11.3 Adaptive Immunity

5 min read•august 28, 2024

Adaptive immunity is your body's specialized defense system. It remembers and targets specific invaders, providing long-lasting protection against diseases. This system works alongside innate immunity to keep you healthy.

T cells and B cells are the key players in adaptive immunity. T cells directly attack infected cells, while B cells produce antibodies to neutralize pathogens. Together, they form a powerful team to fight off various threats.

Adaptive Immunity: Specific Defense

Definition and Characteristics

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  • Adaptive immunity triggers a specific immune response upon exposure to a specific antigen and adapts to the antigen for future encounters
  • Characterized by antigen specificity, diversity, immunological memory, and self/nonself recognition
  • Consists of lymphocytes (B cells and T cells) that recognize and respond to specific antigens through highly specialized receptors

Role in the Body's Defense

  • Provides a targeted defense against pathogens, in contrast to the broad, non-specific protection of innate immunity
  • Adaptive immune response develops slower than the innate response but provides long-lasting protection through immunological memory
  • Works in conjunction with innate immunity to provide comprehensive protection against pathogens (bacteria, viruses, fungi, parasites)
  • Plays a crucial role in the body's ability to fight off infections and prevent disease

Cell-Mediated vs Humoral Immunity

Cell-Mediated Immunity

  • Carried out by T lymphocytes and targets infected cells, cancer cells, and transplanted tissue
  • Cytotoxic T cells directly kill infected or abnormal cells by inducing apoptosis (programmed cell death)
  • Helper T cells secrete cytokines to activate and regulate other immune cells (macrophages, B cells, cytotoxic T cells)
  • Plays a critical role in fighting intracellular pathogens (viruses, some bacteria) and tumors

Humoral Immunity

  • Mediated by B lymphocytes and involves the production of antibodies that neutralize or mark pathogens for destruction
  • B cells differentiate into plasma cells that secrete large quantities of specific antibodies
  • Antibodies neutralize toxins, prevent pathogen entry into cells, and opsonize pathogens for phagocytosis (engulfment by immune cells)
  • Provides protection against extracellular pathogens (bacteria, fungi, parasites) and their toxins

Interaction between Cell-Mediated and Humoral Immunity

  • Cell-mediated and humoral responses often work together to provide comprehensive protection against pathogens
  • Helper T cells activate B cells to produce antibodies and stimulate cytotoxic T cells to kill infected cells
  • Antibodies produced by B cells can also enhance the cell-mediated response by facilitating antigen presentation to T cells

T and B Lymphocytes in Immunity

T Lymphocytes

  • Mature in the thymus and are responsible for cell-mediated immunity
  • Helper T cells (CD4+) activate and regulate other immune cells through cytokine secretion (interleukin-2, interferon-gamma)
  • Cytotoxic T cells (CD8+) directly kill infected or abnormal cells by inducing apoptosis through perforin and granzyme release
  • Regulatory T cells suppress immune responses to maintain self-tolerance and prevent autoimmunity (targeting self-antigens)

B Lymphocytes

  • Mature in the bone marrow and are responsible for humoral immunity
  • Express unique B cell receptors (membrane-bound antibodies) that recognize specific antigens
  • Upon activation, B cells differentiate into plasma cells that secrete large quantities of specific antibodies (IgM, IgG, IgA, IgE, IgD)
  • Memory B cells persist after an infection and provide rapid antibody production upon re-exposure to the antigen

Interaction between T and B Lymphocytes

  • T cells and B cells work together to mount an effective adaptive immune response
  • Helper T cells provide activation signals to B cells, stimulating antibody production and class switching
  • B cells can act as antigen-presenting cells (APCs) to activate T cells by presenting peptide fragments on MHC class II molecules

Antigen Presentation and Adaptive Immunity

Antigen Presentation Process

  • Antigen presentation is the process by which antigen-presenting cells (APCs) display foreign antigens on their surface to activate T cells
  • Major histocompatibility complex (MHC) molecules on APCs present peptide fragments of antigens to T cells
  • MHC class I molecules present intracellular antigens (viral proteins) to cytotoxic T cells
  • MHC class II molecules present extracellular antigens (bacterial proteins) to helper T cells

Antigen-Presenting Cells (APCs)

  • Dendritic cells, macrophages, and B cells are professional APCs that specialize in antigen presentation
  • Dendritic cells are the most potent APCs and play a crucial role in initiating the adaptive immune response
  • Macrophages and B cells can also present antigens to T cells, but their primary functions are phagocytosis and antibody production, respectively

Importance of Antigen Presentation in Adaptive Immunity

  • Antigen presentation is crucial for initiating and directing the adaptive immune response by activating specific T cells
  • The interaction between APCs and T cells occurs in secondary lymphoid organs (lymph nodes, spleen, and mucosa-associated lymphoid tissue)
  • Proper antigen presentation ensures that the adaptive immune response is targeted towards the specific pathogen and not self-antigens

Immunological Memory and Vaccination

Immunological Memory

  • Immunological memory is the ability of the adaptive immune system to respond more rapidly and effectively to a previously encountered antigen
  • Memory B and T cells are long-lived cells that persist after an infection or vaccination and provide swift, robust responses upon re-exposure to the antigen
  • Memory B cells rapidly differentiate into plasma cells, producing high-affinity antibodies
  • Memory T cells quickly expand and differentiate into effector T cells (cytotoxic and helper) to combat the pathogen

Vaccination

  • Vaccination is the administration of a weakened, killed, or component form of a pathogen to stimulate an adaptive immune response and generate immunological memory
  • Vaccines can be live attenuated (weakened pathogen), inactivated (killed pathogen), subunit (pathogen components), toxoid (inactivated toxins), or conjugate (polysaccharide-protein combination)
  • Herd immunity occurs when a significant portion of a population is vaccinated, reducing the spread of the pathogen and protecting those who cannot be vaccinated (infants, immunocompromised individuals)

Booster Shots and Vaccine Efficacy

  • Booster shots are additional doses of a vaccine given to maintain or enhance immunological memory over time
  • Some vaccines require multiple doses (primary series) to generate a strong initial immune response
  • The success of vaccination in preventing infectious diseases relies on the principles of antigen specificity and immunological memory in the adaptive immune system
  • Factors affecting vaccine efficacy include the type of vaccine, route of administration, age, and health status of the recipient, and the prevalence of the pathogen in the population
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