B Lymphocytes and Humoral Immunity
The humoral immune response is mediated by antibodies, which are produced by the B lymphocytes. The primary functions of the B lymphocytes are the elimination of extracellular microbes and toxins and subsequent “memory” for a heightened response during future encounters. Humoral immunity is more important than cellular immunity in defending against microbes with capsules rich in polysaccharides and lipid tox-insbecause only the B lymphocytes are capable of responding to and producing antibodies specific for many types of these molecules. The T cells, which are the mediators of cellular immunity, respond primarily to surface protein antigens.
B lymphocytes are produced in the bone borrow and are classified according to the MHC-II proteins, Ig, and complement receptors expressed on the cell membrane.
During development Ig gene rearrangement takes place to insure that only B lymphocytes are capable of producing antibodies (Ig). At each stage of development, a cell-specific pattern of Ig gene is expressed, which then serves as a phenotypic marker of these maturational stages. The B lymphocyte progenitors are known as pro-B and pre-B cells and develop into both mature and naive B lymphocytes in the bone mar- row. Naïve (or immature) B lymphocytes display IgM on the cell surface. These immature cells respond to antigen differently from a mature B cell. They can be functionally removed from the body as a result of interaction with a self-antigen, by undergoing programmed cell death (apoptosis) or by the process of anergy where they become nonresponsive in the presence of the antigen. Naïve B lymphocytes can leave the bone marrow and migrate to peripheral or secondary lymphoid tissues such as the spleen and lymph nodes where they complete the maturation process. Once B lymphocytes become fully mature, they become capable of expressing IgD, in addition to the IgM on the cell membrane surface. Mature B lymphocytes are fully responsive to antigens and are capable of interacting with T cells.
The commitment of a B-cell line to a specific antigen is evidenced by the expression of the membrane-bound Ig receptors that recognize the specific antigen. Initially, when mature B lymphocytes encounter antigens that are complementary to their encoded surface Ig receptor and in the presence of T lymphocyte antigen presentation, they undergo a series of conformational changes that transform them into antibody-secreting plasma cells or into memory B cells (Fig. 13.8). Both cell types are necessary for the ultimate success of the humoral response. The antibodies produced by the plasma cells are released into the lymph and blood, where they can then bind and remove their specific antigen with the help of other immune effector cells and molecules. The memory B lymphocytes have a longer life span and are distributed to the peripheral tissues in preparation for subsequent antigen exposure.
Antibodies are protein molecules also known as immunoglobulins. Igs are classified into five different categories based upon their role in the humoral defense mechanisms. The five classes include IgG, IgA, IgM, IgD, and IgE (Table 13.4). The classic structure of Igs is comprised of four-polypeptide chains with at least two identical antigen-binding sites (Fig. 13.9). Each Ig is composed of two identical light (L) chains and two identical heavy (H) chains that form a characteristic “Y”-shaped molecule. The “Y” ends of the Ig molecule carry the antigen- binding sites and are called Fab (i.e., antigen-binding) fragments. The tail end of the molecule, which is called the Fc fragment, determines the biologic and functional characteristics of the class of Igs.
The heavy and light chains of the Ig have certain amino acid sequences, which show constant (C) regions and variable (V) regions. The constant regions have sequences of amino acids that vary little among the antibodies of a particular class of Ig and determine the classification of the particular Ig (e.g., IgG, IgE). The constant regions, therefore, determine the effector function of the particular antibody. For example, IgG can tag an antigen for recognition and destruction by phagocytes. In contrast, the amino acid sequences of the variable regions differ from antibody to antibody. They also contain the antigen-binding sites of the particular molecule. The different amino acid sequences found in these binding sites allow this region of the antibody to recognize its complementary epitope (antigen). The variable amino acid sequence determines the shape of the binding site, forming a three-dimensional pocket that is complementary to the specific antigen. When B lymphocytes divide, they form clones that produce antibodies with identical antigen-binding regions. During the course of the immune response, class switching (e.g., from IgM to IgG) can occur, causing the B cell clone to produce one of the different Ig types.
