IgA antibody | Antibodies | Notes | Immunology | Basic Science Series

IgA antibody | Antibodies | Notes | Immunology | Basic Science Series
Immunoglobulin A, IgA antibodies, Mucosal immunity, Secretory IgA, IgA structure, IgA function, Respiratory tract immunity, Gastrointestinal immunity, Antibody neutralization, Pathogen agglutination, Immune system antibodies, Passive immunity, Breast milk antibodies, Salivary antibodies, Mucosal defense mechanism, IgA synthesis, Polymeric Immunoglobulin Receptor, IgA1 and IgA2, Antigen trapping.
Introduction:
Immunoglobulin A (IgA) is one of the five main classes of antibodies found in the human immune system. It plays a crucial role in mucosal immunity, providing protection against pathogens at mucosal surfaces such as the respiratory, gastrointestinal, and genitourinary tracts. IgA antibodies exist in various forms, including monomeric IgA (mIgA) and dimeric IgA (dIgA), each with distinct structural and functional properties.
Structure of IgA Antibody:
IgA antibodies consist of two heavy chains (α-chains) and two light chains (κ or λ chains) arranged in a Y-shaped structure. The basic structure of IgA resembles other antibody classes, with two antigen-binding fragments (Fab) and a crystallizable fragment (Fc).
Heavy Chains: IgA antibodies contain two α-heavy chains that are slightly shorter than those found in IgG antibodies. These chains consist of constant (Cα) and variable (Vα) regions, with the variable region responsible for antigen binding.
Fab Fragments: IgA antibodies have two antigen-binding fragments (Fab) located at the tips of the Y-shaped molecule. Each Fab fragment contains a variable region composed of hypervariable loops that confer antigen specificity.
Functions of IgA Antibody:
Mucosal Immune Defense: IgA antibodies are the primary antibodies found in mucosal secretions such as saliva, tears, breast milk, and mucosal surfaces of the respiratory, gastrointestinal, and genitourinary tracts. They form a crucial component of the mucosal immune defense system, where they neutralize and eliminate pathogens before they can invade host tissues.
Neutralization: IgA antibodies neutralize pathogens by binding to specific antigens, preventing them from adhering to and colonizing mucosal epithelial cells. This prevents the entry of pathogens into the body and limits the spread of infection.
Immune Exclusion: IgA antibodies can form immune complexes with antigens, preventing their penetration into the epithelial barrier and promoting their clearance from mucosal surfaces. This process, known as immune exclusion, helps to maintain mucosal homeostasis and prevent systemic infection.
Immune Regulation: IgA antibodies play a role in immune regulation by modulating the function of immune cells and cytokine production at mucosal sites. They help to maintain the balance between protective immunity and tolerance to harmless antigens such as commensal bacteria and food antigens.
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