Molecular Structure and Function of Antibodies

Molecular Structure of Immunoglobulins

Immunoglobulins are high molecular weight globular proteins consisting of four polypeptide chains: two heavy (H) and two light (L) chains. These chains are joined by disulfide bonds—one between the L and H chains, and two between the H chains. These molecules are glycoproteins containing glucidic protein chains.

Types of Light and Heavy Chains

There are two types of light (L) chains (λ and κ) and five types of heavy (H) chains (α, δ, ε, γ, μ). This results in five distinct immunoglobulin isotypes: IgA, IgD, IgE, IgG, and IgM.

Characteristics of Immunoglobulin Isotypes

Immunoglobulin G (IgG): This is the most abundant isotype, representing 80% of total immunoglobulins. It rapidly binds to macrophages and neutrophils, facilitating the destruction of pathogens. IgG can cross the placental barrier and is secreted in breast milk; therefore, it is responsible for fetal and newborn immunity.

Immunoglobulin A (IgA): This isotype corresponds to 13% of total immunoglobulins. It is found specifically in serous and mucous secretions such as milk and tears. It works by protecting body surfaces and secretory ducts. Together with IgG, it provides immunity to the newborn through breast milk.

Immunoglobulin M (IgM): Representing 6% of total immunoglobulins, IgM appears on the plasma membrane of naïve B cells. It manifests during the primary response by activating the complement system.

Immunoglobulin D (IgD): Found in very low concentrations (1%), IgD is among the first immunoglobulins synthesized by naïve B cells. Its function is likely related to the activation of these cells. Its structure is similar to IgG, but it varies in the position of the glycosidic residues on the protein chains.

Immunoglobulin E (IgE): Found in very low concentrations in serum and external secretions (0.002%), its concentration increases significantly during allergic processes.

Biological Roles of Immunoglobulins

The main function of antibodies is to recognize and bind to antigens to facilitate their destruction. To achieve this, the constant domain of the immunoglobulin can activate the following mechanisms:

• Activation of the complement system: This process ends with the lysis of the microorganism.
• Opsonization of microorganisms: Antibodies bind to antigens, presenting them to macrophages for destruction.
• Precipitation of toxins: Toxins dissolved in the plasma are precipitated, making them easily destroyed by macrophages.
• Agglutination of antigens: Antigens are clumped in a specific area, facilitating the action of phagocytes and lymphocytes.
• Activation of lymphocytes.