Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 43.3 Adaptive Immunity Defends against Infection of Body Fluids and Body Cells
The adaptive immune response is carried out by B and T lymphocytes, which work together to fight pathogens in body fluids and inside infected cells. Helper T cells play a central coordinating role in activating both humoral (antibody-mediated) and cell-mediated responses.
1. Helper T Cells Activate Immune Responses
- Helper T cells are activated when they bind to an antigen presented by a class II MHC on an antigen-presenting cell (macrophage, dendritic cell, or B cell)
- Binding is stabilized by CD4, an accessory protein
- Activated helper T cells release cytokines that stimulate B cells and cytotoxic T cells
- Helper T cells are essential for both humoral and cell-mediated responses
2. B Cells and Humoral Immunity
- B cells internalize antigens via receptor-mediated endocytosis and present fragments with class II MHC
- Activated helper T cells bind and activate B cells with the same antigen specificity
- Activated B cells proliferate and differentiate into plasma cells (secreting antibodies) and memory B cells
- Plasma cells secrete ~2,000 antibodies per second for 4–5 days
- Each exposure activates multiple B cells due to multiple epitopes on antigens
3. Antibody Functions
- Neutralization: Antibodies block virus or toxin entry into cells
- Opsonization: Antibodies enhance phagocytosis by linking pathogens into clumps
- Antibodies activate complement proteins, leading to membrane attack complex and cell lysis
- Some antibodies recruit natural killer (NK) cells to destroy infected cells
- Five antibody classes (IgA, IgD, IgE, IgG, IgM) differ in constant region but have same specificity
4. Cytotoxic T Cells and Cell-Mediated Immunity
- Cytotoxic T cells recognize infected cells via class I MHC presenting foreign antigen fragments
- Stabilized by CD8 protein
- Once activated, they release perforin and granzymes that trigger apoptosis in infected cells
- This destroys the infection site and exposes pathogen to antibody action
5. Summary of Adaptive Immunity
- Humoral immunity defends against pathogens in blood and lymph using secreted antibodies
- Cell-mediated immunity targets infected cells and cancer cells with cytotoxic T cells
- Both systems rely on memory cells for faster secondary responses upon re-exposure
6. Active and Passive Immunity
- Active immunity develops from infection or vaccination and produces memory
- Passive immunity comes from receiving antibodies (e.g. from mother via placenta or breast milk, or injected antivenin)
- Passive immunity is immediate but temporary
7. Immunization
- Vaccines stimulate primary responses and memory cell formation
- Secondary exposure leads to rapid, strong secondary responses
- Widespread vaccination has eradicated smallpox and reduced diseases like polio and measles
- Misinformation about vaccines has caused dangerous declines in immunization rates
8. Immune Rejection and Antibodies as Tools
- Transplants are rejected due to MHC mismatches
- Monoclonal antibodies, made from a single B cell clone, are used for diagnostics and therapy
- New technologies can identify all viruses a person has been exposed to using antibody profiling
In a Nutshell
The adaptive immune system uses helper T cells to coordinate humoral and cell-mediated responses. B cells produce antibodies to neutralize extracellular threats, while cytotoxic T cells kill infected host cells. Vaccines enhance immunity by generating memory cells. Through clonal selection, diversity, and memory, adaptive immunity provides precise, long-lasting protection tailored to each pathogen.