The Innate Defense System

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

Innate Defense System - First Line of Defense

• These are 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 inhibits bacterial growth and sebum has toxic chemicals.
• Stomach mucosa secretes HCl.
• Cilia is present in the upper respiratory system.
• Saliva and lacrimal fluid contain lysozyme, which destroys bacterial walls.
• Mucus in organ system passageways trap microbes.
• Skin secretes antimicrobial peptides called defensins.

Innate Defense System – Second Line of Defense

• Includes cells, fever, inflammation, antimicrobial proteins such as inferferons, and complement
• Phagocytes (WBCs) engulf foreign particles/cells and include macrophages, neutrophils, and dendritic cells.
• Natural killer (NK) cells cause target cells to undergo cell death.
• Fever is a systemic response to invading microbes.
• Body temperature increase is triggered by pyrogens.
• Increased temperature increases repair.
• The liver and spleen take up iron and zinc during fever reducing bacteria's ability to multiply.
• Inflammation is a localized response to infection from physical trauma, heat, chemicals, etc.
• Inflammation prevents the spread of damaging agents, disposes of cell debris/pathogens, and sets the stage for repair.
• Acute inflammation signs: redness, heat, swelling, pain, and sometimes impaired function.
• Interferons (IFNs) are nonspecific antimicrobial proteins secreted by viral-infected cells
• IFNs interfere with viral replication in healthy cells, activating macrophages and NK cells.
• Complement consists of about 30 plasma proteins circulating in the blood.
• Complement activation amplifies inflammation and causes cell lysis of invading cells.

Adaptive Defense System – Third Line of Defense

• The adaptive immune system recognizes foreign substances and acts to immobilize, neutralize, or remove them, and relies on an initial exposure.
• Key aspects: specific, systemic (not restricted to initial site), and has memory
• Humoral immunity/antibody-mediated immunity involves antibodies (produced by lymphocytes).
• Antibodies bind to microbes, inactivating them and acting as markers for destruction.
• Cellular/cell-mediated immunity lymphocytes act against target cells directly by killing them or indirectly by releasing mediators.
• Mediators enhance inflammation or activate other lymphocytes/macrophages.

Antigens

• Antigens mobilize the immune system and provoke an immune response.
• Antigens are large, complex molecules not normally in the body.
• Complete antigens consist of large proteins, polysaccharides, lipids, and nucleic acids.
• These antigens exhibit immunogenicity by stimulating lymphocyte/antibody production.
• They also exhibit reactivity by reacting with activated lymphocytes and antibodies.
• Incomplete antigens (haptens) are small molecules only causing a response when attached to protein carriers.
• These have reactivity but not immunogenicity.
• Antigenic determinants are the parts of an antigen that are immunogenic.
• Most antigens possess a variety of antigenic determinants or epitopes.

Adaptive Defense System - Cells and Receptors

• B cells produce antibodies for humoral immunity.
• T cells do not produce antibodies for cell medicated immunity.
• Both B and T cells possess unique receptors, recognize and bind to specific antigenic determinants.
• Receptors on a single cell are identical.

Self-Antigens: MHC Proteins

• Major histocompatibility complex (MHC) are glycoproteins that are self-antigens.
• MHC proteins have a deep groove that holds a peptide.
• MHC-I is on the surface of all body cells.
• MHC-II is less widespread.

Adaptive Defense System - Antigen-Presenting Cells (APCs)

• APCs do not respond to specific antigens.
• APCs engulf antigens and present fragments on their surface and recognized by T cells.
• Major APCs: dendritic cells, macrophages, and B lymphocytes.