Immune System Overview

Immune System Development and Response

Overview of Immune System Selection Theory

  • Two components of selection theory:

    • Deletion Component:

    • Occurs before birth.

    • Function: Deletes immune system cells that recognize self-antigens to prevent autoimmunity.

    • If self-recognition cells are not deleted, it results in autoimmune disorders, where the immune system may attack the body's own cells.

    • Clonal Selection Component:

    • Takes place after birth.

    • Mechanism: Upon encountering foreign antigens (or epitopes), the immune system can recognize and respond.

    • Generates a heightened immune response by selecting specific immune cells to proliferate.

  • Antigens and Epitopes:

    • Antigen: Any substance that triggers an immune response, including pathogens or foreign bodies.

    • Epitope: The part of the antigen that is recognized by the immune system, initiating the response.

Immune Cells and Their Roles

  • Immune System Cells:

    • A diverse set of immune cells are present before birth, with some retaining the ability for self-recognition.

    • Representation of many markers for immune response at birth.

  • T and B Cells:

    • Important for specific immunity, responsible for targeting specific pathogens once they are identified.

    • Once an antigen is identified, clonal selection occurs:

    • Clonal Selection Theory:

      • A process where the immune system selects which cells to clone in response to an antigen, producing many copies of cells that recognize the pathogen.

      • Leads to an effective immune response against the pathogen.

Lines of Defense in Immune Response

  • First Line of Defense (Innate Immunity):

    • Functions: Prevent pathogens from entering the body, and is nonspecific.

    • Components:

    • Skin (physical barrier), oils, tears (lysozyme), secretions, and cilia.

    • Examples: Sebum (skin oil) and lysosomal enzymes in tears that protect against pathogens.

  • Second Line of Defense (Innate Immunity):

    • Functions: Responds after pathogens have breached the first line; also nonspecific.

    • Components:

    • White blood cells such as lymphocytes, neutrophils, and eosinophils, which attack any invading pathogens.

    • Inflammation, phagocytosis, and chemical responses (cytokines, interferons).

  • Third Line of Defense (Adaptive Immunity):

    • Functions: Involves a specific response to specific pathogens.

    • Components:

    • Primarily B and T cells that generate a targeted immune reaction.

    • Memory Cells: Cells that remain after the immune response and can respond rapidly upon re-exposure to the same antigen.

Mechanisms of Immune Function

  • Phagocytosis:

    • Process where immune cells (e.g., macrophages) engulf and digest pathogens.

    • Involves the fusion of lysosomes with phagosomes to break down the ingested material.

  • Cytotoxic T Cells and Immunoglobulins (Antibodies):

    • Work to identify and destroy infected cells or pathogens.

    • Types of immunoglobulins include IgM, IgG with specific functions:

    • IgM: The first antibody produced in response to an infection.

    • IgG: Produced during secondary immune responses, providing long-lasting immunity.

Clonal Selection and Memory Response

  • Primary Immune Response:

    • Occurs upon first exposure to an antigen, has a lag time, and includes the production of IgM antibodies.

    • Generates memory cells after the initial response.

  • Secondary Immune Response (Anamnestic Response):

    • More rapid and effective response due to the presence of memory cells. IgG antibodies are produced first upon subsequent exposures to the same antigen.

  • Recurrence of Infections:

    • Example: Strep throat may recur due to different strains or incomplete recovery from previous infections, highlighting the adaptability and responsiveness of the immune system.

Additional Immune Components

  • Cytokines: Signaling molecules that mediate and regulate immunity, inflammation, and hematopoiesis.

  • Complement Proteins: Part of the second line of defense, tagging pathogens for destruction, enhancing phagocytosis or directly damaging the pathogen through the complement cascade.

Immune System Testing and Monitoring

  • Phenotypic, Immunologic, and Genotypic Testing:

    • Testing to determine the presence of specific antibodies or antigens, used in clinical diagnostics (e.g., streptococcal tests).

  • Serological Testing: Used to identify the presence of antibodies in response to infections (e.g., IgM for active infection and IgG for past exposure).

Important Terminology and Concepts

  • Agglutination: Clumping of pathogens together by antibodies to facilitate their removal. The immune system uses this process to tag pathogens for destruction.

  • Neutralization: Mechanism where antibodies prevent pathogens from adhering or entering host cells.

  • Complement Fixation: Linking of antibodies to complement proteins, which aides in pathogen destruction through a cascade reaction.

Conclusion

  • The immune system comprises various defenses that work in conjunction to protect the body from intruders. Understanding the pathways and mechanisms is critical for developing effective immunizations and therapies.