11-5 Immune Response of B-lymphocytes (Cambridge AS A Level Biology, 9700)

B Lymphocyte Origin and Characteristics
📌 B lymphocytes are formed in the bone marrow from blood stem cells and undergo maturation there, hence their name.
🧬 Although mature B lymphocytes are genetically identical clones (same chromosomes and base sequences), they develop different types of receptors on their cell surface membranes.
🤏 Initially, the body has a very low concentration of each type of B lymphocyte, dispersed throughout the blood and lymphatic system.
🔗 Each B lymphocyte receptor is complementary to only one specific type of antigen.

Primary Immune Response (First Exposure)
🦠 An immune response is triggered only when antigens, typically located on pathogens, are present and meet the specific B lymphocyte.
📉 In the initial infection, pathogens replicate, causing illness and symptoms (fever, cough, etc.) because the low concentration of B lymphocytes means there is a delay before they meet the pathogen.
🔄 The specific B lymphocyte that binds to the antigen undergoes clonal selection, followed by clonal expansion (mitosis).
🧬 Clonal expansion produces two main cell types: Memory B lymphocytes and Plasma cells.

Role of Plasma Cells and Antibodies
🧪 Plasma cells are responsible for producing and secreting antibodies—proteins complementary to the antigen—at a high rate (estimated at $10^3$ antibody molecules per second).
⚔️ As antibodies bind to the antigens on the pathogen, the pathogens are destroyed, leading to a decrease in their numbers and recovery from illness.
⏳ Plasma cells have a short lifespan (a few days) because producing large amounts of antibodies requires significant energy, necessitating proper nutrient intake (carbohydrates and proteins) during sickness.

Secondary Immune Response (Subsequent Exposure)
🧠 Memory B lymphocytes remain in the body long-term and circulate widely; they are not "remembering" the infection but rather provide a large pool of specific cells ready for action.
⚡ When the same pathogen re-infects the body, the abundant Memory B lymphocytes initiate a much faster immune response (within hours, not days).
📈 This rapid response leads to faster clonal selection and expansion, resulting in the production of significantly more plasma cells and consequently a higher concentration of antibodies immediately.
🛡️ Due to the swift and overwhelming antibody response, the pathogen is immediately eradicated, and the person does not fall ill (no collateral damage).

Key Points & Insights
➡️ B lymphocytes develop diverse receptors even though they originate from genetically identical clones in the bone marrow.
➡️ The Primary Immune Response is slow because initial B lymphocyte concentration is low, allowing pathogens time to cause illness.
➡️ The function of Memory B lymphocytes is to ensure a faster and stronger secondary response by rapidly proliferating upon re-exposure to the same antigen.
➡️ Recovery from illness relies on Plasma cells secreting antibodies that neutralize the pathogen, emphasizing the need for energy and nutrients to sustain this high-output system.

📸 Video summarized with SummaryTube.com on Feb 23, 2026, 17:02 UTC