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Covers the highly specific immune responses involving B-cells, T-cells, and long-term protection.
Imagine your body maintains a 'Most Wanted' database of every germ it has ever fought. How does your immune system recognize a virus it hasn't seen in a decade and destroy it before you even feel a single symptom?
Unlike the innate immune system, which attacks everything foreign, Adaptive Immunity is highly specific. It consists of two main branches: Humoral Immunity and Cell-Mediated Immunity. Humoral immunity is governed by B-lymphocytes (B-cells), which patrol the 'humors' (bodily fluids). When they detect an extracellular pathogen, they transform into plasma cells and secrete antibodies—proteins designed to neutralize specific threats. Cell-mediated immunity is led by T-lymphocytes (T-cells). These cells do not use antibodies; instead, they directly attack infected host cells or coordinate the overall immune response. Think of B-cells as the 'long-range missile' system and T-cells as the 'hand-to-hand combat' units.
Determine which branch of immunity is active in these two scenarios: 1. A bacterium is floating freely in the plasma of the blood. Since it is in the 'humor' (fluid), B-cells will respond via humoral immunity. 2. A virus has successfully entered a lung cell and is replicating inside. Since the pathogen is intracellular, Cytotoxic T-cells will respond via cell-mediated immunity.
Quick Check
Which type of lymphocyte is responsible for producing antibodies to fight pathogens in the blood?
Answer
B-lymphocytes (B-cells).
Lymphocytes aren't born knowing what to fight; they must be 'shown' the enemy. This happens through Antigen Presentation. Specialized cells called Antigen-Presenting Cells (APCs), like dendritic cells, ingest a pathogen and display its pieces (antigens) on their surface using Major Histocompatibility Complex (MHC) proteins. Once a T-cell or B-cell with a matching receptor binds to this antigen, it undergoes Clonal Selection. This is a rapid proliferation process where the single 'chosen' cell clones itself thousands of times. This creates an army of identical cells specifically designed to target that one antigen. The probability of a random match is low, but because we have roughly lymphocytes, the system is incredibly robust.
Suppose a single activated B-cell divides every 8 hours. How many cells will be in the 'clone army' after 48 hours? 1. Calculate the number of divisions: generations. 2. Use the exponential growth formula: , where is the number of generations. 3. cells. While this seems small, in a real immune response, the rate is much higher, leading to millions of cells in just a few days.
Quick Check
What is the name of the process where a specific lymphocyte proliferates after encountering its matching antigen?
Answer
Clonal Selection.
The first time you encounter a pathogen, the Primary Immune Response is slow, taking 7-14 days to reach peak antibody levels. However, during clonal selection, some cells become Memory Cells instead of active fighters. These cells live for decades. If the same pathogen enters the body again, the Secondary Immune Response is triggered. This response is faster, stronger, and longer-lasting because the 'memory' allows the body to skip the initial recognition phase. Vaccines exploit this by introducing a harmless version of an antigen (the 'primary' exposure) so that if you ever face the real disease, your body reacts with a powerful 'secondary' response immediately.
Imagine a graph where the y-axis is antibody concentration () and the x-axis is time. 1. At Day 0, Exposure A occurs. stays at zero for 5 days, then rises slowly to a small peak at Day 14. 2. At Day 28, Exposure A occurs again. skyrockets within 24 hours to a peak 100x higher than the first. 3. This illustrates the anamnestic response. The difference in the slope of the curve () represents the efficiency of memory cells compared to naive cells.
Which molecule is used by Antigen-Presenting Cells to 'show' an antigen to a T-cell?
Why is the secondary immune response much faster than the primary response?
B-cells are the primary cells involved in cell-mediated immunity.
Review Tomorrow
In 24 hours, try to sketch a graph comparing the primary and secondary immune responses. Label the lag phase and the peak antibody concentrations.
Practice Activity
Research the difference between 'Active Immunity' (like a vaccine) and 'Passive Immunity' (like antibodies in breast milk). How does the presence of memory cells differ between the two?