Adaptive immunity is our body's sophisticated defense system against pathogens. It's like a highly trained special forces team, with T cells and B cells as the main players. These cells can recognize specific threats, remember them, and mount stronger responses in future encounters.

Unlike innate immunity, which is quick but general, adaptive immunity is slow but precise. It takes days to kick in but offers long-lasting protection. This system's ability to adapt to new threats and remember old ones is crucial for our survival in a world full of evolving pathogens.

Overview of Adaptive Immunity

Features of adaptive immunity

• Specificity enables recognition and response to specific antigens
  • Mediated by lymphocytes (T cells and B cells) that have unique antigen receptors
• Diversity of vast repertoire of antigen receptors generated through somatic recombination of receptor gene segments
• Memory develops long-lived memory cells after initial exposure to an antigen
  • Enables faster and stronger immune response upon subsequent encounters (secondary response)
• Adaptability allows response to new and evolving pathogens
  • Somatic hypermutation and affinity maturation enhance antibody affinity over time
• Functions include elimination of pathogens and infected cells
  • Generation of antibodies for neutralization and opsonization (tagging for phagocytosis)
  • Regulation of immune responses through cytokines and cell-cell interactions

Adaptive vs innate immunity

• Innate immunity serves as first line of defense against pathogens
  • Rapid response within hours to days
  • Non-specific recognition of pathogen-associated molecular patterns (PAMPs)
  • Mediated by physical and chemical barriers, phagocytes, natural killer cells, and complement system
  • No immunological memory generated
• Adaptive immunity (adaptive immune response) acts as second line of defense
  • Slower response taking days to weeks
  • Highly specific recognition of antigens
  • Mediated by T cells and B cells
  • Development of immunological memory provides long-lasting protection against reinfection

Mechanisms of Adaptive Immunity

Cell-mediated and humoral responses

• Cell-mediated immune response mediated by T cells
  • T cell activation requires:
    1. Antigen-presenting cells (APCs) display antigenic peptides on MHC molecules
    2. T cell receptors (TCRs) recognize specific antigen-MHC complexes (antigen recognition)
    3. Co-stimulatory signals (CD28-B7 interaction) needed for full activation
  • Effector T cells include:
    • Cytotoxic T cells (CD8+) directly kill infected or abnormal cells by releasing cytotoxic granules containing perforin and granzymes
    • Helper T cells (CD4+) activate and regulate other immune cells by secreting cytokines to promote appropriate immune responses
      • Subsets include Th1, Th2, Th17, and regulatory T cells
• Humoral immune response mediated by B cells and antibodies
  • B cell activation involves:
    1. B cell receptors (BCRs) recognize specific antigens
    2. T cell help (CD40-CD40L interaction and cytokines) required for most responses
  • Plasma cells differentiate from activated B cells and secrete large amounts of antibodies
    • Antibodies are immunoglobulins (IgM, IgD, IgG, IgA, IgE) that:
      • Neutralize pathogens and toxins
      • Opsonize pathogens for phagocytosis
      • Activate complement system
  • Memory B cells are long-lived cells that rapidly respond to subsequent antigen exposure

Immune tolerance and autoimmunity

• Immune tolerance is a state of unresponsiveness to self-antigens that prevents the immune system from attacking the body's own tissues
  • Central tolerance occurs during lymphocyte development in primary lymphoid organs (thymus and bone marrow)
    • Elimination of self-reactive T and B cells through negative selection
  • Peripheral tolerance occurs in secondary lymphoid organs and peripheral tissues through mechanisms like:
    • Anergy: functional inactivation of self-reactive lymphocytes
    • Deletion: apoptosis of self-reactive lymphocytes
    • Suppression: regulation by immunosuppressive cells (regulatory T cells)
• Importance of immune tolerance in preventing autoimmune diseases
  • Autoimmune diseases arise from immune responses against self-antigens
    • Examples include type 1 diabetes, rheumatoid arthritis, and multiple sclerosis
  • Maintaining tolerance is crucial for homeostasis and preventing unnecessary inflammation

Lymphocyte Activation and Differentiation

• Primary response occurs upon initial antigen encounter
• Antigen recognition by T and B cells in secondary lymphoid organs (e.g., lymph nodes, spleen)
• Clonal expansion of antigen-specific lymphocytes
• Differentiation of activated lymphocytes into effector cells and memory cells