Each arm of the y shaped antibody molecule is composed of two folded polypeptide chains and the stem is made from two or sometimes more globular masses of similar type. The arm and the stem are connected through extended segments.

The antibody molecule is composed of domains, each of which is a compact but folded structure. The globular domains are equal said sized , made of two folded polypeptide chains, one heavy  and one light in the arms and two heavy chains in the stem. Each domain has about 110 amino acid residues linked by interchain disulphide bonds. Each specific antibody has a binding site for a specific antigen on each of its arm. This specificity is due to different amino acid sequences at the end of the arms. Therefore, the terminal domains contain variable regions of light(V_L) and heavy chains (V_H). The remaining domains are identical and therefore said to be composed of constant regions. Thus the light chain has one variable region (V_L) and one of the constant region(VH) and at least three constant regions (C_H1, C_H2 and C_H3). The antibody binding sites are situated in the cavities of the variable domain, determined by the position and kind of amino acid residues lining the cavities. The moment an antigen combines with the antibody binding site, the antibody starts functioning. Theories concerning the formation of antibodies and their immune reactions will be discussed later.