3 Lines of Immune Defense
The body utilizes 2 types of immunity to defend against microbial invaders: Innate and Acquired Immunity. Innate immunity is a type of natural, built-in defense system that provides non-specific protection against pathogens. Innate immunity includes the 1st line of defense (physical & chemical barriers) and the 2nd line of defense (inflammation, fever, phagocytosis and non-specific immune cells). Acquired immunity is a specific, long-term form of protection that develops over time. Acquired immunity represents the 3rd line of defense and is mediated by specific immune cells (T cells and B cells).
Non-Specific (Innate) Immunity
1st Line of Defense
Non-specific defenses are effective against a wide range of pathogens and require no prior exposure to the pathogen to be effective. The first line of defense includes both physical and chemical barriers. Physical barriers (skin, cilia and mucous membranes) block the entry of pathogens into the body. Chemical barriers (tears, saliva, sweat) help breakdown or destroy the invaders. Lysozyme, a chemical found in tears and saliva, acts by destroying the cell wall of bacteria. Normal flora also provide non-specific protection to the host by competing with pathogens for attachment, nutrients, and by secreting toxins that inhibit microbial growth.
Can you answer the questions below?
2nd Line of Defense
If pathogens do gain entry into the body, the body's 2nd line of defense comes into play. The 2nd line of defense involves granulocytes (basophils, mast cells, eosinophils), phagocytic cells (neutrophils and macrophages), inflammation and fever.
Fever
Fever is an increase in body temperature induced by endogenous (interleukin-1, IL-1) or exogenous (LPS) pyrogens. Fever is induced when chemical messengers such as IL-1 stimulate the hypothalamus to raise body temperature. Fever acts to slow down microbial metabolism while, at the same time, ehances immune cell function.
Inflammation
Inflammation is a non-specific response to tissue injury that limits the spread of pathogens, removes cellular debris, destroys pathogens, and stimulates tissue repair. Inflammation is characterized by 4 signs: rubor, calor, tumor, and dolor. Inflammation begins when injured or infected tissues release chemical distress signals (histamine) that cause vasodilation. Vasodilation leads to increased blood flow and the recruitment of non-specific immune cells to the site of injury. Non-specific immune cells migrate in response to the chemical distress signal released by infected or injured cells via a process called chemotaxis and leave the blood vessel to enter the tissue via diapedesis.
Neutrophils are the 1st cells to arrive at the site of injury or infection and act to destroy invading bacteria by phagocytosis. Phagocytosis involves: 1) chemotaxis; 2) attachment to the microbe; 3)endocytosis; 4)fusion with a lysosome; 5) digestion of the microbe; 6) formation of residual waste; and 7) release of waste by exocytosis. Pus is formed by dead neutrophils. Substances that induce pus formation are called pyogenic. Upon leaving the circulatory system and entering the tissue, monocytes differentiate into macrophages, which are non-specific immune cells that aid in phagocytosis and antigen presentation.
Can you list the steps involved in phagocytosis in the correct order?
Eosinophils are granulocytic cells involved in eliminating allergens and parasites by releasing digestive enzymes. Basophils are involved in allergic responses and act to recruit other immune cells by releasing histamine, which results in increased blood flow to the site of infection and the recruitment of WBCs into the infected or injured tissue.
Antimicrobial Secretions
Non-specific immune cells produce and secrete a variety of antimicrobial substances including interleukins, lactoferrins, peroxides, free radical, lysins, complement and interferon. These substances act to destroy the invading microbe or inhibit microbial metabolism. Interferon is an anti-viral cytokine produced by virally infected cells that helps prevent the spread of infection to neighboring cells.
The complement system is a series of 9 blood proteins that are activated in response to microbial invasion. When activated, these proteins either bind to pathogens or antibody-pathogen complexes to target them for destruction by phagocytosis or form a membrane attack complex to lyse the invading cell. Complement proteins C3 & C5 induce chemotaxis, inflammation, and opsonization. Complement proteins C5-9 form the membrane attack complex (MAC) and induce lysis of the microbe.
