What is a B cell?

| Introduction | Types | Functions | Structure | Stages | Antigen-binding Molecules | Co-receptor Molecules | Signal Transduction Molecules |

1. Introduction to B cell:

B lymphocytes, usually known as B cells, belong to the class of white blood cells known as lymphocytes. They carry out their tasks in the humoral immunity division of the adaptive immune system. Antibody molecules made by B cells can either be secreted or inserted into the plasma membrane and are a part of B-cell receptors. When a naive or memory B cell multiplies and transforms into an effectors cell that produces and secretes antibodies in response to a specific antigen, a plasmablast or plasma cell is created.

B cells or B lymphocytes, which are produced from bursae, are important components of the adaptive immune response that underlies humoral immunity in mammals. Humans produce B cells throughout their lifetimes, first in the fetal liver intrauterine and then in the bone marrow following birth. Their development comes from hematopoietic stem cells.

B-cell maturation includes every stage of early differentiation without antigen engagement to maturity, antigen interaction, and, eventually, antibody production. By going through this process, B cells learn two crucial aspects of adaptive immunity: (1) the ability to distinguish between self and non-self, and (2) memory.

Early chicken tests that show the production of antibodies are where B cells get their name. In the 1960s, Max Cooper discovered that the bursa of Fabricius is necessary for the generation of antibodies in chickens. After the bursa was surgically removed, antibody production was slowed down. The cells that produce antibodies are referred to as B cells or bursa-derived cells.

Humans’ B-cell growth occurred mostly in the bone marrow, unlike chickens. Numerous B-cell development pathways exhibit distinctive immunoglobulin (Ig) gene configurations and particular surface markers. Along the routes, there are also developmental checkpoints that decide whether a cell follows the conventional course or an alternate pathway that leads to cell death.

2. Types of B cells:

There are different types of B cells:

2.1. Plasmablast:

An antibody-secreting cell is with a brief lifespan that develops from B proliferation and differentiation. Compared to plasma cells, plasmablasts are produced early in an infection and their antibodies have a poorer affinity for their target antigen. Plasmablasts can develop from either the extrafollicular response from T cell-dependent stimulation of B cells or from T cell-independent activation of B cells.

2.2. Plasma cell:

A B cell is a derived, long-lived, non-proliferating cell that secretes antibodies. There is evidence to support the idea that B cells first develop into a plasmablast-like cell, and subsequently into a plasma cell. Due to affinity development in the germinal center (GC), plasma cells, which are produced later in infection than plasmablasts, have antibodies with a stronger affinity towards their target antigen and release more antibodies. Though they can also result through T cell-independent activation of B cells, plasma cells mainly result from the germinal center reaction from T cell-dependent activation of B cells.

2.3. Lymphoplasmacytoid cell:

A cell hypothesized to be closely related to or a subtype of plasma cells, having morphological characteristics of both B lymphocytes and plasma cells. This cell type is present in pre-malignant and malignant plasma cell dyscrasias linked to the release of IgM monoclonal proteins; these dyscrasias comprise Waldenstr├Âm’s macroglobulinemia and IgM monoclonal gammopathy of unclear relevance.

2.4. Memory B cell:

Developing from B cell differentiation is the dormant B cell.  If they come across the antigen that had stimulated their parent B cell, they circulate through the body and launch a stronger, quicker antibody response (known as the antibody-mediated immune secondary antibody levels).

Both T cell-dependent activation and T cell-independent stimulation of B1 cells take place through the extrafollicular response and the germinal center reaction. It can result in the production of memory B cells.

3. The function of B cells in the human Body:

A white blood cell is one type of lymphocyte. They are essential to our continued health. We would perish without them.

T cells defend us against infection. We are continually exposed to pathogens including bacteria, viruses, and fungi throughout our daily lives. Every exposure could be fatal without T lymphocytes, commonly known as T cells. T cells can eliminate malignant or contaminated cells. Moreover, they control the immune response by assisting B cells in eradicating foreign infections.

Antibodies are made by B cells. An antibody is a class of proteins produced by B lymphocytes, sometimes known as B cells. To neutralize diseases or foreign substances like poisons, these antibodies bind to them. For instance, an antibody can bind to a virus, preventing it from infecting a healthy cell and spreading. B cells can enlist the aid of other cells in the fight against an infected cell.

