NAJRAN UNIVERSITY College of Medicine

1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 13 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

2. IMMUNOGLOBULINS (ANTIBODIES) ‑ Immunoglobulins are glycoproteins present in the gamma globulin fraction of the serum. ‑ They are produced by B‑lymphocytes in response to exposure to an antigen and react specifically with that antigen. ‑ Immune response by antibodies is called the humoral immune response.

3. Structure of immunoglobulins (Ig) : • 1. Basic structure: • ‑ The basic unit of the Ig molecule is formed of 4 polypeptide chains: 2 light and 2 heavy chains, held together by interchain disulphide bonds. • Light (L) chains: These are 2 identical short chains; each consists of about 200 amino acids. • According to the amino acid sequence, there are 2 types of light chains kappa (K) and lambda ().

4. Heavy chains: These are 2 identical long chains; each consists of about 400 amino acids. According to the amino acid sequence, there are 5 types of heavy chains; gamma (), alpha (), Mu (), delta (), and epsilon ().According to these types to heavy chains, there are 5 types of immunoglobulins (IgG, IgA, IgM, IgD and IgE) Disulphide bonds:Interchain and intrachain bonds

5. Regions of the molecule: Each light and heavy chain consists of 2 regions: The variable (V) region: It is a segment formed of one domain called, the variable domain since it shows marked variations of its amino acid sequence. The constant (C) region: The rest of the Ig molecule shows little variation and is relatively constant in its amino acid sequence. -Each light chain has one variable and one constant domain (VL and ­ CL) ‑ Each heavy chain has one variable domain (VH), and 3 or 4 constant domains (CH1, CH2, CH3 and CH4).

6. 3. Fragments of the Ig molecule: • Fabfragments (fragments of antigen binding): There are 2 fab fragments in each molecule. • Fc fragment (fragment crystallizable):There is one Fc fragment in each Ig molecule. • The biological function of the Fc fragment are : • Opsonization (IgG) • Complement fixation (IgM and IgG). • Transplacental transfer (IgG). • Binding to mast cells (IgE) • Mucosal attachment (IgA).

7. Classes of lmmunoglobulins • IgG: • -It comprises about 75% of normal immunoglobulins in normal human sera. • -There are 4 subclasses of IgG, based on antigenic difference in H chain. • -Each IgG has 2 fab and Fc segment. So it has a valence of 2 (divalent). • -It predominates in secondary antibody response against bacterial, viral or fungal infections and it is the only type that can pass through the placenta.

8. IgM: • -It comprises about 10% of immunoglobulins in normal serum. • -Since each IgM molecule has 10 Fab segments, it can combine up to 10 antigenic sites. It has a valence from 5 to 10. • -IgMmolecules are the earliest antibodies synthesized in response to antigenic stimulation. • -They fix complement well and do not pass through the placenta.

9. lgA: • -The basic structural unit of IgA corresponds to that IgG. • -Two subclasses of IgA are known IgA1 and IgA2 • -A second type of IgA (secretoryIgA) is secreted by B‑lymphocytes of the mucous membranes, e.g. mucous membranes of the intestinal, respiratory and urinary tracts, tears, saliva and milk. The secretory molecule possesses a secretory piece and it plays a major role in local immunity. • -IgA (serum or secretory) does not fix complement, but can neutralize virus and inhibits attachment of bacterial to epithelial cells.

10. IgE: • -In normal serum, IgE is formed in trace concentration. • It has the capacity to fix to tissues and so called cytotropic orcytophilicimmunoglobulin. • -Its main role is in hypersensitivity reactions. • -It is commonly produced against allergens. • -Immunity to certain helminthic infections. • lgD: • It is present in serum in trace amount. Its function is unknown.