The function of the immune system is to protect animals from foreign agents and infectious organisms. It responds in a specific way to pathogens and displays a long term memory of earlier contacts with the disease agents. The immune system consists of two functional components:
- Non-specific defence (innate or non adaptive immune system)
- Specific defence (adaptive immune system)
Non-specific defence prevents penetration and spread of many infectious agents by means of a variety of physical, biochemical and cellular barriers (skin, mucosa, lysozymes, complement and phagocytes). It does not improve with repeated exposure.
Specific defence may be called upon to react against and clear the harmful agent. The adaptive immune system consists of a variety of cells and molecules, among which lymphocytes and immunoglobulins are the key elements. Lymphocytes synthesize cell surface receptors or secrete proteins that specifically bind to foreign molecules. These secreted proteins are known as antibodies. Any molecule that can bind to an antibody is called an antigen (antibody generator). The term immunoglobulin is used interchangeably with antibody.
The interaction of an antibody with an antigen forms the basis of all immunochemical techniques. The region of the antibody that reacts with the antigen is called the paratope. The region of an antigen that interacts with an antibody is defined as an epitope. Affinity is the measure of the strength of the binding of an epitope to an antibody. Avidity is a measure of the overall stability of the complex between antibodies and antigens.
Lymphocytes arise continuously from progenitor cells in the bone marrow. Most functions of the immune system can be described by grouping lymphocytes into three basic types:
- B cells
- Cytoxic T cells (TC cells)
- Helper T cells (TH cells)
The adaptive immune response can be either humoral or cell mediated. Humoral response is mediated by B lymphocytes with the helper T cells. Cell mediated response involves the binding of cytoxic T lymphocytes to foreign or infected cells, followed by the lysis of these cells. The helper T cells are also involved in cell mediated responses.
All three types of lymphocytes carry cell surface receptors that can bind antigens. The specificity of the immune system is controlled by the fact that one cell recognises one antigen. All antigen receptors are glycoproteins and processes ensure that only one type of receptor is synthesized within any one cell.
An antibody response is the culmination of a series of interactions between macrophages, T lymphocytes and B lymphocytes. Antigens are engulfed by antigen presenting cells (APC i.e. macrophages, Langerhans cells, dendritic cells, lymph nodes and monocytes), then they are partially degraded. Fragments of the antigen will appear on the surface of the APC attached to a cell surface glycoprotein known as MHC II (major histocompatibility complex). There are two types of MHC molecules: MHC class I which are expressed on the surfaces of most cells and class II which are expressed exclusively on the surfaces of antigen presenting cells. This antigen-MHC II complex allows helper T cells to bind to the APC which leads to a proliferation of helper T cells. The T cells then bind to the MHC complex on B cells which leads to proliferation and differentiation of the B cells. The B cells change into plasma cells which secrete large quantities of finely tuned antibodies. Some B cells are changed into memory cells which are primed for future challenges.