Introduction to B Lymphocytes: Videos & Practice Problems

B lymphocytes, or B cells, are crucial for active humoral immunity, targeting extracellular pathogens. They develop in the bone marrow and possess thousands of identical B cell receptors (BCRs) that bind free antigens. Upon activation by helper T cells, naive B cells can differentiate into effector plasma cells, which produce antibodies, or memory B cells, which enhance future immune responses. This differentiation is vital for effective immune memory and response to infections, highlighting the importance of B cells in the immune system.

Humoral Immunity

B lymphocytes, commonly known as B cells, are crucial components of the adaptive immune system, specifically involved in humoral immunity. This form of immunity targets and eliminates extracellular pathogens, which are harmful microorganisms located outside host cells. B cells achieve this by producing antibodies, specialized proteins that identify and neutralize these pathogens.

To understand the activation of B cells, it's important to recognize the role of antigen-presenting cells (APCs), such as dendritic cells. These cells express Major Histocompatibility Complex (MHC) class I and class II molecules, enabling them to activate both cytotoxic and helper T cells. When helper T cells are activated, they can subsequently stimulate B cells, initiating their immune response.

B cells develop in the bone marrow, one of the primary lymphoid organs, before migrating to secondary lymphoid organs as naive or inactive B cells. These naive B cells have yet to encounter an antigen. Upon encountering their specific antigen, they internalize and process it, displaying fragments on their MHC class II molecules. This presentation allows effector helper T cells to recognize the antigen and release cytokines, which activate the B cells.

Once activated, B cells undergo proliferation, creating clones, and differentiate into two main types: memory B cells and plasma cells. Memory B cells are essential for a rapid response during subsequent infections, while plasma cells are the effector B cells responsible for producing and secreting antibodies. These antibodies perform various immune functions, including neutralizing pathogens and marking them for destruction by other immune cells.

As the course progresses, further details regarding the activation of B cells, the differentiation into memory and plasma cells, and the specific roles of antibodies will be explored, enhancing the understanding of the adaptive immune response.

Match the correct form of adaptive immunity to the scenarios below.
A.   Cell-Mediated Immunity.         B.   Humoral Immunity


_____   1. A macrophage acting as an APC is activated by a CD4 effector cell.
_____   2. A naive B cell is activated after binding an antigen and differentiates into a plasma cell. 
_____   3. Responds to exogenous antigens. 
_____   4. Responds to endogenous antigens.
_____   5. A liver cell infected with a virus undergoes apoptosis when signaled by a CD8 effector cell.

Adaptive immunity is a crucial aspect of the immune system, consisting of two primary forms: cell-mediated immunity and humoral immunity. Understanding the distinctions between these two types is essential for grasping how the body responds to pathogens.

Cell-mediated immunity primarily involves T cells, particularly CD4 helper T cells and CD8 cytotoxic T cells. For instance, when a macrophage acts as an antigen-presenting cell (APC) and is activated by a CD4 effector cell, this scenario exemplifies cell-mediated immunity. The CD4 T cells play a vital role in recognizing antigens and activating other immune cells.

On the other hand, humoral immunity is centered around B cells. When a naive B cell binds to an antigen, it differentiates into a plasma cell, which is responsible for producing antibodies. This process highlights the role of humoral immunity in targeting exogenous antigens, which are those originating from outside the cell.

Furthermore, the immune response can be categorized based on the origin of the antigens. Exogenous antigens trigger a humoral response, while endogenous antigens, which arise from within infected cells, are recognized and responded to by T cells, thus falling under cell-mediated immunity. For example, when a liver cell infected with a virus undergoes apoptosis due to signals from a CD8 effector T cell, this is another instance of cell-mediated immunity in action.

In summary, the interplay between cell-mediated and humoral immunity is vital for an effective immune response, with T cells and B cells working together to identify and eliminate threats to the body.

B Cell Receptors

B cell receptors (BCRs) are crucial components of the immune system, primarily involved in recognizing and responding to extracellular pathogens. These receptors are embedded in the membranes of B cells, which develop in the bone marrow. Each B cell possesses thousands of identical BCRs, allowing for a robust response to antigens. When BCRs encounter free-floating antigens, they can bind to them, a key distinction from T cell receptors (TCRs), which only recognize antigens presented on Major Histocompatibility Complex (MHC) molecules.

Upon binding to an antigen, BCRs facilitate the internalization and processing of the antigen, enabling the B cell to present it on MHC class II molecules for activation by helper T cells. This interaction is vital for the adaptive immune response. The structure of BCRs closely resembles that of antibodies, as they are essentially membrane-bound antibodies. Both share a Y-shaped structure, with the BCR featuring two antigen-binding sites, allowing it to bind to two antigens simultaneously.

The antigen-binding site of the BCR is specifically designed to recognize unique regions on antigens, known as epitopes. This specificity is fundamental for the immune system's ability to target and eliminate pathogens effectively. As B cells differentiate into plasma cells, they secrete large quantities of antibodies that mirror the structure of their BCRs, further enhancing the immune response.

Understanding the function and structure of BCRs is essential for grasping how the immune system identifies and combats infections. As the course progresses, further exploration of BCRs, plasma cells, and antibodies will deepen this understanding.

What is the difference between a TCR and a BCR?

• A. TCRs must be presented an antigen on an MHC molecule from an APC.
• B. BCRs must be presented an antigen on an MHC molecule from an APC.
• C. TCRs mimic the structure of antibodies and are essentially the same.
• D. BCRs are composed of amino acid chains & TCRs are composed of various carbohydrates.
• E. TCRs are composed of amino acid chains & BCRs are composed of various carbohydrates.

