Immune System Properties and First Line Defence Study Guide

Overview of the Immune System

• Definition: The immune system is a sophisticated defense mechanism designed to protect the host by performing the following actions:
    * Preventing the entry of pathogens into tissues.
    * Recognizing pathogens that have managed to invade.
    * Eliminating invading pathogens.

• Composition: It consists of a coordinated set of components including molecules, cells, tissues, and organs that work together to maintain host health.

• Organization: The system is structured into three main levels of defense:
    1. Barriers: The first line of defense.
    2. Innate Immunity: The second line of defense.
    3. Adaptive Immunity: The third line of defense.

Properties of the Immune System

• Non-reactivity to Self-Antigens (Self-Tolerance):
    * Definition: The immune system distinguishes between foreign antigens and "self-antigens" (molecules found on the host's own cells and tissues).
    * Significance: This property prevents the immune system from launching responses against host organs such as the liver, kidney, spleen, and pancreas, thereby avoiding autoimmune damage.
    * Targeting: The immune system should ideally only react against antigens belonging to microbes like bacteria, viruses, fungi, and parasites.

• Diversity:
    * Definition: The ability of the immune system to recognize and respond to a vast array of different antigens.
    * Significance: Pathogens are diverse; the system can identify antigens from bacteria, viruses, and various other foreign microbes that stimulate an immune reaction.

• Specificity (Adaptive Immunity):
    * Definition: The system generates specific and optimized responses tailored to distinct antigens.
    * Example:
      * Initial exposure to "Bacteria A" triggers lymphocytes specific to the antigens on Bacteria A.
      * A subsequent exposure to "Bacteria B" involves a separate set of lymphocytes specific to Bacteria B.
      * Essentially, the response used for Pathogen A is not the same as the response for Pathogen B.

• Memory (Adaptive Immunity):
    * Definition: The ability to provide an enhanced response to recurrent or persistent infections.
    * Characteristics of Memory Response (2nd Infection by the same pathogen):
      * Speed: The response is faster compared to the primary infection.
      * Level/Intensity: The response magnitude is higher and more effective.
      * Specificity: This enhanced memory response only occurs for the exact same pathogen encountered previously.
      * Timeline Example: If a first infection by Bacteria A occurs at Day 1, a second infection by the same Bacteria A at Day 5 results in a significantly sharper and stronger immune peak than the first.

• Definition of Antigen ($Ag$):
    * An antigen is any substance or molecule capable of inducing a specific immune response against itself.
    * It can be specifically recognized by the products of an immune response, such as antibodies or specialized lymphocytes.

Organization of Immune Defenses

• Barriers (1st Line):
    * Focuses on preventing the initial entry of pathogens.

• Innate Immunity (2nd Line):
    * Provides an immediate response to invasion.
    * Does not possess immunological memory.
    * The nature of the response remains constant regardless of how many times the pathogen is encountered.

• Adaptive Immunity (3rd Line):
    * Characterized by its specificity towards particular pathogens.
    * Possesses long-term memory.
    * The quality and speed of the response improves upon subsequent exposures.

The First Line of Defence: Physical Barriers

• Primary Function: To physically prevent pathogens from entering the host's internal environment.

• Skin:
    * Acts as a tough protective barrier formed by epithelial cells.
    * Condition: The surface is dry, which creates an undesirable environment for microbial growth.

• Mucous Membranes:
    * Found in the epithelial lining of the respiratory, gastrointestinal ($GI$), and urogenital tracts.
    * These areas are more vulnerable than skin and are therefore protected by mucus.
    * Mucus Composition: Secreted by specialized cells, it contains proteoglycans, enzymes, and glycoproteins to protect the cells from damage.
    * Function: Traps microbes in the respiratory and gastrointestinal tracts.

• Saliva:
    * Physically washes and removes microbes from the teeth, gums, and inner lining of the mouth.

• Nose Hairs:
    * Act as a filter to remove microbes and debris from the air entering the nose.

• Expulsion Mechanisms:
    * Defecation and Vomiting: Processes that actively expel harmful microbes from the body.

The First Line of Defence: Chemical Barriers

• Sebum:
    * A complex mixture of lipids produced by the skin.
    * Function: Inhibits the growth of various microbes.

• Perspiration (Sweating):
    * Functions through its acidic $pH$, which creates an inhospitable environment for microbial proliferation.

• Lysozyme:
    * An enzyme found in tears, saliva, nasal secretions, and urine.
    * Function: Kills bacteria by destroying their cell walls.

• Saliva (Chemical components):
    * Contains the antibody $IgA$, which prevents microbes from attaching to host tissues.
    * Also contains lysozyme to break down bacterial membranes.

• Urine:
    * Possesses an acidic $pH$ that inhibits microbial growth.
    * Contains urea and uric acid, both of which inhibit bacterial expansion.

• Gastric Juice:
    * A mixture containing hydrochloric acid ($HCl$) and various digestive enzymes.
    * Function: The extremely high acidity ($low pH$) destroys most bacteria and toxins that enter the stomach.

The First Line of Defence: Normal Flora (Microbiota)

• Definition: Microorganisms that normally reside at specific biological sites and do not cause disease under normal circumstances.

• Mechanisms of Protection:
    * Colonization: They occupy physical space, leaving no room for pathogens to settle.
    * Nutrient Competition: They utilize available nutrients, preventing pathogens from obtaining the resources needed to grow.
    * Bacteriocin Production: They produce chemical substances that kill competing pathogens. For example, $E.\,coli$ in the large intestine produces bacteriocins that inhibit the growth of $Shigella$ and $Salmonella$.

• Pathogenicity Exception: Normal flora usually only cause disease if they migrate to a site that is not their "original home."

• Distribution Examples:
    * Skin:
      * $S.\,aureus$: Predominantly found in moist environments like the armpits and nostrils.
      * $S.\,epidermidis$: Typically found in drier areas of the skin.
      * $Corynebacterium$ (diphtheroids): Common on the palms of the hands.
    * Gastrointestinal Tract:
      * Stomach: Primarily $Lactobacilli$.
      * Small Intestine: Contains $Enterococci$ and $Lactobacilli$.
      * Large Intestine: High diversity including $Enterococcus\,faecalis$, $Bacteroides$, $Bifidobacterium$, $Clostridia$, $Lactobacilli$, and $Peptococcus$.