# The B Lymphocyte Response ## Maturation of B Lymphocytes - **B-lymphocytes (B cells)** remain in the **bone marrow** until they are mature - After maturation, they spread throughout the body - They concentrate particularly in **lymph nodes** and the **spleen** - During maturation, B cells gain specific cell surface receptors called **B cell receptors (BCRs)** - The receptors on B cell surfaces are **antibodies** - They are sometimes referred to as **antibody receptors** - Part of each antibody molecule forms a **glycoprotein receptor** - Each receptor can combine specifically with **one type of antigen**

B cell receptors (BCRs) are antibody molecules on the surface of B cells that act as antigen-specific receptors

- A small number of B cells have the same BCRs - These genetically identical cells are called a **clone** - There are many different mature B cells with different types of antibody receptor on their cell surface membranes - This variety allows B cells to recognise a huge range of different antigens ## B Cells in the Immune Response - B cells need to be **activated** and **increase in number** to play their role in the immune response - This occurs through two main processes: clonal selection and clonal expansion ### Clonal Selection and Activation - B cells with **complementary antibody receptors** bind to antigens on antigen-presenting cells - This is called **clonal selection** - The B cells that "fit" the antigen are selected for replication - Antigen-presenting cells can be: - Phagocytes (e.g. macrophages) - Infected body cells - The pathogens themselves

Only B cells with antibody receptors that are complementary to the specific antigen will be selected and activated

- **Activation** of the B cells requires two signals: 1. Binding to the complementary antigen 2. Interleukins (cytokines) released by **T helper cells** - Both signals are necessary for B cell activation ### Clonal Expansion - Once activated, B cells divide rapidly by **mitosis** to produce clones - This produces large numbers of identical B lymphocytes over several weeks

Clonal expansion is the rapid division of activated lymphocytes to produce many genetically identical copies

- During clonal expansion, B cells differentiate into two types: #### 1. Plasma Cells - Some B lymphocytes differentiate into **plasma cells** - Plasma cells are short-lived (typically only a few days) - They secrete large quantities of **antibody molecules** specific to the antigen - These antibodies are released into: - Blood - Lymph - Linings of the lungs and gut - Plasma cells are essentially "antibody factories" - A single plasma cell can secrete thousands of antibody molecules per second

Plasma cells have extensive rough endoplasmic reticulum and Golgi apparatus to support rapid antibody synthesis and secretion

#### 2. Memory Cells - Other B lymphocytes become **memory cells** - Memory cells remain circulating in the blood for a long time (often for life) - They provide the basis for **immunological memory** - If the same antigen is encountered again: - Memory cells rapidly divide to produce more plasma cells and memory cells - This leads to a faster and stronger **secondary immune response** ## Summary Table: B Cell Response | Stage | Process | Outcome | |-------|---------|---------| | **Maturation** | B cells develop BCRs in bone marrow | Many B cells with different antibody receptors | | **Clonal Selection** | B cells with complementary receptors bind to antigens | Specific B cells are selected for activation | | **Activation** | Binding to antigen + T helper cell interleukins | Selected B cells become activated | | **Clonal Expansion** | Activated B cells divide by mitosis | Large clone of identical B cells produced | | **Differentiation** | B cells differentiate into two cell types | Plasma cells (antibody secretion) and Memory cells (long-term immunity) | ## The Role of Cell Signalling - **Cell signalling** is crucial for coordinating the B cell response - **T helper cells** release **interleukins** (a type of cytokine) - These chemical signals are essential for B cell activation - Without interleukins from T helper cells, B cells cannot become fully activated

Both antigen binding and interleukin signals are required for B cell activation—neither alone is sufficient

- This requirement for two signals prevents inappropriate B cell activation - It acts as a "safety mechanism" to ensure the immune response is appropriate ## Key Features of the B Cell Response - The response is **antigen-specific** - Only B cells with complementary receptors respond to a particular antigen - The response involves **amplification** - A few activated B cells produce thousands of plasma cells - Each plasma cell produces thousands of antibodies - The response creates **immunological memory** - Memory cells persist after infection - They enable faster responses to subsequent infections with the same pathogen - The response is part of the **specific immune system** - Unlike non-specific defences, it targets particular pathogens - It improves with repeated exposure to the same antigen