and are tailored to an individual threat. Antibody-mediated and cell-mediated responses are two types of specific response. The immune system is associated with defense against disease-causing agents, problems in transplants and blood transfusions, and diseases resulting from over-reaction (autoimmune, allergies) and under-reaction (AIDS).

Non-specific Vs Specific Mechanisms Active defensive mechanisms function to avoid and limit damage and disease or to facilitate repair and recovery. Some active mechanisms are triggered whenever tissue damage or disease occurs and so are called non-specific. When they operate together in a co-ordinated way the result is called inflammation. Non-specific mechanisms provide for innate (inborn) immunity, i.e. they protect us effectively without any previous exposure to the agents of damage or disease (like viruses and bacteria). Specific mechanism provide for acquired immunity, i.e. they become effective only after the body has been exposed to specific agents of disease.

General Defenses Physical barriers are the first line of defense. Barriers to entry are the skin and mucous membranes. The skin is a passive barrier to infectious agents such as bacteria and viruses. Tears and saliva secrete enzymes that breakdown bacterial cell walls. Skin glands secrete chemicals that retard the growth of bacteria. Mucus membranes lining the respiratory, digestive, urinary, and reproductive tracts secrete mucus that forms another barrier.

Non Specific Defense The inflammatory response. 1.Damaged cells release chemical signals such as histamine that increase capillary blood flow into the affected area (causing the areas to become heated and reddened). White Blood Cells can move out of the blood vessels to the site of infection. 2.Phagocytes are a type of White Blood Cell that will recognize and engulf bacteria and other foreign substances, including cell debris left behind by. 3.Macrophages are white blood cells that function BOTH in non-specific and specific defenses. They attack and engulf viral-infected cells in a non-specific manner, but then PRESENT parts of the foreign virus particle to the immune system in order to trigger an immune response.

Inflammation The basic symptoms of inflammation are: -redness, -swelling, -heat, -pain.

Inflammatory Response - 1 Three major events occur during this response : 1.An increased blood supply to the tissue ''in danger''. It is performed by vasodilation. 2.Increased capillary permeability caused by retraction of the endothelial cells. Enables larger molecules than usual to escape from the capillaries, and thus allows the soluble mediators of immunity to reach the site of inflammation. 3.Leukocytes migrate out of the capillaries into the surrounding tissues. In the earliest stages of inflammation, neutrophils are particularly prevalent, but later monocytes and lymphocytes also migrate towards the site of infection.

Inflammatory Response - 2 Neutrophils, the most active and phagocytic of the white blood cells, become sticky and begin to adhere to the inside of the vessel wall. Adherence slows the cells down, making them "roll" on the inside of the vessel. The neutrophils then become superadherent and squeeze out between endothelial cells that line the vessel, a phenomenon called "diapedesis". From there, the white cells must FIND, EAT, and KILL foreign microbes. CHEMOTAXIS... how white cells FIND the bad guys PHAGOCYTOSIS... how white cells EAT microbes OXIDATIVE BURST... how white cells KILL bacteria

Foreign Bodies A n t i g e n s ( Ag )- term coined long ago…because they are antibody generating, usually large, foreign molecule (hopefully), can be proteins and carbohydrates. E p i t o p e s - specific portion of the antigen that is recognized by an antibody. H a p t e n s - are small organic molecules that are bound by antibodies, but do not illicit immune response, are antigenic when combined with a larger molecule.