Immune System Overview

Immune System

• The immune system functions to protect the body against pathogens, which are disease-causing organisms such as bacteria and viruses.

Innate Defenses

• The innate defense system is a non-specific line of defense that is always active.
• Surface Barriers:
  • The skin, the body's largest organ, serves as a physical barrier against pathogens.
  • Mucous membranes, found in lining body cavities and openings, provide additional protection.
  • Mucous membranes often contain acidic secretions, lysozymes (enzyme that breaks down peptidoglycan), and defensins (antimicrobial peptides), contributing to pathogen inhibition.
• Internal Defenses:
  • Phagocytosis: Macrophages, derived from white blood cells, and other phagocytes engulf pathogens or cellular debris.
    • The phagocytic vesicle merges with a lysosome, releasing enzymes that break down the engulfed material.
  • Natural Killer (NK) Cells: NK cells circulate in blood and lymph and target and destroy cancer cells and virus-infected cells.
    • NK cells recognize abnormalities on the surface of infected cells and induce apoptosis (programmed cell death).
  • Inflammatory Response:
    • Triggered by physical trauma, heat, or infection, leading to the release of inflammatory chemicals such as histamine.
    • Macrophages and specific cells in boundary tissues possess receptors that recognize pathogens and release cytokines (another type of inflammatory chemical).
    • Inflammatory chemicals cause vasodilation (dilation of arterioles) and vascular permeability (increased leakage of capillaries), leading to redness, swelling, and pain.
    • These reactions help recruit phagocytes to the site of injury, deliver clotting proteins for repair, and help clear foreign material via lymphatic vessels.
    • Neutrophils (first responders) followed by macrophages arrive at the site of inflammation.
    • Margination refers to phagocytes adhering to capillary walls at the site of injury, recognizing signals on inflamed cells.
    • Diapedesis is the process of phagocytes squeezing out of the capillary.
    • Chemotaxis is the movement of phagocytes following a gradient of molecules toward the source of injury.

Adaptive Defenses

• The adaptive defense system offers a specific response to each foreign substance encountered.
• Antigens:
  • Foreign substances recognized as "non-self" are called antigens.
  • Examples include proteins, polysaccharides, lipids, and pathogens.
  • The antigenic determinant is the specific part of the antigen that is recognized by the immune system.
  • Antibodies and lymphocytes bind to specific antigens, similar to enzyme-substrate interactions.
• Lymphocytes:
  • Lymphocytes originate in the red bone marrow from hematopoietic stem cells.
  • They become immunocompetent, gaining the ability to recognize a specific antigen.
  • They undergo clonal selection, meaning they proliferate and produce thousands of copies of themselves, all with the same antigen-specific receptors.
• B and T Lymphocytes:
  • B lymphocytes (B cells) are responsible for humoral immunity.
    • Their surface receptors are membrane-bound antibodies.
  • T lymphocytes (T cells) are responsible for cellular immunity.
    • They possess T cell receptors that recognize specific antigens.
• Antigen-Presenting Cells (APCs):
  • APCs present antigens to T cells, aiding in the T cell activation process.

Humoral Immunity

• The humoral immune response relies on antibodies produced by B cells.
• Primary Immune Response:
  • When a new B cell encounters its antigen, it undergoes endocytosis followed by proliferation and differentiation into plasma cells.
  • Plasma cells produce large amounts of antibodies, which circulate in blood and lymph, targeting the antigen.
  • This takes several days, making it slow to respond to an initial invasion.
• Secondary Immune Response:
  • Subsequent exposures to the same antigen result in a rapid and robust antibody production due to memory B cells generated during the primary response.
• Active Humoral Immunity:
  • Develops after exposure to an antigen through infection or vaccination.
• Passive Humoral Immunity:
  • Acquired when antibodies are introduced directly into the body through sources like mother's milk or gamma globulin injections.
• Antibodies (Immunoglobulins):
  • Large, Y-shaped proteins composed of four polypeptide chains linked by disulfide bridges.
  • They consist of two heavy chains and two light chains.
  • Variable regions (V regions) at the tip of the Y arms are responsible for antigen binding.
  • Constant regions (C regions) are similar within each class of antibody.
  • Five classes of antibody: IgM, IgA, IgD, IgG, and IgE, differing in structure, function, and location.
  • Antibodies tag antigens for destruction.

Cellular Immunity

• Cellular immunity is carried out by T cells, which directly target and destroy infected cells.
• T Cell Receptors:
  • Receptors on T cells recognize specific antigens presented by APCs.
• CD4+ T Cells (Helper T cells):
  • Activate B cells, T cells, and macrophages.
  • Release cytokines that coordinate immune responses.
• CD8+ T Cells (Cytotoxic T cells):
  • Destroy foreign cells or body cells infected by viruses or bacteria.
• Activation and Differentiation of T cells:
  • T cell receptors interact with antigens presented by APCs.
  • Co-stimulation signals from other molecules on the APC are needed for full T cell activation.
  • Activated T cells proliferate and differentiate into various effector T cell types.

Immune System Summary

• The innate immune system is the first line of defense, providing immediate and non-specific protection against pathogens.
• The adaptive immune system is specific and highly tailored to each encountered antigen.
• Humoral immunity relies on antibodies produced by B cells, while cellular immunity involves T cells directly targeting and destroying infected cells.
• Understanding the interplay between these systems is key to comprehending the body's defenses against diseases.