Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 43.2 In Adaptive Immunity, Receptors Provide Pathogen-Specific Recognition
Adaptive immunity, found only in vertebrates, relies on highly specific receptors on B and T cells that recognize particular molecules on pathogens. It develops more slowly than innate immunity but provides stronger and faster responses upon re-exposure due to immunological memory.
1. Antigens and Lymphocytes
- An antigen is any molecule that elicits a response from B or T cells
- Each lymphocyte (B or T cell) has receptors specific to one antigen epitope
- B cells mature in bone marrow; T cells mature in thymus
- A single B or T cell has ~100,000 identical antigen receptors
2. Antigen Receptors
- B cell receptors: Y-shaped proteins with 2 identical binding sites
- Binding leads to secretion of antibodies (immunoglobulins), which neutralize pathogens
- Antibodies recognize intact antigens in body fluids or on pathogen surfaces
- T cell receptors: bind to antigen fragments presented by MHC molecules on host cells
- Class I MHC: on all nucleated cells; Class II MHC: on antigen-presenting cells
3. Diversity of Receptors
- Immense diversity (~1 million B cell types, 10 million T cell types) arises from gene recombination
- V (variable), J (joining), and C (constant) gene segments combine randomly
- Recombinase enzyme joins segments during lymphocyte development
- Each receptor is unique and permanent once rearranged
4. Self-Tolerance
- Developing lymphocytes are tested for self-reactivity
- Those that bind self-molecules are either eliminated by apoptosis or inactivated
- Ensures immune system does not attack its own cells
5. Clonal Selection and Proliferation
- When a receptor binds its specific antigen, the cell proliferates into a clone
- Clone includes effector cells (e.g., plasma cells, helper T cells) and memory cells
- This process is called clonal selection
6. Immunological Memory
- Primary response: slower, begins ~10–17 days after first exposure
- Secondary response: faster, stronger, and longer due to memory cells (2–7 days)
- Memory cells persist for decades and respond rapidly upon re-infection
In a Nutshell
Adaptive immunity uses millions of unique receptors on B and T cells to recognize specific pathogen components. It includes antigen recognition, clonal expansion, and memory formation. The result is a targeted, efficient defense that improves with repeated exposure, forming the basis for long-term immunity.