Introductory Chapter: B-Cells

Mourad Aribi

## 1. Introduction

Etymologically, the "B" from B-cells, also referred to as B lymphocytes, stands from the name of bursa of Fabricius, a lymphoid organ found only in birds, as reported, in 1956, by Bruce Glick and Timothy Chang [1, 2], but not from the bone marrow as it has been believed.

B-cells represent about 5–15% of circulating blood lymphocytes and are responsible for the humoral immune response, as a critical component of adaptive immune system. Their roles are not limited only to the production of antigen-specific antibodies after antigen binding with high affinity via their membrane Ig but also to antigen presentation. This allows them to interact with cells involved in cellmediated immunity and to produce cytokines [3] within immunological synapses (IS) [4, 5] that they create with both CD4<sup>+</sup> T-cells [6] and CD8+ T-cells [7] (for review, see [8–10]).

The current chapter presents a brief overview on Igs and phases of B-cell ontogeny and B-lymphoid lineage markers. The end of the chapter summarizes the main types of diseases related to B-cell abnormalities.

#### 2. Immunoglobulins: transmembrane and secreted B-cell receptors

#### 2.1 Structure of Igs

#### 2.1.1 Ig chains

The most common form of Igs in the blood has a heterodimeric structure, about approximately 150 kDa [11], with two antibody sites—paratopes—that bind to the epitope of a specific antigen, located in the fragment antigen-binding [F(ab)]. This structure is composed of two identical heavy (H) and two identical light (L) chains, these being either kappa (Lκ) or lambda (Lλ). The H and L chains are associated with each other by disulfide bridges (Figure 1).

#### 2.1.2 Ig domains

Each chain of Igs is composed functionally of constant (H; CH, L; CL) and variable (H; VH, L; VL) domains. The constant region of H chain is composed of three (for IgG, IgA, IgD) or four (for IgM and IgE) constant domains, designated, respectively, CH1, CH2, CH3, and CH4. Except for IgM and IgE, the region between CH1 and CH2 domains is called the hinge "H" region, permitting flexibility in the chain [12], which is longer and more flexible in IgG3 than the other IgG subclasses [13]. Ig L chains are composed of two separate domains, each having an approximate molecular weight of 12 kDa [14]. The association of the variable domains of the H and L chains defines the site of attachment to the antigen (Figure 1).

#### Figure 1.

Molecular structure of a typical Ig molecule. Ig molecules have a symmetric structure that is stabilized by interchain disulfide bonds. The heavy chain determines the isotypes, i.e., the classes (IgM, IgG, IgA, IgD, IgE) and subclasses (IgG1, IgG2, IgG3, IgG4, IgA1, IgA2) of Igs. Panel (A) shows a simplified schematic representation of an antibody molecule. Panel (B) illustrates a schematic representation of the four-chain composition and the separate domains comprising each chain. This representation is based on the X-ray crystallography of an IgG antibody. Three globular regions form a Y. The two antigen-binding sites are at the tip of the arms, which are attached to the trunk of the Y by a flexible hinge region [16]. Incubation of Igs with papain (papaya proteinase I), in the presence of a reducing agent, results in the production of two monovalent fab fragments (50 kDa each) and one intact fc fragment (about 50–70 kDa). Igs digestion by pepsin leads to the production of one F(ab')2 fragment and numerous smaller peptide fragments of the fc portion. CDRs, complementarity-determining regions; CH, heavy chain constant domain; CHO, carbohydrate; CL, light chain constant domain; fab, fragment antigen binding; fc, fragment crystallizable; H, hinge region; Ig, immunoglobulin; VH, heavy chain variable domain; VL, light chain variable domain.

Constant domains have specify effector functions such as activation of complement or binding to FcRs [15].

binding specificity of the antigen is therefore determined by the loops present at one end of VL and VH domains of Ig chains; the difference in specificity between

Subdivision of the variable region of the Ig molecule. The Ig V region contains seven amino acid regions, four of which are FRs and three of which are CDRs. The FRs are located on the tips of the Y-shaped molecule and act as a scaffold for the CDRs. Of note, this representation shows the physical location of the VH region and the VL region. N region between the V and D regions is called N1, and that between the D and heavy J regions is called N2. CDR, complementarity determining region; FR, framework region; C, invariant constant domain; V,

There are five isotypes (classes) of Igs that are structurally and functionally distinct, IgM, IgD, IgG, IgA, and IgE. The difference between the classes of Igs lies in the constant portion of the H chains: mu (μ) determines the IgM class, gamma (γ) determines the IgG class, alpha (α) determines the IgA class, delta (δ) determines the IgD class, and epsilon (ε) determines the IgE class. So the DNA encoding the constant part of an H chain contains several constant sequences. Thus, there are four subclasses of IgG in both humans (IgG1, IgG2, IgG3, IgG4) and mice (IgG1, IgG2a, IgG2b, and IgG3) and two subclasses of IgA (IgA1 and IgA2) in humans,

Molecular formula (μ2κ2)5 or (μ2λ2)5

Structure mIgM; monomer, sIgM: pentamer

Subclasses None MW 900–970 kDa

Valency 10

H chain typejMW μj65–70 kDa

antibodies is therefore related to these loops [21].

variable domain; D, diversity domain; J, joining domain.

which are unequally distributed in the body fluids [22].

Key Point 1jMain features of human Ig classes and subclasses mIg sIg Properties

2.1.4 Isotypes and subisotypes of Igs

Figure 2.

