B Cell Activation

• The process of stimulating the B cells to produce antibodies and humoral immune response is called B cell activation.
• B cells are bursal or bone marrow derived lymphocytes. They attack extracellular antigens (bacteria, viruses) by secreting antibodies. So they bring about humoral immune response.
• B cells are activated by infection or vaccine.
• The activation of B cell begins when the antigen enters the body. The antigen Entering through the tissues reaches the lymph nodes and that entering through the blood reaches the spleen.
• B cells are activated in two ways. They are:
  • Thymus independent activation
  • Thymus dependent activation.
• The thymus independent antigens (polysaccharides or lipopolysaccharides) can activate the B cells directly. Some thymus independent antigens are handled by macrophages which present the antigen to B cells. This is called thymus independent activation.

• The thymus dependent antigens (proteins) need the co-operation of T helper Cells. The T cells receive the antigen from B cell and APCs. Then the T helper cells activate the B cells. This is called thymus dependent activation.
• The B cell on its surface contains antigen binding sites called B cell receptors (BCR). Similarly the antigen contains on its surface antigenic determinants.
• The B cell on encounter with the antigen, binding takes place between the receptors of B cells and the antigenic determinants of antigen.

The activation of B cell involves the following events:

• The antigen such as viruses, bacteria, fungus, etc. enters the body through infection. They reach secondary lymphoid organs. Blood borne antigens enter spleen. Antigen coming through skin and epithelia reach lymph nodes. The pathogens inhaled and ingested reach mucosal lymphoid tissues.
• The non-protein antigens such as polysaccharides, lipids, nucleic acids etc. are recognized by B cells as well as macrophages. The B cell gets attached to the polysaccharide antigen. This binding activates the B cell. It is the T cell independent activation as the T cell is not involved in the activation of B cells. The B cell also recognizes the antigen present on the macrophage.
• The protein antigen is recognized by B cells as well as antigen presenting cells (APC) such as dendritic cells, macrophages, etc. The activation of B cell by the protein antigen requires the help of T cells. This activation is called T cell dependent activation.

• The B cell or APC gets attached to the antigen.
• The antigen is taken into the cell by endocytosis.
• The antigen is enclosed inside an endosome.
• The antigen is degraded into peptide fragments.
• A peptide fragment is loaded on class II MHC molecule to form peptide-class II MHC complex.
• The peptide-class II MHC complex is deposited on the surface of the B cell or APC.
• The T helper cell (TH) recognizes this antigen and binds to it.
• The binding of T cell with the B cell produces a T-B cell conjugate. The junction between the TH and B cell is called immunological synapse. In the immunological synapse the partner cells are interconnected by receptor molecules and adhesion molecules.
• This binding activates TH cell.
• The TH cell secretes autocrines such as interleukin. The autocrines bind to the same TH cell. The bound autocrines stimulate the TH cell to proliferate into a clone of TH cells containing effector TH cell and memory TH cell.
• The TH cells also get activated by making immunological synapse with macrophages and dendritic cells.
• The effector TH cell secretes paracrines such as cytokines.
• The paracrines activate B cell.
• The activated B cell undergoes proliferation and produces a clone of B cells.
• The clone of B cell produces two types of cells such as many plasma cells and a few memory B cells.
• The plasma cells are the effector B cells. They secrete antibodies.
• The antibodies move to the site of infection and get attached to the pathogen. The antibodies mark the pathogen for killing by phagocytes and complement system.
• The plasma cells move to the bone marrow and continue secreting antibodies.
• The antibodies remain circulating in the blood.
• The memory B cells mount a quick response when the same antigen enters in future.

Mechanism of B Cell Activation

The activation of B cell involves the following steps:

1.    Recognition of Antigens

The identification of antigen is the first step in B cell activation. The antigen is recognized by B cells, dendritic cells and macrophages.The surface of these cells contains surface receptors. The receptors aresample antibodies attached to the surface of B cells.When these cells encounter the antigen, surface receptor binds with the matching antigen.

2.    Clonal Selection of B Cells

Clonal selection is the selection of one B cell out of the numerous B cells by a specific antigen. The body contains numerous B cells. Each cell has a specific type of B cell receptor. Similarly there are numerous types of antigens. Each has a specific epitope. The B cell, containing the receptor which matches with the antigen, binds with the antigen. Thus one B cell is selected out of the numerous B cells. This is called clonal selection.

3.    T Cell Independent Activation

The activation of B cells without the involvement of T cells is called T cell independent activation or thymus independent activation. The non-protein antigens such as polysaccharides, lipopolysaccharides, Nucleic acids, etc. bring about thymus independent activation. These non-protein antigens are called thymus independent antigens (TI).

The TI antigens are polyvalent containing many identical epitopes. They bind jointly adjacent surface receptors of B cells. This brings about cross-linking of surface receptors. This cross-linking phenomenon of TI antigens activates B Cells directly.

The TI antigens are not internalized, not processed and not presented on class II MHC molecules.

The binding of B cell with the thymus independent antigen activates the B cell. The activated B cell undergoes proliferation and develops into plasma Cells and memory B cells.

4.    T Cell Dependent Activation

The activation of B cells by T cells is called T cell dependent activation or Thymus dependent activation.

The protein antigens bring about T cell dependent activation. The protein antigens are called T cell dependent antigens.

The B cell or APCs recognize the protein antigen and bind with it.

The antigen attached to the B cell or APCs is engulfed by endocytosis. It is enclosed in an endosome. The antigen is degraded into peptide fragments. This is called antigen processing.

The peptide fragments are loaded on class II MHC molecule to form peptide- class II MHC complex. The peptide-class II MHC complex is deposited on the surface of B cell and APCs. This is called antigen presentation.

The antigen presented by the B cell or APCs is recognized by T helper cell (TH cell).