IgG (gamma globulin) is the most abundant of the Igs making up 75% of the total circulating antibodies. It is a large molecule with a molecular weight of approximately 150 kDa and is composed of two different kinds of polypeptide chain. IgG possesses antiviral, antibacterial, and antitoxin properties. It is present in all body fluids, readily enters the tissues, and is capable of crossing the placenta where it confers immunity upon the fetus. Intact IgG functioning requires the help of APCs. It binds to target cells as well as Fc receptors on NK cells and macrophages, leading to lysis of the target cell. There are four subclasses of IgG (i.e., IgG1, IgG2, IgG3, and IgG4) with specificity for certain types of antigens. For example, IgG2 appears to be responsive to bacteria that are encapsulated with a lipopolysaccharide layer, such as Streptococcus pneumoniae, Neisseria gonorrhoeae, and several strains of Salmonella.
IgA possesses a dimeric structure and is the second most common Ig found in serum accounting for approximately 15% of all antibodies. It is primarily a secretory Ig that is found in saliva, tears, colostrum (i.e., first milk of a nursing mother), and bronchial, gastrointestinal, prostatic, and vaginal secretions. Because it is primarily found in secretions, its primary function is in local immunity on mucosal surfaces. IgA prevents the attachment of viruses and bacteria to epithelial cells.
IgM accounts for approximately 10% of all circulating antibodies. It normally exists as a pentamer with identical heavy chains and identical light chains. Because of its structure, it is an efficient complement fixing Ig and is instrumental in the ultimate lysis of microorganisms. It also functions as an effective agglutinating antibody, capable of clumping organisms for eventual lysis and elimination. IgM is the first anti- body to be produced by the developing fetus and by immature B lymphocytes.
IgD is a monomer found primarily on the cell membranes of B lymphocytes where it functions as a receptor for antigen. It circulates in the serum in extremely low levels where its function is essentially unknown. IgD on the surface of B lymphocytes contains extra amino acids at C-terminal so that it can successfully anchor to the membrane. It also associates with the Ig-alpha and Ig-beta chains.
IgE is the least common serum IgE because it binds very tightly to the Fc receptors on basophils and mast cells. It is involved in inflammation and allergic responses by causing mast cell degranulation and release of chemical mediators including histamine. IgE is also essential for combating parasitic infections.
Humoral immunity requires the presence of mature B lymphocytes capable of recognizing antigen and which can ultimately mature into antibody-secreting plasma cells. The ultimate response of the antigen–antibody complex formation can take several forms including antigen–antibody complex precipitation, agglutination of pathogens, neutralization of toxins, phagocytosis or lysis of invading organisms, immune cell activation, and complement activation.
Two separate but interrelated responses occur in the development of humoral immunity: a primary and a secondary response (Fig. 13.10). A primary immune response develops when the body encounters the antigen for the first time. The antigen comes in contact with various APCs including macrophages, DCs, and B lymphocytes. The antigen is processed by these cells in association with the MHC-II molecules on the cells surface and then presented to the lymphocytes (i.e., CD4+ T-helper cells) to initiate the immune process. APCs such as macrophages also secrete ILs, which are essential for CD4+ helper T cell activation. The activated CD4+ helper T cells trigger B cells to proliferate and differentiate into clone plasma cells that produce antibody. The primary immune response takes 1 to 2 weeks, but once generated, detectable antibody continues to rise for several more weeks even though the infectious process has resolved. The memory phase or secondary immune response occurs on subsequent exposure to the antigen. During the secondary response, the rise in antibody occurs sooner and reaches a higher level because of available memory cells.
During the primary response, B lymphocytes proliferate and differentiate into antibody-secreting plasma cells. A fraction of the activated B cells do not undergo differentiation but rather remain intact to form a pool of memory B lymphocytes that then become available to efficiently respond to invasion during subsequent exposure. Activated T cells can also generate primary and secondary cell-mediated immune responses and the concurrent development of T memory cells.
The immunization process makes use of the primary and secondary immune responses. The initial vaccination causes production of both plasma cells and memory cells. The plasma cells destroy the invading organism or toxin, and the memory cells provide defense against future exposure. “Booster” immunizations produce an immediate antigen–antibody response that simulates an immediate rise in antibody levels. Current phase I clinical immunization trials for cancer treatment show dense concentrations of CD4+ and CD8+ T lymphocytes and plasma cells in preexisting tumors after vaccination with irradiated malignant cells.