Immunoglobulins, also known as antibodies, are Y-shaped glycoproteins produced by B cells that function to recognize and bind to foreign objects like antigens or pathogens. There are five classes of immunoglobulins - IgG, IgM, IgA, IgD, and IgE - which differ in size, sugar content, and biological function. Each immunoglobulin molecule contains two light chains and two heavy chains that form variable and constant regions. The variable regions are responsible for binding to antigens while the constant regions mediate different immune functions.

1. Immunoglobulins

2. Antibodies are also called Immunoglobulins

4. 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

5. The Antibodies Fig 1

6. 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

7. 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

8. 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

9. 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.

10. 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

11. 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