The immune system is designed to recognize foreign agents and infectious microbes invading the animal body and resist them. It responds to disease-causing agents with great specificity, and preserves a memory of the exposure over the long-term. It is composed of two parts with complementary functions:
- Innate or nonspecific immunity
- Adaptive or specific immunity
The innate immune system consists of several components that constitute the primary defense against infectious agents. It includes physical barriers, such as the skin, which prevents the entry of disease agents, and fluids secreted by the body such as mucus, which collect the pathogens and clear them.
It also includes cellular components such as some white cells and phagocytes, as well as biochemical factors, such as proteins belonging to the complement system, which recognize any foreign particle or cell as not belonging to the body, and seek to remove it.
The innate immune system functions with unaltered efficiency and in the same way, no matter how often it is exposed to a foreign disease agent.
The adaptive immune system comes into play when a pathogen is not cleared by the innate immune system alone. It constitutes both cells and molecules which include lymphocytes and antibody molecules, as elements of primary importance.
Lymphocytes are immune cells that are generated from their progenitors within the bone marrow. They produce cell surface receptors which can bind foreign molecules, or produce secreted proteins called antibodies that do the same thing. An antigen is any molecule that can bind to an antibody. Antibodies are also called immunoglobulins.
Once an antibody binds a pathogen, it marks the latter for destruction or elimination from the body.
Lymphocytes are classified into three groups, which best describe the functions of the adaptive immune system:
- B cells
- Cytotoxic T cells (TC cells)
- T helper cells (Th cells)
In short, the response of the adaptive immune system may be mediated by cells or by molecules. The latter is called humoral immunity, and the former cell-mediated. Humoral immunity is carried out via B cells, a type of lymphocyte which releases antibodies that specifically bind the infectious agent. Cell-mediated immunity, on the other hand, binds TC cells to the cells which are recognized to have been infected or to be foreign, and this is followed by cell lysis.
Th cells take part in both divisions of adaptive immunity via the secretion of cytokines, or cell messenger proteins. All lymphocytes have receptors on the surface that can bind antigens. These are all glycoproteins, but only a single receptor type is produced by any cell. Since this is the fact, each cell recognizes only a single antigen, and this is why the immune system is so specific.
When an antibody interacts with an antigen, the immune response occurs. This is also the basic principle of immunohistochemistry.
The antibody-antigen reaction occurs between a region of the antibody, called the paratope, and a corresponding region on the antigen, called the epitope. The strength with which an epitope binds to an antibody is called the affinity of the antibody. On the other hand, avidity describes the stability of the antigen-antibody complex, which is shown by the dissociation constant Kd.
Macrophages react with T cells and B cells to produce an antibody response. Antigen-presenting cells (APCs) also engulf and partly break down antigens on infectious agents. APCs include macrophages, dendritic cells, Langerhans cells, lymph nodes and monocytes.
Some antigen fragments are found on the surface of the APC in combination with a glycoprotein, also found on the cell surface, which is called the MHC II (major histocompatibility complex). Most cells express another class of MHC called MHC I, while MHC class II molecules occur only on the surface of APCs. When MHC II binds with antigen, the resulting complex makes it possible for Th cells to bind to the APC.
The result is Th cell proliferation with subsequent cytokine release. This then stimulates the binding of T cells to the MHC complex present on B cells which results in the proliferation and maturation of the latter into plasma cells.
These generate large quantities of very specific antibodies against the foreign particle or cell. Among the B cells, some become memory cells. By preserving the memory of the specific immune response over the long term, they produce a rapid antibody response the next time the body is exposed to the same infectious agent.
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