Innate Immunity and Susceptibility to Infection

Mechanisms of Innate Immunity and Factors Affecting Susceptibility to Infection

• Infectious diseases are more severe in the very young and the elderly due to the immaturity of the immune system in children and physical abnormalities in the elderly.
• Bad nutrition decreases resistance to infection, while hormonal factors like diabetes mellitus increase susceptibility to infection.
• Mechanical barriers like intact skin and mucous membranes provide effective protection against most organisms, while fatty acid secretions of sebaceous glands and other acids in sweat glands are bactericidal and fungicidal.
• Chemical barriers like lysozyme found in tissue fluids and antimicrobial cationic peptides destroy bacteria, while complement components have various biological effects like cell lysis and opsonization.
• Normal bacterial flora compete with pathogens for essential nutrients and secrete antimicrobial agents, but long-term antibiotic use can destroy this flora, leading to infection.
• Phagocytic cells like microphages and macrophages engulf microorganisms or inert particles that enter tissue fluids or the bloodstream.
• Phagocytosis involves chemotaxis, recognition of foreign particulates by surface receptors, binding, and internalization of the microorganism or particle.
• Inflammatory response involves the release of mediators like histamine and prostaglandins, which cause vasodilation, increased vascular permeability, and chemotaxis of leukocytes.
• Neutrophils are the first leukocytes to arrive at the site of infection, followed by macrophages, which engulf and digest the microorganisms.
• Natural killer cells recognize and destroy virus-infected cells and tumor cells, while interferons provide protection against viruses and tumor cells.
• Males are generally less susceptible to infection than females, except in the case of hepatitis.
• Areas of the body with high flow rates of secretions, like the urinary and biliary tracts, are almost sterile.

Overview of Immune System Cells and Mechanisms

• The immune system defends against invading microorganisms (MO) through various mechanisms.
• Phagocytosis is one mechanism by which MO are engulfed by pseudopodia and destroyed by enzymes and bactericidal agents.
• The oxygen-dependent system and oxygen-independent system are the two main systems that kill ingested MO.
• Nitrogen reactive species also play a role in killing MO.
• Inflammatory response occurs due to tissue injury caused by invading MO.
• The major changes during the inflammatory response include increased blood supply, capillary permeability, and leukocyte infiltration.
• The immune system consists of organs and different cell types that recognize "non self" antigens and eliminate them.
• Haemopoeitic stem cells differentiate into three main lines: common lymphoid progenitor, common myeloid progenitor, and common erythroid progenitor.
• The primary lymphoid system is responsible for the evolution and maturation of immune cells, while the secondary lymphoid system is responsible for trapping foreign antigens and producing antibodies and effector T-lymphocytes.
• The immune response cell types include neutrophils, eosinophils, basophils, monocytes, dendritic cells, natural killer cells, and lymphocytes.
• Monocytes differentiate into macrophages, which migrate to the site of infection and possess high phagocytic activity and produce inflammatory and immune mediators.
• Dendritic cells digest MO and present them as antigens to T lymphocytes, making them antigen-presenting cells.