General Structural Principles
All immunoglobulins share a common basic structural plan. They consist of two types of peptide chains: heavy (Heavy, $H$) and light (Light, $L$). Depending on the immunoglobulin class, the number of these basic structural units ranges from one to five.
Each chain is composed of functional units called domains. An immunoglobulin molecule always contains one variable domain and one to three constant domains. The variable regions of both light and heavy chains together form unique antigen-binding sites (ABS). Each antibody monomer has at least two such sites, allowing it to securely bind an antigen.
Classification of Immunoglobulins
Classification is based on the type of heavy chain, structural features, and specific functions:
- Class IgM (contains $\mu$ heavy chains). Exists in two forms. On the membrane of B lymphocytes, it functions as a monomeric receptor. Early plasma cells secrete it as a large pentamer. Five basic molecules are linked by a specialized J-chain, forming a structure with 10 antigen-binding sites.
- Class IgG ($\gamma$ heavy chains). These are classical monomers and the most abundant antibody pool in the blood. They are actively produced at the end of the primary immune response and dominate during secondary infections. An important feature is that their $F_c$-fragment is readily recognized by macrophage and neutrophil receptors, facilitating phagocytosis.
- Class IgA ($\alpha$ heavy chains). The primary defender of mucosal surfaces. Inside plasma cells, it assembles into a dimer via a J-chain. During transport across covering or glandular epithelium, a secretory component (S) is added to the molecule. This component protects the antibody from degradation by aggressive enzymes in mucosal secretions.
- Class IgE ($\epsilon$ heavy chains). Practically absent in free plasma (less than 0.01%). Almost all IgE is pre-bound via its $F_c$-regions to the surface of circulating basophils and mast cells, awaiting contact with an allergen.
- Class IgD ($\delta$ heavy chains). A monomer that, together with membrane-bound IgM, resides on the surface of naive (unstimulated) B lymphocytes, serving as a receptor.
Effector Mechanisms: Post-Binding Events
Antibody-antigen binding is only the first step. Subsequent destruction mechanisms include:
- Opsonization and Phagocytosis. If the antigen is soluble, specific IgG binds it into immune complexes. These complexes bind to the surface of macrophages and neutrophils (via $F_c$-receptors), leading to internalization (endocytosis) and digestion.
- Complement Activation. When a microorganism or foreign cell is coated with antibodies, a cascade of plasma proteins is triggered. The membrane attack complex (MAC) assembles on the target cell membrane, forming pores that cause osmotic shock and cell lysis (destruction).
- Hypersensitivity Reactions. If an antigen (allergen) binds to IgE already fixed on mast cells, the cells undergo immediate degranulation, releasing histamine and other inflammatory mediators into the tissues.
Dynamics of the Humoral Immune Response
During the initial encounter with an infection (primary response), peak immunoglobulin production is reached only by the end of the second week. This is followed by a decline: plasma cells have a short lifespan (2–3 weeks), and if the antigen is cleared, antibody levels drop.
If the same antigen enters the body again (secondary response), immunological memory is engaged. The reaction develops much faster and more powerfully, with the body immediately synthesizing specialized molecules (IgG, IgA, IgE) and bypassing the prolonged lag phase of IgM production.
There is also a specialized pathway—humoral response to T-independent antigens. Large polymers (such as bacterial lipopolysaccharides) can activate B cells independently, without T-helper cell assistance, by cross-linking multiple B-cell receptors on the membrane. This type of response does not involve class switching, and cells exclusively synthesize IgM.