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

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.

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 ( ).

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

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 (CH 1, CH 2, CH 3 and CH 4 ).

3. Fragments of the Ig molecule: Fab fragments (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).

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.

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. -IgM molecules are the earliest antibodies synthesized in response to antigenic stimulation. -They fix complement well and do not pass through the placenta.

lgA: -The basic structural unit of IgA corresponds to that IgG. -Two subclasses of IgA are known IgA 1 and IgA 2 -A second type of IgA (secretory IgA) 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.

IgE: -In normal serum, IgE is formed in trace concentration. It has the capacity to fix to tissues and so called cytotropic or cytophilic immunoglobulin. -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.