Lecture 4 Specific Immunity:
• B and T cells bind and respond to foregin or abnormal molecules
• Antigens are typically complex protein or polysaccharide components of
viruses, bacteria, fungi, protozoa, parasitic worms, pollen, transplanted
tissue, and tumor cells.
• Each antigen as a unique structure and contains different recognition sites
called epitopes (antigen determinants), each of which can be detected by
specific lymphocytes, which then target that invader for destruction.
• A typically antibody is a Y-shaped molecule consisting of 4 protein
chains: 2 identical heavy chains, 2 identical light changes (joined by
Figure23.5a BasisofAntibody–Antig en
• Each contains variable regions (V) and constant regions (C). C consists of
portions of heavy and light chains that make up the tail of the Y.
• The V of an antibody have amino acid sequences that vary extensively from
antibody to antibody, consist of portions of heavy and light chains that make up
the top of the Y. 2 identical V regions form 2 identical antigen-binding sites (bind
to two epitopes of the same kind).
• An enzyme and its substrate show specificity.
• Antigen receptors on the B cells are similar to antibody molecules except that the
receptors are bound to the plasma membrane, whereas antibodies are secreted into
the extracellular fluid. B cell antigen receptors are often called membrane
antibodies (membrane immunoglobulins). A B cell’s membrane antibodies have
the same specificity as the antibodies it later secretes as a plasma cell
• T cells have T cell receptors (TCRs). Different in structure. Act as cell surface
receptors for antigen.
2) DIVERSITY :
• A single T lymphocyte or B lymphocyte has about 100 000 antigen receptors, all
with the same specificity.
• Antigen receptor molecules lymphocyte products are determined by random
genetic events that occur early in the development of lymphocytes.
• When a particular microorganism invades the body, it interacts with and activates only those lymphocytes that have receptors specific for the antigens its possesses.
• Foreign antigen triggers an immune response against itself. This antigen-driven
activation of lymphocytes is called clonal selection (necessary for immune
• Lymphocyte differentiation gives rise t