Immunity and Immune System Functions

Immunity

• Immunity refers to the body's defense against diseases.
• The immune system comprises cells and molecules that combat pathogenic microbes.
• The immune response is a coordinated reaction to infectious agents.

Key Functions of the Immune System

• Distinguishing 'self' from 'non-self' antigens.
• Launching a specific response against non-self antigens.
• Remembering previously encountered non-self antigens.
• Antibody formation.
• Cell-mediated reactions.

Types of Immunity

• Innate (Natural/Native) Immunity:
  • First line of defense.
  • Present at birth.
  • Immediate initial protection against pathogens.
  • Independent of prior antigen exposure.
  • Lacks specificity, but highly effective.
  • Triggers the adaptive immune response.
  • No memory.
• Adaptive Immunity:
  • Acquired through exposure.
  • Second line of defense.
  • Takes more time to develop, but is more potent.
  • Requires prior antigen exposure.
  • Recognizes both microbial and non-microbial substances.
  • Offers long-lasting protection with memory.
  • Specific.

Components of Innate Immunity

• Epithelial Barriers: Skin, gastrointestinal and respiratory tracts act as mechanical barriers. Also produce antimicrobial molecules like defensins.
• Humoral: Complement system.
• Cellular: Neutrophils, macrophages, and natural killer (NK) cells.

Components of Adaptive Immunity

• Humoral Immunity: Protects against extracellular microbes and toxins.
  • Mediated by B lymphocytes and secreted antibodies (immunoglobulins).
• Cell-mediated (Cellular) Immunity: Defends against intracellular microbes and cancers.
  • Mediated by T lymphocytes.
• Both B and T cells express highly specific receptors for diverse substances called antigens.

Organs of the Immune System

• Primary Lymphoid Organs:
  • Thymus: Site of T cell development.
  • Bone marrow: Site of production of all other blood cells, including naïve B cells.
• Secondary Lymphoid Organs: Sites where adaptive immune responses occur.
  • Lymph nodes.
  • Spleen.
  • Mucosal and cutaneous lymphoid tissues.

Immune System Cells

• Lymphocytes:
  • T lymphocytes (T cells):
    • Originate in bone marrow, mature in thymus.
    • Involved in cell-mediated immunity.
    • Subsets:
      • CD4+ T cells (Helper T cells): Recognize antigens presented by MHC Class II molecules.
      • CD8+ T cells (Cytotoxic T cells): Recognize antigens presented by MHC Class I molecules and directly kill infected or cancerous cells.
      • Regulatory T cells: Suppress the immune response.
  • B lymphocytes (B cells):
    • Originate and mature in bone marrow.
    • Involved in humoral immunity.
    • Produce antibodies.
• Monocytes, Macrophages, and Dendritic Cells:
  • Phagocytize pathogens.
  • Present antigens to T cells.
  • Secrete cytokines.
• Mast Cells and Basophils:
  • Release histamine and other inflammatory mediators.
• Neutrophils and Eosinophils:
  • Involved in phagocytosis and killing of pathogens.

Antibodies (Immunoglobulins)

• Five main classes: IgM, IgG, IgA, IgE, and IgD.
• Functions:
  • Neutralize pathogens.
  • Opsonize pathogens for phagocytosis.
  • Activate complement.
• Structure:
  • Two heavy chains and two light chains.
  • Variable region: Recognizes specific antigens.
  • Constant region: Determines the isotype of the antibody.

Natural Killer Cells (NK cells)

• Non-phagocytic lymphocytes.
• Larger than T and B cells, containing azurophillic granules.
• Lack markers for T or B cells.
• Identified by CD16 and CD56 markers.
• Kill virus-infected cells and tumor cells without prior exposure.
• Recognize abnormal cells through:
  • Antibody-dependent cellular cytotoxicity (ADCC): NK cells bind to IgG-coated target cells through CD16.
  • Perforin-granzymes system: NK cells release perforins (create pores in target cell membranes) and granzymes (induce apoptosis).

Monocytes and Macrophages

• Monocytes are immature macrophages, circulating in the blood.
• Function as:
  • Antigen recognition.
  • Phagocytosis.
  • Secretion of cytokines:
    • Cytokines.
    • Wound Healing proteins (e.g., collagenase, elastase, fibroblast growth factor, angiogenesis factor).
    • Acute phase reactants (e.g., fibronectin, microglobulin, complement components).
  • Antigen presentation.

Soluble Mediators of Immunity

• Soluble molecules that regulate innate immunity and effector cell behavior.
• Opsonins:
  • Coats microbes for phagocytosis.
  • Examples: Immunoglobulins and complement proteins.
• Cytokines:
  • Short-acting soluble proteins.
  • Secreted by hematopoietic and non-hematopoietic cells.
  • Immune system messenger molecules.
  • Types:
    • Innate Immunity Cytokines: TNF, IL-1, IL-12, chemokines.
    • Adaptive Immunity Cytokines: IL-2, IL-4, IL-5, IL-17, IFN-.
    • Colony-Stimulating Factors: stimulate hematopoiesis and increase leukocyte numbers.
• Proteins of the Complement System:
  • Part of the innate immune system.
  • A series of proteins that work together to eliminate pathogens.
  • Functions:
    • Opsonize pathogens.
    • Attract phagocytes.
    • Form membrane attack complexes (MACs) to lyse pathogens
    • Initiate inflammation.

Major Histocompatibility Complex (MHC)

• Membrane-bound proteins encoded by the MHC gene.
• Serve as peptide display molecules for recognition by T lymphocytes.
• Critical for inducing T-cell immunity.
• In humans, MHC genes are located on the short arm of chromosome 6.
• Also called the human leukocyte antigen (HLA) complex.

Importance of the MHC

• Organ/tissue transplantation: MHC compatibility is crucial for successful transplant outcomes.
• Autoimmune diseases: Certain HLA alleles are linked to autoimmune diseases.

Classification of MHC Molecules

• MHC Class I Molecules:
  • Expressed on all nucleated cells and platelets, except erythrocytes and trophoblasts.
  • Encoded by HLA-A, HLA-B, and HLA-C loci.
  • Function:
    • Important for responses to intracellular infections, tumors, and allografts.
    • Interact with CD8+ T lymphocytes during antigen presentation for cytotoxic reactions.
• MHC Class II Molecules:
  • Expressed only on professional antigen-presenting cells (B cells, macrophages, dendritic cells, and activated T cells).
  • Encoded by HLA-DP, HLA-DQ, and HLA-DR.
  • Function:
    • Involved in antigen processing and presentation.
    • Interact with CD4+ T cells (helper cells) for immune responses.
• MHC Class III Molecules:
  • Encoded by genes that express complement system components, cytokines, and some proteins with unknown immune functions.

HLA and Disease Association

• Certain HLA alleles are associated with specific diseases:
  • Inflammatory Diseases: Ankylosing spondylitis is strongly linked to HLA-B27.
  • Autoimmune Diseases: Autoimmune endocrinopathies are associated with HLA-DR alleles.
  • Inherited Errors of Metabolism: Specific HLA alleles are often linked to metabolic disorders.

Explore the intricate workings of the immune system with this quiz. Learn about the key functions of immunity, including the distinctions between innate and adaptive immunity. Test your understanding of how the body defends against diseases and the mechanisms involved.