Antibody

An antibody (from Latin anti — against and corpus — body) is a complex specialized glycoprotein produced by B-lymphocytes. It is the main effector weapon of the humoral (acquired) immunity, designed for high-precision binding and neutralization of specific foreign antigens.

Etiology and pathophysiology

Antibodies are synthesized by plasma cells and circulate in the blood plasma and tissue fluid. Structurally, the antibody molecule resembles the letter ‘Y’. Its base (Fc fragment) is identical for all antibodies of one class, while the two branched ends (Fab fragments) have a unique, genetically programmed spatial configuration. It fits perfectly, like a key to a lock, to only one specific antigen.

The mechanism of action of antibodies is multifaceted:

• neutralization — antibodies can physically “coat” a virus, blocking its ability to penetrate healthy cells;
• opsonization — they act as markers, covering the bacteria and making them visible to phagocytes;
• complement system activation — they activate a cascade of blood proteins which help in defense against pathogens by stimulating a localized inflammatory response.

Clinical significance

Antibodies are classified into 5 classes (IgM, IgG, IgA, IgD, IgE):

• Immunoglobulins M (IgM) — they appear first after exposure to a new antigen and often indicate an acute phase of the disease;
• Immunoglobulins G (IgG) — they are produced later, providing long-term, often lifelong immunity; they are the only immunoglobulins that can cross the placenta, important for neonatal protection;
• Immunoglobulins A (IgA) — they are found on mucous membranes, in serum, and in biological fluids (saliva, tears, colostrum, secretions of the respiratory, gastrointestinal, and urogenital tracts), providing antibacterial and antiviral protection;
• Immunoglobulins D (IgD) — their function is currently not completely understood;
• Immunoglobulins E (IgE) — they are used for diagnosing allergies, autoimmune diseases, infections, and other conditions.

Artificially created monoclonal antibodies are also used for treating severe oncological and autoimmune diseases.