The Antibodies Antibodies are synthesized by B lymphocytes in 2 forms: soluble and cell bound . The cell-bound form is the BCR . The soluble form is released into the circulation as antibodies Each B cell produces Ab of a single specificity and expresses on the cell surface only one BCR specificity The specificity of the Ab produced by a B cell is the same as of its BCR
Antibodies are also called Immunoglobulins Immunoglobulins (Ig) are divided into 5 classes: IgG IgM IgA IgD IgE Ig are glycoproteins They differ in size, amount of CHO and biologic functions
Ig structure Fig 2 The prototypic Ig molecule is IgG. It is made up of 4 polypeptide chains held together by disulfide bonds Two Light (L) and two Heavy (H) chains form the letter Y in which the two arms and the stem are linked by a Hinge region L chain 212-213 amino acids long H chain 440 amino acids The chains are held together by disulfide bonds S-S Hinge
Variable and Constant regions Fig 3 VL CL CH CH The first 110 amino acids in each chain are variable (V regions) blue and purple The remaining 110 aa in the L chain and the remaining 330 aa in the H chain are constant (C regions) (green and yellow) The V regions contact the antigen. The C regions are involved in biological functions V V C C C C
Ig Domains Each segment of about 110 amino acids is tightly packed and forms a domain . Each Ab molecule has therefore 4 V domains (one in each H and one in each L chain), one C domain in each L chain and either 3 or 4 C domains in each H chain. Each IgG H chain has 3 C domains but IgM and IgE H chains each has 4 C domains.
Ig Fragments Fig 4 The IgG molecule can be dissected by proteolytic enzymes: Papain 2 Fab and 1 Fc fragment. Pepsin 1 large F(ab) 2 and several small fragments from the Fc segment
The function of the Ig segments The distinct Ig segments are involved in different functions: The Fab binds Ag The Fc is involved in: Transplacental passage of Ab Enhanced phagocytosis by macrophages and PMN (opsonization) Activation of Complement: Kills foreign cells, triggers inflammatory reactions. Enhances phagocytosis