Innate Defenses

Surface Barriers as the First Line of Defense

The immune system protects the body from pathogens through a series of defenses. Surface barriers are the body's first line of defense, preventing the entry of harmful microorganisms.

• Dermis: The outer layer of skin acts as a physical barrier.

• Lysosome: Enzyme found in secretions that destroys bacterial cell walls.

• Ciliary escalator: Moves mucus and trapped particles out of the respiratory tract.

• Gastric juice: Acidic environment in the stomach kills pathogens.

• Acid mantle: Skin secretions lower pH, inhibiting microbial growth.

• Normal microbiota: Compete with pathogens for resources and space.

Example: The skin and mucous membranes are primary barriers that block pathogens from entering the body.

Innate Internal Defenses: Cells and Chemicals

If pathogens bypass surface barriers, the body relies on innate internal defenses, which include various cells and chemicals that act quickly to contain and eliminate invaders.

• Phagocytes: Cells that engulf and digest pathogens (e.g., neutrophils, macrophages).

• Natural killer cells: Destroy infected or abnormal cells by releasing perforin and granzymes.

• Proteins of the innate immune system:

  • Complement system: Enhances the ability of antibodies and phagocytic cells to clear microbes. Includes activation (classical, alternative, lectin pathways), opsonization, and formation of the membrane attack complex (MAC).

  • C-reactive protein (CRP): Binds to pathogens and activates complement.

• Inflammation: Local response to infection or injury. Characterized by redness, heat, swelling, and pain. Involves histamine, prostaglandins, leukotrienes, and other mediators.

• Fever: Elevated body temperature that enhances immune function and inhibits pathogen growth. Pyrogens are substances that induce fever.

Example: Neutrophils and macrophages migrate to sites of infection, engulfing and destroying pathogens.

Adaptive Defenses

Antigens and the Adaptive Immune Response

Antigens are substances that trigger the body's adaptive immune defenses. The adaptive immune system is highly specific and provides long-lasting protection.

• Antigen: Any molecule that can be recognized by the immune system and elicit an immune response.

• Epitope: The specific part of an antigen recognized by antibodies or lymphocyte receptors.

• Antibody: Protein produced by B cells that binds to specific antigens.

Example: The influenza virus contains antigens that stimulate the production of antibodies.

B and T Lymphocytes & Antigen-Presenting Cells

B lymphocytes and T lymphocytes are the main cells of the adaptive immune system. Antigen-presenting cells (APCs) display antigens to lymphocytes, initiating the adaptive response.

• B cells: Produce antibodies and mediate humoral immunity.

• T cells: Mediate cellular immunity; include helper T cells (CD4+) and cytotoxic T cells (CD8+).

• Antigen-presenting cells: Include dendritic cells, macrophages, and B cells. They process and present antigens to T cells.

Example: Dendritic cells capture antigens and present them to T cells in lymph nodes.

Humoral Immunity

Antibodies and Their Functions

In humoral immunity, antibodies are produced by B cells and target extracellular antigens. Antibodies have variable and constant regions that determine their specificity and function.

• Structure of an antibody: Y-shaped molecule with two antigen-binding sites (variable regions) and a constant region.

• Classes of IgG: Five major classes of immunoglobulins (IgG, IgA, IgM, IgD, IgE), each with distinct functions.

• Mechanisms of antibody protection:

  • Opsonization: Antibodies coat pathogens, enhancing phagocytosis.

  • Agglutination and precipitation: Antibodies clump pathogens together for easier removal.

  • Neutralization: Antibodies block pathogen attachment to host cells.

  • Complement activation: Antibodies trigger the complement cascade, leading to pathogen lysis.

  • ADCC (Antibody-dependent cellular cytotoxicity): Antibodies recruit immune cells to destroy infected cells.

Example: IgG antibodies neutralize bacterial toxins and viruses in the bloodstream.

Table: Major Cells and Molecules of Innate and Adaptive Immunity

Key Equations and Terms

• Antibody Structure:

• Complement Activation (Classical Pathway):

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.