Template:Polyclonal response-difficult terms KC Panchal/Sandbox

(Gk. for one giving rise to suffering) is any biologic agent that causes a disease in the host.

is a molecule consisting of less than 50 repeating subunits called amino acids, which are in turn nitrogen-containing organic compounds. Smaller peptides are known as di-, tri- or tetra-peptides for two, three and four amino acids that constitute them, and henceforth.^[1] Peptides with more than 10 amino acids are known as polypeptides. Usually peptides in multicellular organisms are derived from degradation of proteins outside or within the cells, or sometimes by their synthesis (production) within the body to serve various functions.

is any substance that can be recognized specifically by the cells or proteins of the immune system. These are most usually proteins. But, they can also be polysaccharides (polymers of sugars). This recognition occurs through the B cell receptor or the T cell receptor present respectively on B cells and T cells (see below).^[3]

is that part of a protein molecule belonging to the immune system that specifically binds with the epitope. A paratope typically contains 15 to 22 amino acids.

are complex protein molecules also known as immunoglobulins that may be cell membrane-bound or freely circulating in the blood—secreted or soluble antibodies. Antibodies have various isoforms, meaning the various antibody molecules specific to the same epitope can have slightly different structures allowing them to perform different functions or possess different properties—remaining membrane bound—(immunoglobulin D or IgD);^[5] greater water-solubility—(immunoglobulin A or IgA);^[6] better penetration into interstitial fluid—(immunoglobulin G or IgG);^[7] and binding with many antigen molecules simultaneously, remaining membrane-bound, and activating complement system strongly—(immunolglobulin M or IgM).^[8] There is another class of immunoglobulin called immunoglobulin E (or IgE) that is involved in allergic reactions.