Overview Introduction Antibodies are immunologically active circulating proteins that are composed of two heavy chains paired with two light chains serve as a primary component of humoral immunity are produced by B-cells that can further differentiate into plasma cells that specialize in secreting antibodies mature to make antibodies with higher affinity remain dormant as memory cells bind antigens from a wide variety of pathogens are also known as immunoglobulins (Ig) Antibodies are able to fight infections through multiple mechanisms including opsonization of the surface of the pathogen leading to phagocytosis by innate immune cells like macrophages cytotoxicity by triggering release of toxic compounds by innate immune cells neutralization of pathogens and viruses by blocking interaction of pathogenic proteins with host receptors inactivating virulence factors expressed by pathogens activation of the complement cascade through the classical pathway Antibody Structure Antibodies are composed of two heavy chains paired with two light chains Together these chains create distinct regions of the antibody such as the constant fragment (Fc) two identical variable antigen binding fragments (Fab) These regions differ in both structure and function Differences Between Antibody Regions Feature Antibody Binding (Fab) Constant (Fc) Composition One heavy and one light chain Linked by disulfide bonds Two heavy chains Linked by disulfide bonds Attached to carbohydrate chains Terminus N-terminus of protein chains C-terminus of protein chains Function Binds to antigen in specific manner Binds to complement proteins Binds to effector regions of innate immune cells Variability Site of idiotype diversity Changed in affinity maturation Unique for every antibody Site of isotype diversity Changed in isotype switching 5 types that are shared among all antibodies Antibody Variation and Diversity Antibodies are able to fight an incredible range of infections because of idiotype diversity which governs what antigens can be recognized by antibodies is generated by multiple diversity mechanisms including random recombination of VDJ regions of antibody coding regions random addition of nucleotides to hypervariable regions by TdT random assortment of heavy chains with light chains affinity maturation through somatic hypermutation after antigen exposure ensures that any moiety can be recognized by the variable region of an antibody isotype diversity through five types of constant regions Antibody Isotypes Feature IgA IgD IgE IgM IgG Constant chain type α chain δ chain ε chain μ chain γ chain Concentration High in mucus membranes and in secretions Low Low High in serum during early response High in serum during late response Valence Dimer Monomer Monomer Pentamer Monomer Function Mucosal immunity Transported into mucosal lumens by poly Ig Present on B-cell surfaces Unclear role Defense against parasites Mediates allergies Low affinity Main antibody in early response High affinity Main antibody in late response Recognized by Innate immune cells Complement Unknown Mast cells Basophils Innate immune cells Phagocytes Complement