| Pathogen |
(Gk. for one giving rise to suffering) is any biologic agent that causes a disease in the host. |
| Host |
is any organism (or cell) harboring the pathogens that may (or may not in case of a strong immune response or depending upon the stage of infection) suffer from the disease. |
| Peptide |
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. |
| Protein |
is a large (50 or more subunits) polymer of amino acids.[2] |
| Antigen |
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] |
| Epitope |
or antigenic determinant is a portion of a protein that can be recognized by the immune system. It consists of a single stretch of amino acids known as linear or sequential epitope, or widely separated amino acids or short stretches of amino acids that are brought close together because of three-dimensional protein-folding known as conformational or nonsequential epitope. |
| Paratope |
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. |
| Phagocytosis |
literally means cell eating which involves engulfing the whole of a small (at microscopic scale) particle through the formation of small projections of the cell known as pseudopodia literally meaning "false feet". This process is followed by degradation (breaking down into simpler molecules) of the engulfed entity (very often a pathogen). Any particle that is engulfed by phagocytosis is said to be "phagocytosed".[4] |
| Antibodies |
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. |
| Actions of soluble antibodies [9] |
* Coat the pathogen not allowing it to adhere to the host cell.
- Precipitating (making the particles "sink" by attaching with them) the soluble antigens and promoting their clearance by other cells of immune system from the various tissues and blood.
- Coating the microorganisms to attract phagocytosing cells. This is known as opsonization. And thus the antibody acts as an opsonin.
- Activating the complement system.
- Marking up host cells infected by viruses for destruction in a process known as Antibody-dependent cell-mediated cytotoxicity (ADCC)
|
| Complement system |
is a group of proteins present in the blood that help fight infections. One of their major functions is to make pores in the cell membranes of pathogens. Other functions include attracting WBCs to the site where these proteins form get cleaved to their acitvate forms (chemotaxis), and opsonization.[10] |
| White blood cells or WBCs |
are types of cells found apart from blood in various organs and tissues like the bone marrow, lymph nodes, spleen, liver, lymph, etc. They are involved in functions of immunity. An important property of many white blood cells is that they can move over from one tissue to other (including blood) and then return back still retaining their functions in the process. They are also known as leukocytes (leuko=white and cyte=cell).[11] |
| B lymphocyte or B cell |
is a type of WBC that produces soluble antibodies and also recognizes a specific organism (through epitope present on the antigen present on pathogen's surface). Upon doing so, the B cell engulfs the pathogen.[12] |
| T lymphocyte or T cell |
is a type of WBC that produces soluble growth factors necessary for survival and proliferation (rapid division and multiplication) of B cells and other types of WBCs. They also stimulate the B cells by direct contact.[13] |
| Macrophage |
is a type of WBC specializing in relatively nonspecific phagocytosis and antigen presentation.[14] |
| Major histocompatibility complex |
is region on the DNA (the material responsible for transmission of various properties of a cell as well as the entire organism from one generation to the next) that codes for the synthesis of Major histocompatiblity class I molecule, Major histocompatiblity class II molecule and other proteins involved in the function of complement system (MHC class III)[15]. MHC-compatibility is a major consideration in organ transplantation.[16] |
| Antigen processing |
T cells can recognize antigens only when they are present on the surface of a cell. Another prerequisite is that these antigens must be complexed (loosely attached to) with MHC molecules. The "preparation of antigen" so that it can be brought to the surface of the cell possessing the antigen is known as antigen processing. This usually involves breaking down the protein (antigen) into smaller peptides. After the antigen is processed, it complexes with MHC molecule and migrates to the cell membrane. This is antigen presentation. The cells that are earmarked for this purpose are known as Antigen presenting cells, in which case they phagocytoze the antigen, and complex the processed antigen with MHC class II molecules. Examples are B cells and macrophages. All the nucleated cells can also process and present antigens synthesized within the cell (in case of viral infections or tumors) by complexing them with MHC class I molecules.[17] |
| B cell receptor (BCR) |
is an immunoglobulin present on the surface of the B cells that can bind specifically to its epitope.[18] |
| T cell receptor (TCR) |
is a protein present on the T cells that can specifically recognize an antigen bound to the MHC molecule. |
| Cluster of differentiation |
Lymphocytes at various stages of development express molecules typical to them. The monoclonal antibodies that can specifically bind with a particular surface molecule would be regarded as one cluster of differentiation. Any monoclonal antibody or a group of monoclonal antibodies that do not react with known surface molecules of lymphocytes, but rather to a yet-unrecognized surface molecule would be clubbed as a new cluster of differentiation and numbered accordingly. Each cluster of differentiation is abbreviated as "CD", and followed by a number (usually indicating the order of discovery).[19] |