The TH cell binds to the B cell or APCs to form a conjugate.

The binding is enhanced by the receptor molecules, accessory molecules and adhesion molecules present on the surface of the cells. These bring the Conjugates in an intimate contact with each other.

The junction between the TH cell and B cell is called immunological synapse.

The binding of TH cell with the B cell or APCs activates the TH cell.

The TH cell secretes cytokines such as IL-2, IL-4 and IL-5. They bind to interleukin receptors (ILR) of B cell.

                                              IL-2 → IL-2R

                                              IL-4 → IL-4R

                                              IL-5 → IL-5 R

This binding activates the B cell.

5.    Proliferation

The B cell, triggered by the antigen or TH cell, undergoes a series of divisions.

All the daughter cells of an activated B cell are identical and form a clone. This occurs in 5 days with 8 generation cell divisions.

As a result a clone of identical cells is formed.

The proliferation is stimulated by the BCSF (B cell stimulation factor) and BCGF (B cell growth factor) secreted by TH cell.

6.    Affinity Maturation

The process of producing antibodies by B cells with increased affinity for its antigen is called affinity maturation. Affinity maturation is the fine tuning of the shape of Antibody for better fit with the original antigen.

The binding ability of an antibody produced to its antigen at the end of immune response is higher than the antibodies that first produced to this antigen.

The affinity of antibody for antigen increases when the antigen is exposed to the B cell repeatedly. It also increases during the course of immune response. The affinity of antibody for antigen increases several log folds during the secondary immune response.

Affinity maturation occurs in the secondary lymphoid organs such as lymph nodes, spleen, etc.

Affinity maturation is brought about by two processes:

a)   Somatic hyper mutation (SHM)

The genes producing the BCR molecule undergo mutation in the activated B cells. This is called somatic hyper mutation (SHM).

The mutation occurs at every division of activated B cell.

The mutation rate of B cell is 10,00,000 times greater than that of ordinary cells.

SHM produces a diversity of antibodies against the antigen. This mutation produces slight changes in the regions of antigen binding site of the antibody called the complementarity determining region (CDR) (Teng and Papavasiliou (2007).

b)   Affinity Selection

The selection of B cell producing highly specific antibody to the antigen is called affinity selection.

The dendritic cells of lymph node follicles contain complement receptors such as CRI, CR2 and CR3, Fc receptors for antibodies and CD40L.

The dendritic cells display antigen on their surface.

The proliferating B cells come in contact with the dendritic cells. The B cells bind to the antigen displayed by dendritic cells.

The B cells that bind to these antigens firmly are selected to survive.

The unselected cells undergo apoptosis.

The B cell, producing the very suitable antibody against the antigen, will be selected every time and this cell gets the opportunity to proliferate.

Thus hyper mutation produces variations in antibody production and the antigen selects the suitable antibody producing B cell.

This is something like Darwinian principle: Random mutation produces phenotypic variation; favorable variation is selected to survive (survival of the Fittest). The unfavorable variation is not selected and is allowed to die.

This selection process produces a population of B cells producing antibodies with high affinities for the antigen. The signal, generated from the firm binding of B cell, and dendritic cell Blocks the apoptosis of B cell. These selected cells leave the lymph node and enter bone marrow. In the Bone marrow they continue to secrete antibodies for months to years.

7.    Class Switching

The process of formation of new type of antibodies is called isotype switching or class switching. It is a process of switch recombination. The proliferating B cells develop into two types of cells namely plasma cells and memory B cells.

Plasma cells are the effector B cells. They actively secrete antibodies. The cytokines activate the transcription of immunoglobulin genes. The antibodies, secreted by the newly formed plasma cells, are exactly similar to the antibodies (BCR) that captured the antigen, but vary only in their carboxyl terminal.

The antibodies produced initially are predominantly IgM isotype with µ heavy chain. But later, new classes of antibodies are produced .The new classes of antibodies include IgG, IgA and IgE with heavy chains γ α and ε respectively.

In this process, the rearranged VDJ gene segments recombine with a C Region gene and the intervening DNA is deleted.

This leads to the production of antibodies of different classes with heavy chains.

              IgG with γ heavy chain

              IgA with α heavy chain

              IgE with ε heavy chain

The IgD class with ε heavy chain is rarely produced.

8.    Production of Plasma Cells and Memory Cells

The proliferating B cells finally produce two types of cells namely plasma Cells and memory B cells.

a)   Plasma Cells

Plasma cells are antibody producing cells derived from B cells. They are the effector B cells. They are produced in 5 days of B cell activation with 8 generations. They are elliptical in shape with an eccentric nucleus. The cytoplasm is accumulated at one side. The cytoplasm contains large amount of rough endoplasmic reticulum. The Golgi body is well developed. The surface receptors are absent. They have a short life span of 2 to 4 days. The cytoplasm contains plenty of immunoglobulin.

b)   Memory B Cell

A few cells of B cell proliferation develop into memory B cells. Memory cells remember the antigen which activated the B cell. When the same antigen enters the body for the second time, the memory B cells respond quickly. They proliferate quickly when the same antigen encounters in future. They rapidly generate a population of antibody producing plasma cells. Memory cells can live for years or even lifetime.

9.    Secretion of Immunoglobulin

The plasma cells synthesize Immunoglobulin vigorously. They synthesize Immunoglobulin specific to the antigen. During the first response, the plasma cells synthesize IgM class of immunoglobulin. Then IgM is replaced by IgG during the second response. At a later stage IgE is produced. The change of production of one class of immunoglobulin to other class is called class switching. Class switching is brought about by BCDF. The antibodies are released into the blood and lymph. They go in search of their specific antigen and bind with it. This binding will not kill or eliminate the antigen. But they mark the antigens for killing by T killer cells or complements.