Chapter 17: Adaptive Immunity (Part 2)

Overview of Adaptive Immune Response

• Also known as the third line of defense.
• Involves lymphocytes: T-cells and B-cells, and antibodies.
• Responsible for fighting an infection by recognizing and responding to specific antigens.

Two Parts of Adaptive Immunity

1. Humoral Immune Response
   • Involves B cells and antibodies.
2. Cell-Mediated Immune Response
   • Involves T cells.
   • T cells kill infected or abnormal eukaryotic cells.

Review of Antibodies

• Basic Structure: Y-shaped monomer composed of four chains (2 heavy, 2 light) held by disulfide bonds.
• Antigen Binding Site: Formed by the variable regions of the heavy and light chains.
• Recognition: Antibodies recognize antigens via the epitope.

Classes of Antibodies

1. IgG
   • Monomer, 80% of antibodies in blood, small enough to cross into tissues.
   • Second class of antibody made by activated B-cells.
2. IgA
   • Dimer, found in mucus and secretions like breast milk.
   • Prevents attachment of pathogens.
3. IgM
   • Large, pentamer, found only in blood.
   • Important for transfusion reactions.
4. IgD
   • Monomer, acts as a receptor on B cells.
5. IgE
   • Monomer, acts on mast cells and basophils, triggers histamine release.

Benefits of Antibody-Antigen Binding

1. Agglutination
   • Clumping of microbes, preventing spread.
   • IgM is highly effective.
2. Activation of Complement
   • Classical pathway, leads to opsonization, inflammation, and lysis.
3. Opsonization
   • Coating with antibodies enhances phagocytosis.
4. Neutralization
   • Prevents viruses and bacteria from binding to cells/tissues.
5. Antibody-Dependent Cell-Mediated Cytotoxicity
   • Antibodies tag infected cells for destruction by other immune cells.

B Cell Development and Activation

• Origin: Derived from stem cells in red bone marrow.
• Maturation: Occurs in bone marrow, involves genetic shuffling.
• Genetic Shuffling: Determines structure of antigen binding site.
• Migration: Mature B cells move to lymph nodes/spleen.
• Activation: Occurs upon antigen encounter; leads to clonal expansion.

Clonal Expansion

• Memory Cells: Wait in lymph nodes for 10 years.
• Plasma Cells: Produce antibodies, first IgM, then IgG.

Primary and Secondary Responses

• Primary Response: First exposure involves naive B cells; slow and weak.
• Secondary Response: Involves memory B cells; fast and strong, prevents disease.

Types of Adaptive Immunity

1. Naturally Acquired
   • Active: Natural exposure (e.g., infection).
   • Passive: Antibody transfer from mother (placenta/breast milk).
2. Artificially Acquired
   • Active: Vaccination (antigen injection).
   • Passive: Antibody infusion (e.g., gamma globulin).

Differences Between Active and Passive

• Active Immunity: Generates memory cells.
• Passive Immunity: Temporary, relies on transferred antibodies.