Diagram Note — Immunoglobulin Structure — Notes | NoteTube

Antibody Structure

Structure	Description	Function
Heavy chains (2)	Longer polypeptide chains forming the backbone	Determine antibody class (IgG, IgM, etc.)
Light chains (2)	Shorter polypeptide chains on outer arms	Contribute to antigen-binding site
Variable region ( $V_H$ + $V_L$ )	N-terminal end; unique for each B-cell clone	Antigen-binding specificity (paratope)
Constant region ( $C_H$ + $C_L$ )	C-terminal end; class-specific	Effector functions
Fab fragment	Each arm of the Y = 1 heavy + 1 light chain	Antigen binding (2 sites per monomer)
Fc fragment	Stem of the Y = constant heavy chain regions	Complement activation (IgM, IgG); Fc receptor binding; placental transport (IgG)
Hinge region	Flexible middle region of heavy chains	Allows flexibility for bivalent antigen binding
Disulfide bonds	Covalent S-S bonds	Link H-H chains; link H-L chains

Class	Structure	Key Function	Special Feature
IgG	Monomer	Most abundant serum Ig; secondary response	ONLY Ig that crosses placenta
IgM	Pentamer	First antibody produced; complement activation	Largest Ig; 10 antigen-binding sites
IgA	Dimer (secretory)	Mucosal immunity; in colostrum, saliva, tears	Secretory component protects from digestion
IgE	Monomer	Allergy; binds mast cells → histamine release	Least abundant serum Ig
IgD	Monomer	B-cell surface receptor; largely regulatory	Function not fully clarified