Features of molecular structure
All the proteins under consideration have a distinct domain structure. The simplest antibody molecule consists of four polypeptide chains, which are united by numerous non-covalent interactions and four disulfide bonds:
- Light chains (L): two identical structures, each about 220 amino acids long.
- Heavy chains (H): two identical structures, from 440 to 700 amino acids long.
Each immunoglobulin domain represents a $\beta$-pleated super-structure that is additionally stabilized by a disulfide bridge. Light chains have two domains: a variable domain ($V_L$) at the N-terminus and a constant domain ($C_L$) at the C-terminus. Heavy chains typically contain one variable domain ($V_H$) and three or four constant domains ($C_H$).
To ensure molecular mobility, a hinge region is located between the $C_{H1}$ and $C_{H2}$ domains. It contains a high number of proline amino acid residues. This prevents the formation of a rigid secondary structure, inhibits the sticking together of adjacent heavy chains, and imparts the necessary flexibility to the entire complex.
Antigen-binding sites and mechanism of action
Antibodies are a frontline defense against foreign agents. The ability to recognize a target resides in the antigen-binding sites (active centers), which are localized at the N-termini of the molecule—between the $V_H$ and $V_L$ variable domains.
The formation of the active center involves not the entire protein sequence, but only 20–30 amino acids located in the hypervariable regions. Since the basic molecule has two such identical sites, antibodies are called bivalent.
The functioning process proceeds in two stages:
- Recognition and binding. Occurs on the surface of the pathogen. The attachment of the target to the antigen-binding site induces conformational changes in the constant domains.
- Initiation of inactivation. The subsequent fate of the complex depends on the class to which the antibody belongs. This stage triggers the mechanisms of antigen destruction and clearance.
Immunoglobulin classes
The classification of immunoglobulins is based on the structural features of the constant domains of their heavy chains (types $\alpha, \delta, \epsilon, \gamma, \mu$ are distinguished). It is the structure of the heavy chains and hinge regions that determines the unique conformation of each class.
- IgG (immunoglobulin G). Predominate in the blood (75%). Exist as monomers. These are the main antibodies of the secondary immune response. They activate the complement system, bind to macrophage receptors, and induce phagocytosis. Special property: this is the only class capable of crossing the placental barrier for intrauterine fetal protection.
- IgM (immunoglobulin M). Occur in two forms. The monomeric form (possessing a 25-hydrophobic-amino-acid tail) is embedded in the membrane of B lymphocytes as a receptor. The secretory form is a huge pentamer held together by disulfide bridges and a J chain. It has 10 binding sites. This is the first class of antibodies secreted during the primary immune response; it effectively activates complement to disrupt bacterial membranes.
- IgA (immunoglobulin A). The main antibodies of secretions (saliva, milk, respiratory and intestinal mucus). Secreted as dimers linked by a J chain. They do not activate complement and macrophages, but effectively bind microbes, blocking their adhesion to the epithelium.
- IgE (immunoglobulin E). Monomers whose heavy chain has one variable and four constant domains. Via their C-termini, they attach to the receptors of mast cells and basophils. Upon antigen binding, the cell receives a signal to release histamine and serotonin, leading to inflammation and allergies (urticaria, hay fever, asthma).
- IgD (immunoglobulin D). Monomers present in trace amounts in serum. Heavy chains contain three constant domains. They serve as receptors on the surface of B lymphocytes; upon antigen binding, they transmit a signal inside the cell, triggering the proliferation of this clone.
Briefly: answers to topical questions
Which heavy chain types correspond to each of the five classes of immunoglobulins (IgG, IgA, IgM, IgE, IgD)?
The heavy chain type determines the class (isotype) of the immunoglobulin:
- Immunoglobulin G (IgG) — $\gamma$ (gamma) chains;
- Immunoglobulin A (IgA) — $\alpha$ (alpha) chains;
- Immunoglobulin M (IgM) — $\mu$ (mu) chains;
- Immunoglobulin E (IgE) — $\epsilon$ (epsilon) chains;
- Immunoglobulin D (IgD) — $\delta$ (delta) chains.
What is the structural difference between the secretory form of IgA and the serum form?
Serum IgA exists predominantly as a monomer. Secretory IgA (sIgA) exists as a dimer in which two monomers are linked to each other via an additional J chain. In addition, the secretory dimer contains an attached secretory component (S peptide chain) that protects the molecule from enzymatic degradation.
Which class of immunoglobulins is capable of crossing the placenta, and by means of what specific receptor?
The only class of immunoglobulins capable of crossing the placental barrier is IgG. Information sources do not provide details on the specific receptor used to mediate this transport.
Into which fragments is the immunoglobulin G molecule cleaved upon hydrolysis by papain and pepsin?
Upon cleavage of immunoglobulin G by pepsin, a single bivalent antigen-binding $F(ab')_2$ fragment (two Fab parts remaining connected by disulfide bonds) and a shortened Fc' fragment are formed. Information sources do not provide details on the fragments formed upon hydrolysis by papain (it is only noted that pepsin acts "unlike papain").
Which specific non-covalent bonds are involved in holding the polypeptide chains of immunoglobulins together?
The antibody molecule contains numerous non-covalent bonds, but their specific types for uniting the polypeptide chains are not specified in the sources.
To memorize the immunoglobulin classes, use the English word GAMED: IgG, IgA, IgM, IgE, IgD.
Class and exam questions
Why is the antibody molecule called bivalent?
Because its simplest monomeric structure contains two completely identical antigen-binding sites formed by variable domains.
Which class of antibodies provides immunity to the newborn?
Intrauterine protection of the fetus is provided by IgG class antibodies. Due to its structural features, this is the only class of immunoglobulins capable of crossing the placental barrier.
How do IgE antibodies participate in the development of an allergic reaction?
IgE molecules attach via their C-termini to mast cells and basophils. When an antigen-allergen binds to them, the cell degranulates, releasing serotonin and histamine into the tissues, which causes allergy symptoms.
What else to study on this topic
- Organization of the $\beta$-pleated superstructure of immunoglobulin domains
- Mechanism of complement system activation by the antigen-antibody complex
- Structural features of the J chain in polymeric immunoglobulins (IgM and IgA)
- Intracellular signal transduction from the receptor forms of IgM and IgD
- The process of phagocytosis and destruction of immune complexes in phagosomes
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