B cell

What is The Function of B Cell?

The function of B cell mainly includes two sections. On the one hand, they function in the humoral immunity component of the adaptive immune system by secreting antibodies. These antibodies serve as flags or the flares over a battle site. They recruit other defensive molecules in the bloodstream to the site, working toward killing the infection-causing organism. They also signal other immune cells to, in turn, wage war on the invader.

In addition to their essential role in humoral immunity, B cells also regulate many other functions essential for immune homeostasis. Of major importance, B cells are required for the initiation of T-cell immune responses by B cells antigen presenting and cytokines secreting. In mammals, B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, and not from bone marrow as commonly believed.

What is The B Cell Differentiation?

The differentiation process of mammalian B cells can be divided into five stages: pre-B cells, immature B cells, mature B cells, activated B cells, and plasma cells. Among them, the differentiation of pre-B cells and immature B cells is antigen-independent, and the differentiation process takes place in the bone marrow. The antigen-dependent phase refers to mature B cells that are activated after antigen stimulation and continue to differentiate into plasma cells. This phase is performed in lymph node (Figure 1).

Figure 1. Overview of B cell lineage differentiation

Plasma cells, also called plasma B cells, are specialized terminally differentiated B cells that synthesize and secrete large quantities of proteins named antibodies in response to invading antigens. Plasma cells are the key cells involved in humoral immunity. The antibodies produced by a plasma cell are specific to a particular pathogen. They are transported from the plasma cells by the blood plasma and the lymphatic system to the site of the target antigen, where they bind and either neutralize the pathogen or mark it for destruction by other immune components. Plasma cells do not have the ability to differentiate and proliferate. During the differentiation process, specific plasma cell antigens are acquired, which are the main membrane markers that distinguish plasma cells from lymphocytes, such as BCMA⁺, CD10^-, CD19^-, CD20^-, CD27⁺, CXCR4⁺, etc.

Additionally, some of the activated B-cells become memory B-cells, which have very long lives in the bone marrow, lymph nodes and spleen. They remember the antigen they are specific for and are ready to respond quickly if they see it again. These are the cells that give us long-lasting immunity to different invaders.

How B-Cells Give Us Immunity?

A young B-cell, also called a naive B-cell, circulates in the bloodstream, usually ending up in the spleen or lymph nodes. It gets activated by an antigen, which can be any substance the body thinks is foreign, such as a piece of a virus, or a patch of a bacterium's cutter capsule. Once activated, the B-cell begins to transform into a plasma B-cell, whose specialized job it is to mass-produce the antibodies that match the activating invader. Each plasma B-cell makes antibodies to only one antigen. They are very specific. Luckily, there are millions of them in our body so we can fight many different types of infection. Throughout the life of a B-cell, it makes these antibodies. They settle down mostly in the spleen and lymph nodes to pump out antibodies.

How are B Cells Activated?

Among of these processes, B cell activation is a key step in B cell function. B cell activation requires two distinct signals, and results in B cell differentiation into memory B cells or plasma cells. The first signal, also known as antigen-specific signal, is initiated by the binding of specific antigen to the B cell receptors (BCRs), and introduced into the cell by Igα/Igβ with the assistance of co-receptors (such as CD19, CD21, CD81). Upon binding to the BCR, the antigen is internalized by receptor-mediated endocytosis; then it presents peptides from the antigen bound to class II MHC molecules expressed on the B cell surface.

The second signal is able to either take place in a thymus-dependent (TD) or thymus-independent (TI) manner. Most B cell responses to antigen require the interaction of B cells with T helper cells, that is thymus-dependent activation.

In a thymus-dependent manner, presentation of a peptides-class II MHC complex on a B cell enables it to act as an antigen-presenting cell (APC) to T cells. T cell receptors (TCR) on T helper cells recognize and bind to the antigen-complexed class II MHC molecule on the B cell surface resulting in T cell activation. The activated T cell then provides a second activation signal to the B cell, which can occur through a variety of proteins.

However, some antigens are able to directly activate B cells without the help of T cells, so they are called thymus-independent (TI) antigens, and TI B cells are abundant in the spleen. B cells do not fully develop until later in childhood, which is one reason children are susceptible to repeated bacterial infections. TI antigens include repeating polymers (eg, bacterial polysaccharides) and certain bacterial cell wall components (eg, lipopolysaccharides). These antigens all have multiple repeat motifs and bind to several BCRs, thus overcoming the need for a second signal. Note that B cells activation in thymus-independent (TI) manner can only induce the production of IgM antibodies, and cannot form memory cells.

The Markers of B Cell

In the last section of this article, we list the most common markers of B Cell on the following table. B Cell markers refer to several special proteins which distinguish B Cell from other cell types.