The Antibody

The Antibody-Mediated Immune Response Antigen activates naïve B cells and dendritic cells Naïve T cell binds to APC and differentiates into effector and memory helper T cells Helper T cells bind antigen-MHC complexes on activated B cell and secrete cytokines B cell differentiates into effector B cells, which produce antibodies targeting a specific antigen, and memory B cells

Antibodies Immunoglobulins (Glycoproteins) The heavy and light chains are polypeptides The chains are held together by disulphide bridges Each ab has 2 identical ag binding sites – variable regions. The order of amino acids in the variable region determines the shape of the binding site

Antibodies and Other Antigen Receptors Antigen receptors on B and T cells have the potential to recognize billions of different antigens Antibody Y-shaped antigen receptor (protein), made only by B cells, that binds only to the antigen that prompted its synthesis Activates complement, facilitates phagocytosis, or neutralizes pathogens or toxins

Five Classes of Antibodies Constant regions determine 5 classes of antibodies (immunoglobins IgG, IgA, IgE, IgM, and IgD), each with different functions B cell receptors are membrane-bound IgM or IgD antibodies

Different Immunoglobulins

Five Classes of Antibodies

How Abs work Some act as labels to identify antigens for phagocytes Some work as antitoxins i.e. they block toxins for e.g. those causing diphtheria and tetanus Some attach to bacterial flagella making them less active and easier for phagocytes to engulf Some cause agglutination (clumping together) of bacteria making them less likely to spread

Number of ag binding sites Site of action Functions Type Number of ag binding sites Site of action Functions IgG 2 Blood Tissue fluid CAN CROSS PLACENTA Increase macrophage activity Antitoxins Agglutination IgM 10 IgA 2 or 4 Secretions (saliva, tears, small intestine, vaginal, prostate, nasal, breast milk) Stop bacteria adhering to host cells Prevents bacteria forming colonies on mucous membranes IgE Tissues Activate mast cells  HISTAMINE Worm response