Antibodies agains e at part erial tox nde den are the lymph nodes and the spleen. where antiger ucture (pneu ibility comple (MHC)for to T play an im The a e against bacterial Activation of B lymphocytes requires antigen recognition pathogens with cell wall polysaccharides. through the B-c cell receptor.but also a second signal ered by T een T and Structure blasts and begin to divide.leading to formation ofgerminal centres.When they recogniz helper cells Each millilitre of normal serum contains approximately B o mmunoglo ules.Even anti odes specif eted antibodies ted via lym phatic ised aga ontains many diffe antibodies which bind to the antigen in slightly different associated some of them ct with related re even able of stimulating an IgA response.The bulk of IgA found in the requires use of homogeneous antibodies produced either secretions roduce by by neoplastic plasma cells,termed myeloma cells,or by plasm ng in a propria pre hybridoma cells erate and It is a polymeric form of lA containing an additional ed to as my teins component M conta ng a myeloma cell line to ound or an The rom a basolateral membrane recentor for I chain containins only one kind of antibody.referred to as a monoclonal polymeric immunoglobulins,referred to as the polymeric During tran Basically.the immu molecule is a Y-shap o. po to the menic protein c of two release of slgA The rest of the IgA produced at the mucosal sites is released to the systemic circulation where it mmunoglobulins with enzymes and chemical reagents will oioutenoeher apable of cl eaving peptide bonds breaks them up into ating Ig secondary agents that cleave disulfide bonds generate two H-chain surface of mucosal tissues sic molecule into th cytes,the called helper T cell indenendent can tri an antibody eferred to as Fab fragments (for 'fragn ent antiger response in the absence of T lymphocytes.Antigens of the binding).Each fragment possesses only one combining nt me precipi ate,the antigen.The nt er ependent type agment ess called polyclonal Fefragment (forfragment crystalline).Finally,treatment activation.However,they cannot induce isotype switch with pepsin gives rise to several small fragments and a large ragle tha the ial lipopolysa F(abb ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group /www.els.net the protective effect of antibodies present at low concen￾trations, particularly those directed to soluble antigens active at low concentrations, such as bacterial toxins. The major sites of formation of high-affinity antibodies are the lymph nodes and the spleen, where antigen￾presenting cells degrade the protein antigen into peptides that bind to class II molecules of the major histocompat￾ibility complex (MHC) for presentation to T cells. Activation of B lymphocytes requires antigen recognition through the B-cell receptor, but also a second signal delivered by T cells. Most of the interaction between T and B cells occurs in the primary follicle of secondary lymphoid organs. After cell interaction, lymphocytes transform into blasts and begin to divide, leading to formation of germinal centres. When they recognize antigen, helper T cells activate B cells to proliferate and differentiate into antibody-producing cells and memory lymphocytes. Se￾creted antibodies are transported via efferent lymphatics into the venous system and circulate through the peripheral blood to accomplish their effector functions. Some antigens are able to reach the mucosa-associated lymphoid tissue (present in the gut, lung, mammary, lachrymal and salivary glands) in a form capable of stimulating an IgA response. The bulk of IgA found in the secretions, called secretory IgA (sIgA), is produced by plasma cells residing in the lamina propria. It predomi￾nates in exocrine secretions of humans and most mammals. It is a polymeric form of IgA containing an additional glycoprotein chain, designated ‘secretory component (SC)’. Secretory IgM (sIgM), a form of IgM containing bound SC, also occurs in secretions. The SC is synthesized in epithelial and glandular cells. It is first expressed as a basolateral membrane receptor for J chain-containing polymeric immunoglobulins, referred to as the polymeric immunoglobulin receptor (pIgR). During transepithelial transport, polymeric IgA is disulfide bonded to the SC, which is cleaved from its transmembrane segment before release of sIgA. The rest of the IgA produced at the mucosal sites is released to the systemic circulation where it will constitute, together with IgA produced in secondary lymphoid organs, systemic IgA. Circulating IgA does not contain SC, but may occur in both monomeric and J-chain polymeric forms. IgA antibodies neutralize toxins at the surface of mucosal tissues. Because antibody production to some antigens requires the help of T lymphocytes, these antigens are called helper T cell dependent. However, a second group of antigens, called helper T cell independent, can trigger an antibody response in the absence of T lymphocytes. Antigens of the second group activate B cells by different mechanisms and fall into two classes. Helper T cell-independent type 1 antigens can induce proliferation and differentiation of immature and mature B cells, a process called polyclonal activation. However, they cannot induce isotype switch￾ing, affinity maturation and generation of memory B cells. The prototype of these antigens are bacterial lipopolysac￾charides, which play an important protective role against extracellular pathogens. The second class, termed helper T cell-independent type 2 antigens, can activate only mature B cells. The response is dominated by a subset of B cells bearing the CD5 marker. They generally comprise a repetitive structure (pneumococcal polysaccharide, Sal￾monella-polymerized flagellin). The antibodies produced play an important early protective role against bacterial pathogens with cell wall polysaccharides. Structure Each millilitre of normal serum contains approximately 1016 immunoglobulin molecules. Even antibodies specific for the same epitope are heterogeneous. An antiserum raised against a given immunogen contains many different antibodies which bind to the antigen in slightly different fashions, and some of them may cross-react with related antigens and even with antigens exhibiting no obvious structural similarity with the immunogen. Because of this heterogeneity, study of the immunoglobulin structure requires use of homogeneous antibodies produced either by neoplastic plasma cells, termed myeloma cells, or by hybridoma cells. Myeloma cells proliferate and secrete immunoglobulins indefinitely and their products are referred to as myeloma proteins, found in the serum of diseased subjects and animals. Hybridoma cells are obtained experimentally by fusing a myeloma cell line to a normal B lymphocyte obtained from a human or an animal. The hybridoma grows continuously and secretes only one kind of antibody, referred to as a monoclonal antibody. Basically, the immunoglobulin molecule is a Y-shaped tetrameric protein characteristically composed of two H and two L polypeptide chains held together by covalent (disulfide) and noncovalent bonds (Figure 1). Treatment of immunoglobulins with enzymes and chemical reagents capable of cleaving peptide bonds breaks them up into fragments. Three types of treatment are particularly important in understanding antibody structure. First, agents that cleave disulfide bonds generate two H-chain polypeptides of approximately 50 kDa and two L chains of 25 kDa. Second, papain splits the basic molecule into three fragments of approximately 45 kDa. Two of them retain the antibody ability to recognize the antigen and are referred to as Fab fragments (for ‘fragment antigen binding’). Each fragment possesses only one combining site and can bind, but cannot precipitate, the antigen. The third fragment produced by papain digestion can be crystallized from a solution and is therefore referred to as Fc fragment (for ‘fragment crystalline’). Finally, treatment with pepsin gives rise to several small fragments and a large fragment, the F(ab)’2 fragment, of molecular weight double that of one Fab fragment, and capable of binding Antibodies ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3