Introduction to Adaptive Immunity

Introduction to Adaptive Immunity

• Adaptive immunity is the third line of defense, activated when innate defenses are insufficient.
• This system responds specifically to antigens, utilizing T cells and B cells to combat infections.
• It develops immunological memory, enhancing responses to subsequent infections.
• Key properties of adaptive immunity include:
  • Specificity
  • Versatility
  • Memory
  • Tolerance

Types of Immune Responses

• T cells mediate cellular immunity against infected or abnormal cells.
• B cells mediate humoral immunity focusing on pathogens in body fluids.

Properties of Immunity

• Specificity: Individual T or B cells react only to unique antigens through antigen recognition.
• Versatility: Diverse lymphocyte types are produced to target various antigens; activated lymphocytes clone themselves.
• Memory: Inactive memory cells persist after initial exposure, enabling quicker responses upon reinfection.
• Tolerance: The immune system typically disregards normal self-antigens.

Development of T and B Cells

• Lymphoid stem cells originate in the bone marrow; some mature into B cells, others migrate to the thymus to become T cells.
• Activated T cells differentiate into:
  • Cytotoxic T cells (TC) - attack infected or abnormal cells.
  • Regulatory T cells - include helper T (TH), memory T (TM), and suppressor T (TS) cells.

Function of T Cells

• Cytotoxic T Cells: Directly target and destroy infected or foreign cells.
• Memory T Cells: Remain in circulation and can rapidly activate during re-exposure to antigens.
• Helper T Cells: Activate to stimulate B cells and further immune responses upon encountering antigens presented by phagocytes.
• Suppressor T Cells: Modulate or inhibit immune responses after effective pathogen clearance.

Activation of Lymphocytes

• Lymphocyte activation occurs upon exposure to antigens, predominantly protein-based substances.
• Haptens are partial antigens that induce immune responses when bound to body proteins.
• Major histocompatibility complex (MHC) proteins on cell surfaces help identify self from non-self.
• Two classes of MHC proteins:
  • MHC I: Present on all nucleated cells; signals abnormal cells to be destroyed.
  • MHC II: Found on specialized antigen-presenting cells (APCs); signals presence of pathogens.

Antigen Presentation

• Class I MHC proteins display abnormal peptides on infected cells to alert the immune system.
• Class II MHC proteins present processed antigens on APCs, indicating the presence of harmful pathogens.

Antibody-Mediated Immunity: Antibodies

• Five classes of antibodies (immunoglobulins): IgG, IgA, IgM, IgE, IgD, each with distinct functions.
  • IgM: First antibody produced; effective in agglutinating pathogens.
  • IgG: Most abundant; crosses the placenta and provides passive immunity to the fetus.
  • IgA: Found in secretions like tears and saliva; protects mucosal surfaces.
  • IgE: Involved in allergic responses; binds to mast cells and basophils.
  • IgD: Present on B cells, facilitates antigen binding.

Actions of Antibodies

• Neutralize pathogens by blocking binding sites.
• Induce precipitation and agglutination, enhancing pathogen clumping.
• Activate the complement system leading to lysis of pathogens.
• Enhance phagocyte activity and attract them to infection sites.
• Increase inflammation and prevent microbial adhesion.

Primary vs. Secondary Immune Responses

• Primary Response: Slower, takes about two weeks to peak after initial exposure; antibodies gradually rise.
• Secondary Response: Faster and stronger due to memory cells, capable of quick activation even at lower antigen levels; may last for decades.

Revision Questions

• Key terms to define: adaptive immunity, cell-mediated immunity, antibody-mediated immunity, antigen, antibody, hapten, MHC protein.
• Explain the origins of T and B lymphocytes.
• Compare the two forms of specific immunity.
• Describe the functions and types of T lymphocytes.
• Detail the lymphocyte activation process upon antigen exposure.
• Identify the locations of MHC class I and II proteins.
• Describe the immune response assistance provided by class I and II MHC proteins.

Learn about the adaptive immune system, its functions, and the key types of immune responses. This quiz covers T cells and B cells, their roles in immunity, and the essential properties that define adaptive immunity. Test your understanding of how the body develops memory and tolerance against invaders.