The Innate Defense System

• The immune system shields the body from infectious microorganisms, cancer cells, and transplanted organs.

Innate Defense System: First Line of Defense

• The first line of defense involves external body membranes like skin and mucosa.
• Keratin in the skin's epithelial membrane resists weak acids, bases, bacterial enzymes, and toxins.
• Intact mucosa lines body cavities open to the exterior.
• Epithelial membranes produce protective chemicals.
  • Skin acidity (pH 3-5) inhibits bacteria; sebum has toxic chemicals.
  • Stomach mucosa secretes HCl.
  • Cilia is present in the upper respiratory tract.
  • Saliva and lacrimal fluid contain lysozyme to destroy bacterial walls.
  • Mucus in organ systems traps microbes.
  • Skin secretes defensins, which are antimicrobial peptides.

Innate Defense System: Second Line of Defense

• The second line of defense consists of cells, fever, inflammation, and antimicrobial proteins.
• Phagocytes, including macrophages, neutrophils, and dendritic cells, engulf foreign particles.
• Natural killer (NK) cells induce target cell death.
• Fever is a systemic response that elevates body temperature in response to pyrogens and increases repair processes.
  • The liver and spleen take up iron and zinc during fever, limiting bacterial multiplication.
• Inflammation is a localized response to infection or injury by physical trauma, heat, chemicals.
  • Inflammation functions to prevent the spread of damaging agents and dispose of cell debris and pathogens.
  • Acute inflammation has cardinal signs: redness, heat, swelling, pain, and sometimes impaired function.
• Interferons (IFNs) are antimicrobial proteins that are non-specific and secreted by virally infected cells.
  • They interfere with viral replication in healthy cells and activate macrophages and NK cells.
• Complement is a system of about 30 plasma proteins circulating in the blood.
  • Activation amplifies inflammation and causes cell lysis for invading cells.

Adaptive Defense System: Third Line of Defense

• The adaptive immune system recognizes foreign substances and acts to immobilize, neutralize, or remove them.
• Priming is required through initial exposure to a foreign substance.
• The three aspects are that it is specific, systemic, and has a memory.
• Humoral immunity involves antibodies produced by lymphocytes.
  • Antibodies bind to microbes, inactivating them and marking them for destruction.
• Cellular immunity involves lymphocytes acting against target cells: directly by killing them
  • Indirectly by releasing chemical mediators to enhance inflammation or activate other lymphocytes or macrophages.

Antigens: The Trigger

• Antigens mobilize the immune system and provoke a response.
• These are large complex molecules which aren't normally in the body.
• Complete antigens include large proteins, polysaccharides, lipids, and nucleic acids.
  • They have immunogenicity, stimulating lymphocyte and antibody production, and reactivity. Reacting with activated lymphocytes and antibodies.
• Incomplete antigens (haptens) are small molecules needing a protein carrier to cause a response.
  • They have reactivity but lack immunogenicity.

Antigenic Determinants

• Antigenic determinants trigger the immune response.
• Most antigens possess a variety of antigenic determinants or epitopes.
  • The more complex the molecule is, the greater the number.

Cells - Lymphocytes

• B cells produce antibodies for humoral immunity.
• T cells do not produce antibodies which is cell mediated immunity.
• They possess unique receptors that recognize and bind to specific antigenic determinants.
• All receptors on a single cell are identical.

Self-Antigens: MHC Proteins

• Major histocompatibility complex (MHC) molecules are glycoproteins and are self-antigens.
• Each MHC protein has a deep groove that holds a peptide.
• MHC Class I are found on all body cells.
• MHC Class II are less widespread.

Adaptive Defense System: Antigen-Presenting Cells

• Antigen-presenting cells (APCs) do not respond to specific antigens.
• APCs engulf antigens and present fragments on the surface for recognition by T cells.
• Major APC types include dendritic cells, macrophages, and B lymphocytes.