B1 and B2 cells are B cell subsets. B1 cells are distinguished by the CD5 marker, appear to form a self-renewing set, respond to a number of common microbial antigens, and occasionally generate autoantibodies. B2 cells account for most of the B lymphocytes in adults. This subset generates a greater diversity of antigen receptors and responds effectively to T-dependent antigen.

B cells are derived from progenitor cells through an antigen-independent maturation process occurring in the bone marrow and GALT. Participation of B cells in the humoral immune response is accomplished by reacting to antigenic stimuli through division and differentiation into plasma cells. Plasma cells or antibody-forming cells are terminally differentiated B cells. These cells are entirely devoted to antibody pro duction, a primary host defense against microorganisms.

The specific antibodies produced are able to bind to infected cells, free organisms bearing the antigen, and then inactivate those cells or organisms and destroy them. The condition of hyperacute rejection of transplanted organs is also mediated by B cells. In addition, antigenic stimulation prompts B cells to multiply.

Cell Surface Markers

 B lymphocytes are best known to express CD19 but not CD3 surface membrane markers. During B-cell differentiation in the bone marrow, the surface molecule CD19 appears early and remains on the B cell unit until it differentiates into a plasma cell. Four proteins on the surface of mature B cells-CD19, CD21, CD81, and CD225—from the CD19 complex.

Primitive B cell precursors have δ chains in their cytoplasm and no Ig on their surface. More differentiated (but still immature) B cells have intact cytoplasmic IgM and surface IgM. Mature B cells lose their cytoplasmic IgM and add surface IgD to the surface IgM. These changes appear to occur in the absence of antigen and depend on cytokines.

In humans, there is evidence of four types of B cell surface markers:

 1. Ig receptor is the best studied B cell surface marker. This receptor is actually an antibody molecule with antigenic specificity. According to the clonal selection theory, B cells exist in the body with Ig receptors specific for antigen before exposure to the antigenic substances. When specific antigen exposure does occur, the antigen will select the B cell having an Ig receptor with the best fit.

After binding and cooperative interaction with T cells, B cells undergo transformation into plasma cells. The secreted antibody, in turn, has the same specificity as the Ig receptor on the B cell. Almost all the antibody produced by plasma cells is secreted (plasma cells have few Ig receptors), but 90% of the antibody produced by B cells is expressed as surface Ig receptors. Some antigens (e.g., lipopolysaccharides from some gram-negative organisms) can bind to the Ig receptor and also stimulate an antibody response independent of T cell cooperation (T-independent antigens). This type of response is generally of low intensity and is class-restricted to the production of IgM antibody.

B cells have surface immunoglobulin (sIg), except for very immature lymphocytes and mature plasma cells, that are normally polyclonal (i.e., kappa and lambda light chains are present on the cytoplasmic membrane of B cells). Mu and delta heavy chains are usually found with kappa or lambda chains on any one cell surface. Gamma and alpha chains are rarely found on the surface of properly prepared, normal lymphocytes.

 2. An Fc receptor that specifically binds the Fc portion of IgG antibody may function to aid B cells in binding to antigen already bound to antibody.

3. Receptors that bind fragments of the cleaved complement component C3 have been reported on the surface of approximately 75% of B cells. This receptor binds C3b, iC3b (inactivated C3b), and C3d, but the function of these receptors is not completely understood.

4. B cell surface antigens coded by the MHC class II genes are a fourth type of human B cell marker.

B Cell Activation

 B cells can be stimulated in their resting state to enlarge, develop synthetic machinery, divide, mature, and secrete anti body. The proper signals for this sequence depend on the type of triggers, which can be specific or nonspecific and polyclonal. Specific activation involves the antigen that is complementary to the particular Ig on the surface. Nonspecific activation occurs with B cell mitogens.

Efficient antibody production to complex protein antigens requires T cell help, which in turn develops from APCs presenting antigen to the T cell. Activated T cells secrete a variety of cytokines that together with the specific antigen, trigger the B cell to develop into an antibody-secreting cell. This process also involves class switching.

In the immune response to a foreign protein, the first anti bodies to appear are of the IgM class (or isotype). As the response proceeds, other isotypes (IgG, IgA, and IgE) emerge from Ig class switching. The isotype switch has considerable clinical importance because each of the four major isotypes has specialized biologic properties. IgG is the principal class of antibody in interstitial fluids and IgA is the protective antibody of mucosal surfaces. Isotype switching requires collaboration between antibody-synthesizing B cells and helper CD4+ T cells. The B cell uses IgM molecules on its surface to capture the antigen and present the antigen to the T cell. Contact between the collaborating lymphocytes is enhanced by complementary pairs of CAMs. Some CAMs (e.g., CD4, MHC class II anti gens) are constitutively expressed on the surface of T and B cells, whereas others are induced. For example, contact between B and T cells induces the T cell to express a ligand for the B cell surface molecule CD40. In turn, CD40 interacts with the newly expressed CD40 ligand on the T cell, which leads to the expression of another B cell surface molecule, B7. The latter’s partner on the surface of the T lymphocyte is CD28. These cooperative and synergistic interactions between T and B cells induce the secretion of cytokines such as IL-2 and IL-4.

Isotype switching requires two signals. The first is delivered by an interleukin and the second by the binding of CD40 to its ligand on the T cell. In the process of switching from IgM synthesis to IgE synthesis, IL-4 makes the IgE gene in the B cell accessible to the switch machinery initiated when CD40 binds to its ligand. In this process, the gene that encodes the variable region (the part of the antibody molecule that contains the antigen-binding site) moves from its position near the gene that encodes for IgM to a position near the gene that encodes for IgE.

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Cytotoxic T Lymphocytes

T Regulatory Lymphocytes

Helper T Lymphocytes

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Pathogenesis of AIDS