B Cells and T Cells
Lymphocytes are one of the five kinds of white blood cells or leukocytes), circulating in the blood.
Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are:
• B lymphocytes (often simply called B cells) and
• T lymphocytes (likewise called T cells
The specificity of binding resides in a receptor for antigen:
• The B cell receptor (BCR) for antigen and
• The T cell receptor (TCR) respectively.
Both BCRs and TCRs share these properties:
• They are integral membrane proteins.
• They are present in thousands of identical copies exposed at the cell surface.
• They are made before the cell ever encounters an antigen.
• They are encoded by genes assembled by the recombination of segments of DNA.
How antigen receptor diversity is generated
• They have a unique binding site.
• This site binds to a portion of the antigen called an antigenic determinant or epitope.
• The binding, like that between an enzyme and its substrate depends on complementarity of the surface of the receptor and the surface of the epitope.
• Successful binding of the antigen receptor to the epitope, if accompanied by additional signals, results in:
Stimulation of the cell to leave G0 and enter the cell cycle.
Repeated mitosis leads to the development of a clone of cells bearing the same antigen receptor; that is, a clone of cells of the identical specificity.
BCRs and TCRs differ in:
• Their structure;
• The genes that encode them;
• The type of epitope to which they bind.
Epitopes
An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. For example, the epitope is the specific piece of the antigen that an antibody binds to. The part of an antibody that binds to the epitope is called a paratope.
The epitopes of protein antigens are divided into two categories, conformational epitopes and linear epitopes, based on their structure and interaction with the paratope. A conformational epitope is composed of discontinuous sections of the antigen's amino acid sequence. These epitopes interact with the paratope based on the 3-D surface features and shape or tertiary structure of the antigen. The proportion of epitopes that are conformational is unknown.
By contrast, linear epitopes interact with the paratope based on their primary structure. A linear epitope is formed by a continuous sequence of amino acids from the antigen.
T cell epitopes are presented on the surface of an antigen-presenting cell, where they are bound to MHC molecules. In humans, professional antigen-presenting cells are specialized to present MHC class II peptides, whereas most nucleated somatic cells present MHC class I peptides.
MHC class I and II epitopes can be reliably predicted by computational means alone.
Epitopes are often used in proteomics and the study of other gene products. Using recombinant DNA techniques genetic sequences coding for epitopes that are recognized by common antibodies can be fused to the gene. Following synthesis, the resulting epitope tag allows the antibody to find the protein or other gene product enabling lab techniques for localization, purification, and further molecular characterization.
Vaccination is generally considered to be the most effective method of preventing infectious diseases. All vaccinations work by presenting a foreign antigen to the immune system in order to evoke an immune response. The active agent of a vaccine may be intact but inactivated (‘attenuated’) forms of the causative pathogens (bacteria or viruses), or purified components of the pathogen that have been found to be highly immunogenic. The increased understanding of antigen recognition at molecular level has resulted in the development of rationally designed peptide vaccines. The concept of peptide vaccines is based on identification and chemical synthesis of B-cell and T-cell epitopes which are immunodominant and can induce specific immune responses. The accelerating growth of bioinformatics techniques and applications along with the substantial amount of experimental data has given rise to the probability of development new vaccines.