An antibody molecule consist of two alike light chains (220 amino acids each) ad two similar heavy chains (about 440-450 amino acids each) held together by disulfide bridges; this create the monomeric form of antibody. Enzymes papain cleaves particular monomeric form into two fragments that bind to the antigen (designated as FAB; fragment with antigen binding) and one fragment which does bid to antigen but make crystals (so it's called Fc, crystal forming fragment). Approximate 100 amino acid long amino-terminal ends of both light and heavy chains add up to their variable region denoted VL and VH, correspondingly; the amino acid sequence of this regions of the heavy and light chains are known constant region (designated as CH and CL, correspondingly) as there is little variation in the amino acid sequence of this region between the antibodies belonging to the same class. Every antibody molecule has two antigen-binding domains or sites, particular domain being constituted by the variable regions of one light and one heavy chains of an antibody molecule form its effectors function domain that determines its interaction with the other components of the immune system. The light chains are of two types: and lambda (λ) and Kappa (K) ; the type of a light chain is find out by its constant region.
Different genes encode the Kappa (located in human chromosome 2) and lambda (chromosome 22) light chains, and the heavy chain (chromosome 14). The variable region of particular chain contains 3 highly variable regions known hyper variable regions and denoted as CDR1, CDR2and CDR3 (CDR = complementarily-determining region) divided by 4 constant regions known framework regions (this is designated as FR1, FR2, FR3, FR4). The constant region of particular heavy chain has 3 homologous regions (CH1, CH2 and CH3) which most likely originated from a common parental gene (3 tandem repeats of the parental gene, by following mutations).