Answer: (A)

A naive B cell is activated when:

• A. The B cell’s BCR binds to an endogenous antigen presented by an APC.
• B. The B cell is told to differentiate into a plasma cell or memory B cell by a CD4 effector cell.
• C. The B cell’s BCR binds to a “free” antigen that is not bound to an APC.
• D. When A and B occur.
• E. When B and C occur.

Answer: (E)

Naive B cells Become Effector (Plasma) Cells & Memory Cells

B lymphocytes, or B cells, play a crucial role in the immune response, particularly in their transformation from naive B cells to effector plasma cells and memory B cells. Initially, naive B cells exist in an inactive state, having not yet encountered their specific free antigen. Upon encountering this antigen, the naive B cell internalizes and processes it, presenting it on MHC class II molecules. This presentation is essential for the activation of the B cell by helper T cells, which are vital for the immune response.

Once activated, B cells undergo proliferation, creating identical clones, and differentiate into two main types: effector plasma cells and memory B cells. Effector plasma cells are short-lived and are responsible for producing large quantities of antibodies—thousands per second. These antibodies are crucial for the immediate response to infections, as they can neutralize pathogens and mark them for destruction.

In contrast, memory B cells are long-lived and do not respond immediately to the first infection. Instead, they retain a 'memory' of the encountered antigen, allowing for a faster and more efficient response during subsequent infections. This memory response is a key feature of the adaptive immune system, enhancing the body's ability to combat pathogens it has previously encountered.

Each B cell is equipped with B cell receptors (BCRs), which are membrane-bound forms of the antibodies they will produce upon differentiation. This connection between BCRs and the antibodies underscores the specificity of the immune response, as each B cell is tailored to recognize a particular antigen.

In summary, the journey of naive B cells from inactivity to becoming either effector plasma cells or memory B cells is fundamental to the adaptive immune response, enabling the body to respond effectively to infections and remember past encounters with pathogens.

Antibodies are made by:

• A. Red blood cells.
• B. Plasma cells.
• C. Dendritic cells.
• D. Helper T cells.

Answer: (B)

In which of the following sites in the body can B cells be found?

• A. Lymph nodes.
• B. Spleen.
• C. Red bone marrow.
• D. Intestinal wall.
• E. All of the above.

Answer: (E)

The antibody-secreting progeny of an activated B cell are called:

• A. Antibodies.
• B. Sensitized T cells.
• C. Activated macrophages.
• D. Plasma cells.

Answer: (D)

Which of the following are properties of B cells?

• A. Naive B cells, like naive T cells, can differentiate into effector and memory B cells once activated.
• B. Effector B cells when activated secrete antibodies which fight a novel infection.
• C. Memory B cells when activated secrete antibodies which fight subsequent infections more effectively.
• D. To become activated, a naive B cell must present an immunogenic antigen to an effector CD4 cell.
• E. All are properties of B cells.

Answer: (E)

Here’s what students ask on this topic:

What role do B lymphocytes play in adaptive humoral immunity?

B lymphocytes, or B cells, are essential for adaptive humoral immunity, which targets extracellular pathogens. These cells develop in the bone marrow and possess B cell receptors (BCRs) that can bind to free antigens. Upon encountering an antigen, B cells internalize and process it, presenting it on MHC class II molecules. Helper T cells recognize these antigens and activate the B cells through cytokine release. Activated B cells then proliferate and differentiate into plasma cells, which produce antibodies, or memory B cells, which provide long-term immunity by responding more rapidly to future infections.

How do B cell receptors (BCRs) function in the immune response?

B cell receptors (BCRs) are membrane-bound proteins on B cells that recognize and bind to free antigens. Each B cell has thousands of identical BCRs. When a BCR binds to an antigen, the antigen is internalized, processed, and presented on MHC class II molecules. This presentation allows helper T cells to recognize the antigen and activate the B cell. Activated B cells can then differentiate into plasma cells, which secrete antibodies, or memory B cells, which provide long-term immunity. BCRs are structurally similar to the antibodies that plasma cells eventually produce.

What is the difference between naive B cells, effector plasma cells, and memory B cells?

Naive B cells are inactive B cells that have not yet encountered an antigen. Upon encountering an antigen and being activated by helper T cells, naive B cells can differentiate into either effector plasma cells or memory B cells. Effector plasma cells are short-lived cells that produce and secrete large quantities of antibodies to fight the current infection. Memory B cells, on the other hand, are long-lived cells that do not respond immediately but provide a faster and more robust response to future infections by the same pathogen.

How do helper T cells activate B cells?

Helper T cells activate B cells through a process involving antigen presentation and cytokine release. When a B cell encounters an antigen, it internalizes and processes it, presenting the antigen on MHC class II molecules. Helper T cells recognize these antigens via their T cell receptors (TCRs) and bind to the MHC class II-antigen complex. This interaction, along with the release of cytokines by the helper T cells, activates the B cells. Activated B cells then proliferate and differentiate into plasma cells, which produce antibodies, or memory B cells, which provide long-term immunity.

What are the primary and secondary lymphoid organs involved in B cell development?

The primary lymphoid organs involved in B cell development are the bone marrow and the thymus. B cells fully develop in the bone marrow, while T cells fully develop in the thymus. Once B cells are fully matured, they migrate to secondary lymphoid organs, such as the lymph nodes, spleen, and mucosal-associated lymphoid tissues (MALT). In these secondary lymphoid organs, B cells encounter antigens and undergo activation, proliferation, and differentiation into plasma cells and memory B cells, which are crucial for the adaptive immune response.