Introductory Chapter: B-Cells

DOI: http://dx.doi.org/10.5772/intechopen.90636

5

Each Ig domain contains roughly 100–110 amino acids long [14, 17] and consists of a two-layer sandwich of seven to nine antiparallel beta-strands arranged in two beta-sheets/-barrels with a Greek topology [18].

Ig domains play well-defined roles, which depend on the location of each one. So the CH1 domain, located within the F(ab) region, interacts with the constant domain of L chains. The remaining CH domains (CH2-CH3 or CH2-CH4) comprise the Fc region, which defines the isotype, classes, and subclasses of the Ig. The CH2 (CH3 for IgM and IgE) domain allows an important role in mediating the effector functions, including interaction with FcRs and antibody stability thanks to the presence of N-linked glycan, which is conserved in mammalian IgGs at Asn297 as well as in homologous regions of other antibody isotypes [19]. The importance of N-glycosylation is well-known for IgGs, but little is known for other isotypes [20]. The CH3 domain allows dimerization and participates in the stabilization of the binding of the heavy chains to one another through interactions between the CH3 domains. For both IgM and IgA, the CH3 domains have short tailpieces to which the J-chain binds via disulfide bonds, whereas the secretory component is disulfide bonded to one of the CH2 domains of the dimer. CH3 domain of IgG binds to FcγR and of IgE to FcεRI and CD23 [15].

#### 2.1.3 Framework and complementarity determining regions

Each Ig V domain contains three hypervariable regions, corresponding to the site of recognition of the antigen, thus forming the paratope (complementarity determining regions, CDR1, CDR2, and CDR3). CDRs separate four highly conserved segments with less variability, termed the framework regions and designated FR1, FR2, FR3, and FR4 [16] (Figure 2). As for the T-cell antigen receptor (TCR), the

#### Figure 2.

Constant domains have specify effector functions such as activation of complement

constant domain; fab, fragment antigen binding; fc, fragment crystallizable; H, hinge region; Ig, immunoglobulin; VH, heavy chain variable domain; VL, light chain variable domain.

Molecular structure of a typical Ig molecule. Ig molecules have a symmetric structure that is stabilized by interchain disulfide bonds. The heavy chain determines the isotypes, i.e., the classes (IgM, IgG, IgA, IgD, IgE) and subclasses (IgG1, IgG2, IgG3, IgG4, IgA1, IgA2) of Igs. Panel (A) shows a simplified schematic representation of an antibody molecule. Panel (B) illustrates a schematic representation of the four-chain composition and the separate domains comprising each chain. This representation is based on the X-ray crystallography of an IgG antibody. Three globular regions form a Y. The two antigen-binding sites are at the tip of the arms, which are attached to the trunk of the Y by a flexible hinge region [16]. Incubation of Igs with papain (papaya proteinase I), in the presence of a reducing agent, results in the production of two monovalent fab fragments (50 kDa each) and one intact fc fragment (about 50–70 kDa). Igs digestion by pepsin leads to the production of one F(ab')2 fragment and numerous smaller peptide fragments of the fc portion. CDRs, complementarity-determining regions; CH, heavy chain constant domain; CHO, carbohydrate; CL, light chain

Each Ig domain contains roughly 100–110 amino acids long [14, 17] and consists of a two-layer sandwich of seven to nine antiparallel beta-strands arranged in two

Ig domains play well-defined roles, which depend on the location of each one. So the CH1 domain, located within the F(ab) region, interacts with the constant domain of L chains. The remaining CH domains (CH2-CH3 or CH2-CH4) comprise the Fc region, which defines the isotype, classes, and subclasses of the Ig. The CH2 (CH3 for IgM and IgE) domain allows an important role in mediating the effector functions, including interaction with FcRs and antibody stability thanks to the presence of N-linked glycan, which is conserved in mammalian IgGs at Asn297 as well as in homologous regions of other antibody isotypes [19]. The importance of N-glycosylation is well-known for IgGs, but little is known for other isotypes [20]. The CH3 domain allows dimerization and participates in the stabilization of the binding of the heavy chains to one another through interactions between the CH3 domains. For both IgM and IgA, the CH3 domains have short tailpieces to which the J-chain binds via disulfide bonds, whereas the secretory component is disulfide bonded to one of the CH2 domains of the dimer.

Each Ig V domain contains three hypervariable regions, corresponding to the site of recognition of the antigen, thus forming the paratope (complementarity determining regions, CDR1, CDR2, and CDR3). CDRs separate four highly conserved segments with less variability, termed the framework regions and designated FR1, FR2, FR3, and FR4 [16] (Figure 2). As for the T-cell antigen receptor (TCR), the

CH3 domain of IgG binds to FcγR and of IgE to FcεRI and CD23 [15].

2.1.3 Framework and complementarity determining regions

or binding to FcRs [15].

Normal and Malignant B-cell

Figure 1.

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beta-sheets/-barrels with a Greek topology [18].

Subdivision of the variable region of the Ig molecule. The Ig V region contains seven amino acid regions, four of which are FRs and three of which are CDRs. The FRs are located on the tips of the Y-shaped molecule and act as a scaffold for the CDRs. Of note, this representation shows the physical location of the VH region and the VL region. N region between the V and D regions is called N1, and that between the D and heavy J regions is called N2. CDR, complementarity determining region; FR, framework region; C, invariant constant domain; V, variable domain; D, diversity domain; J, joining domain.

binding specificity of the antigen is therefore determined by the loops present at one end of VL and VH domains of Ig chains; the difference in specificity between antibodies is therefore related to these loops [21].