4. Structure of B cell:

Since there are sadly few investigations on the interior environment and composition of b lymphocytes, this section will concentrate on their exterior surface and related structures. The plasma membrane of B cells is made up of carbohydrates and protein glycosphingolipids in an equal weight ratio. The most important B-cell exterior structures for cell activation, antigen detection, and cell signaling will be discussed in the following paragraphs.

5. Stage-specific Markers:

On B-cells at various phases of formation, maturation, and activation, several chemicals are shown. For instance, CD10 is expressed on B cell lineage pro-, pre-, and germinal center cells, which are first-stage cells. Except for plasma cells, all b cell lineage cells display CD19 and C20. However, memory and plasma cells are the only cells that express CD27. The CD5 molecule is another characteristic of B-1 cells.

6. Antigen-binding Molecules:

At the cell surface, the antigen receptor on B-cells is a functional component of multimolecular protein molecules. The membrane transporter for b-cell antigens extends into the cytoplasm. However, the cytoplasmic sequences are quite brief (tails). These tails are useless for sending signals and turning on B-cells. As a result, the process of signaling and activating lymphocytes involves another protein.

The multimolecular protein complex known as the B-cell receptor (BCR) is joined to other proteins by noncovalent bonds. (1) Membrane-bound immunoglobulin receptor (mIg), Ig-alpha (CD79a), and Ig-beta are involved in the function of BCR (CD79b). Signaling molecules Ig-alpha and Ig-beta attach to mIg as a disulfide-linked heterodimeric complicated. Immune receptor tyrosine-based activation motif (ITAM), which is present in Ig-alpha and Ig-beta, is crucial for signal transduction in both B and T cells. Tyrosine amino acid, which is phosphorylated by protein tyrosine kinases (PTKs) during cellular activation to engage with cytoplasmic signaling proteins, is used by ITAM to transmit activation signals from the cell membrane to the cytoplasm.

According to the theory, genetic diversity and organization provide enough B cell receptor antigens for every bacterium. B-cell receptors are inherited as gene fragments, which each growing cell assembles differently to produce a variety of receptors. A person’s b lymphocytes have the potential to produce up to 1011 distinct antibodies. Additionally, somatic hypermutation of B-cell receptors produces distinct receptors.

7. Co-receptor Molecules of B Cells:

Co-receptors arise on the cell membrane because BCR alone is insufficient for optimal signaling in B-cell activation. Clusters of molecules called B-cell co-receptors can multiply the effectiveness of signaling by a thousand. The complement receptor 2 (CD21), CD19, CD81, and CD225 are proteins that function as co-receptors on B-cells. Although they are not a component of BCR, these proteins are near to it.

Ig-alpha and beta phosphorylation of co-receptor proteins will increase the activation of singles from the cell surface to the cytoplasm. Co-receptors also lower the BCR stimulation threshold, which implies fewer antigens are required for BCR stimulation. The best way to show how the co-receptor works is when a microbial pathogen activates complement and then connects to a B cell.

The antigen can attach to CD21 and BCR simultaneously when it binds to complement protein C3d, preventing the co-receptors complex from clustering, cross-linking with BCR, and phosphorylating CD19 tail (figure 2). This procedure will boost the concentration of signals near BCR.

8. Signal Transduction Molecules of B cell:

When T-B cells interact during Thyms required B-cells activation, surface chemicals such as the following are needed:

Major Histocompatibility Complex Class II Molecule: T helper cells (CD4+) are driven to T-dependent protein antigens by these current molecules, which are peptides. Except for pro-B cells, all B cell lineage cells express MHC II.

Co-stimulator molecules: Following antigen binding, these molecules are needed as a second signal to go along with the first signal. In contrast to naive B-cells, co-stimulators are highly expressed in activated cells. The most well-known co-stimulatory have been examined, and they include B7 and CD40. A class of molecules known as B7 interacts with CD28 on the surface of T-cells. B-cell CD40 interacts with activated T-cell CD40 ligand (CD40L or CD154). In somatic hypermutation and class switching, this relationship is essential. Additionally, the interaction between ICOS on activated T-cells and inducible co-stimulatory ligands on B-cells is essential for the formation of germinal centers. As a result, individuals with dysfunctional ICOSL or ICOS produce low levels of IgG, IgA, and IgE.

Cytokines receptors: B-cells respond to protein antigens through cytokines released by activated T lymphocytes (CD4+). The interaction between the CD40 molecule and the CD40 ligand on the surface of TH lymphocytes promotes the transformation of B lymphocytes into antibiotic plasma cells.