Immune System Essentials: Barriers, Responses, and Key Concepts

Posted on Jun 30, 2025 in Biomedical Sciences

Immune System Fundamentals

Natural Resistance

  • Non-Specific Species Resistance:
    1. Absence of specific tissue or receptors.
    2. Second host or target site.
    3. Absence of toxins.
  • Individual Natural Resistance:
    1. Nutrition
    2. Fever
    3. Age
    4. Genetic factors
    5. Stress

Physical Barriers

  • Skin: Keratinized cell layer, scaling, dryness, normal flora, fats, washing.
  • Respiratory Tract: Normal flora, microvilli, mucous surfaces.
  • Gastrointestinal Tract: Stomach pH, peristalsis, secretions, normal flora.
  • Urogenital Tract: Urine flow, normal flora.
  • Eye: Tears.

Chemical Barriers

  • Lysozyme: Found in serum, saliva, sweat, tears.
  • Transferrin and Lactoferrin: Found in sweat, serum, tissues; capture iron (bacteriostatic effect).
  • Interferon: Produced by virus-infected cells.
  • Complement: Serum bacteriolytic proteins.

Biological Barriers

  • Microbial Antagonism:
    • Competition
    • Specific antagonism (bacteriocins destroy or inhibit bacteria)
    • Non-specific antagonism
  • Complement System Pathways:
    • Classical Pathway: Antibodies destroy bacteria.
    • Alternative Pathway: Independent of classical pathway (C3b convertase: C3bBDP; C5 convertase: C3bBDC3b).
    • Lectin Pathway: Recognizes mannose.
    • MASP1: Joins C4b and C2a fragments to form C3 convertase.
    • MASP2: Helps release C3b by C3 breakdown.
  • Phagocytosis:
    • Neutrophils (PMNs): Short half-life, 30-70% of total leukocytes, engulf and die (forming pus).
    • Monocytes (APCs): 3-7% of leukocytes, phagocytose but do not die, process agents and become Antigen-Presenting Cells (APCs).

Key Immune Concepts

  • Opsonization: Union of proteins on the surface of microorganisms.
  • Opsonin: Immunoglobulins, C3b; identify bacteria in a non-specific way.
  • Anaphylatoxin: Cationic peptides (C3a, C4a, C5a) that indicate to the immune system where a bacterial infection is present.
  • Anaphylaxis: An exaggerated immune response.
  • Diapedesis: Transition of blood cell elements through fenestrations in capillaries to reach the site of infection without injury.
  • Chemotaxis: Directed movement when there is a difference in chemical compounds.
  • Cytotoxic T Lymphocytes (CTLs): Recognize the constant part of MHC I and destroy infected cells.
  • Helper T Cells: Recognize microorganisms via MHC II and do not kill them.

Adaptive Immune Responses

Types of Adaptive Immunity

  • Antibody Response: Antibody-mediated immunity.
  • Cellular Response: Cell-mediated immunity.

Key Cell Interactions

  • TH (Helper T Cells): CD4+ cells recognize MHC II.
  • TC (Cytotoxic T Cells): CD8+ cells recognize MHC I.

Characteristics of Immune Response

  1. Tolerance
  2. Memory
  3. Specificity

Antigens and Epitopes

  • Antigens: Substances that react with antibody molecules and cellular receptors for antigens.
  • Epitope: The portion that reacts with antibodies or B and T lymphocytes.

Antibody Functions

  1. Capable of immunomodulation.
  2. Opsonize.
  3. Activate the classical complement pathway.
  4. Neutralize viruses and toxins.
  5. Antibody-mediated cytotoxicity.
  6. Blockade effect.
  7. Generate oxidants.

Lymphocyte Activation and Proliferation

  • Clonal Selection: Lymphocyte activation by contact with the antigen.
  • Clonal Expansion: Rapid proliferation of activated B cells.
  • B Lymphocyte Activation:
    1. Indirect contact.
    2. Direct contact.
    3. Chemical mediators (e.g., Interleukin-2 (IL-2) for Th2 lymphocyte activation).

Primary vs. Secondary Immune Response

  • First Antigen Contact (Primary Response): Primarily IgM (more efficient).
  • Second Antigen Contact (Secondary Response): Primarily IgG (faster, more robust, and rapid in synthesis).

Cellular Immune Response Steps

  1. Antigen presentation by APCs (via MHC I).
  2. Activation of TCD4+ (Helper T cells, TH1).
  3. Proliferation of TCD4+ (TH1).
  4. Activation of TCD8+ (Cytotoxic T Lymphocytes, CTL).
  5. Proliferation of CTLs.

Cytokines

  • Pleiotropic: Acts on more than one cell type or site.
  • Redundant: Several different cytokines perform the same function.
  • Multifunctional: The same cytokine regulates several different roles.

Functional Types of Cytokines:

  1. Regulate Innate Immune Response: Produced by macrophages (e.g., TNF-alpha, IL-1, IL-12, IL-6, IL-10, Type I Interferon).
  2. Regulate Adaptive Immune Response: Produced by Th lymphocytes; stimulate B and T lymphocyte proliferation (e.g., IL-2, IL-4, IL-5, IL-13).
  3. Stimulate Hematopoiesis: Produced in the bone marrow; stimulate growth of immature white blood cells (e.g., IL-3, IL-7).

Hypersensitivity Reactions

  1. Type I: Anaphylactic or IgE-mediated.
  2. Type II: Antibody-mediated cytotoxicity (IgG, IgM) recognizing self as foreign.
  3. Type III: Immune complex-mediated, often associated with food.
  4. Type IV: Delayed-type hypersensitivity (DTH), cell-mediated immunity (e.g., tuberculosis, leprosy).
  5. Type V: Superantigens.