User:HT416/Humoral immunity

Humoral Immunity
"Humoral" redirects here. Not to be confused with humeral.

Humoral immunity is the aspect of immunity that is mediated by macromolecules - including secreted antibodies, complement proteins, and certain antimicrobial peptides - located in extracellular fluids. Humoral immunity is named so because it involves substances found in the humors, or body fluids. It contrasts with cell-mediated immunity. Humoral immunity is also referred to as antibody-mediated immunity that uses assistance of helper T cells to differentiate B cells into plasma B cells, producing antibodies against a specific antigen. The humoral immune system is what deals with pathogens circulating in the immune as antibodies produced will bind to antigens causing lysis or phagocytosis of the foreign bacteria.

The study of the molecular and cellular components that form the immune system, including their function and interaction, is the central science of immunology. The immune system is divided into a more primitive innate immune system and an acquired or adaptive immune system of vertebrates, each of which contain both humoral and cellular immune elements.

Humoral immunity refers to antibody production and the coinciding processes that accompany it, including: Th2 activation and cytokine production, germinal center formation and isotype switching, and affinity maturation and memory cell generation. It also refers to the effector functions of antibodies, which include pathogen and toxin neutralization, classical complement activation, and opsonin promotion of phagocytosis and pathogen elimination.

Antibodies
Main article: Antibody

Immunoglobulins are glycoproteins in the immunoglobulin superfamily that function as antibodies. The terms antibody and immunoglobulin are often used interchangeably. They are found in the blood and tissue fluids, as well as many secretions. In structure, they are large Y-shaped globular proteins. In mammals there are five types of antibody: IgA, IgD, IgE, IgG, and IgM. Each immunoglobulin class differs in its biological properties and has evolved to deal with different antigens. Antibodies are synthesized and secreted by plasma cells that are derived from the B cells of the immune system.

An antibody is used by the acquired immune system to identify and neutralize foreign objects like bacteria and viruses. Each antibody recognizes a specific antigen unique to its target. By binding their specific antigens, antibodies can cause agglutination and precipitation of antibody-antigen products, prime for phagocytosis by macrophages and other cells, block viral receptors, and stimulate other immune responses, such as the complement pathway.

An incompatible blood transfusion causes a transfusion reaction, which is mediated by the humoral immune response. This type of reaction, called an acute hemolytic reaction, results in the rapid destruction (hemolysis) of the donor red blood cells by host antibodies. The cause is usually a clerical error, such as the wrong unit of blood being given to the wrong patient. The symptoms are fever and chills, sometimes with back pain and pink or red urine (hemoglobinuria). The major complication is that hemoglobin released by the destruction of red blood cells can cause acute kidney failure.

Antibody Production
In humoral immune response, the B cells first mature in the bone marrow and gain B-cell receptors (BCR's) which are displayed in large numbers on the cell surface.

These membrane-bound protein complexes have antibodies which are specific for antigen detection. Each B cell has a unique antibody that binds with an antigen. The mature B cells then migrate from the bone marrow to the lymph nodes or other lymphatic organs, where they begin to encounter pathogens. The B cells within humoral immunity require signals to initiate activation. Antigens present are T-dependent, meaning that the T cells are required to help with the antibody production. The T-dependent antigen signal comes from the antigen cross linking BCR as well as the Th2 cell. These T-dependent antigens carry proteins that allow peptides to be present on B cell class II MHC to the Th2 cells, providing a co-stimulatory trigger to B cell proliferation and differentiation into the plasma cells.

B cell maturation
B cells are formed through multipoint hematopoietic stem cells found in bone marrow that follow a mechanism through the lymphoid stem cell and lymphoblast. Those destined to become B cells coninue to reside in the bone marrow and will travel to the thymus for maturation where the B cells will continue to mature. One of the first steps of maturation of the B cell is an evaluation of the functionality of antigen-binding receptors via positive selection of B cells that present normal receptors. Negative selection is then used to eliminate B cells that self react through mechanisms sof apoptosis, or the modification of receptors so self-reactivity is no longer capable. Immature B cells that skip through the process of selection within the bone marrow will travel to the spleen for the last degrees of maturation, there they come to be naïve mature B cells.

B cell Activation
When a B cell encounters an antigen a signal is activated, the antigen binds to the receptor and is taken inside the B cell by endocytosis. The antigen is processed and presented on the B cell's surface again by MHC-II proteins. The MHC-II proteins are recognized by helper T cells, stimulating the production of proteins, allowing for B cells to multiply and the descendants to differentiate into antibody-secreting cells circulating in the blood. B cells can be activated through certain microbial agents without the help of T-cells and have the ability to work directly with antigens to provide responses to pathogens present.

B cell proliferation
The B cell waits for a helper T cell (TH) to bind to the complex. This binding will activate the TH cell, which then releases cytokines that induce B cells to divide rapidly, making thousands of identical clones of the B cell. These daughter cells either become plasma cells or memory cells. The memory B cells remain inactive here; later, when these memory B cells encounter the same antigen due to reinfection, they divide and form plasma cells. On the other hand, the plasma cells produce a large number of antibodies which are released freely into the circulatory system.

Antibody-antigen reaction
In an antigen-antibody reaction, here the antibody will look to attach itself with the antigen. The combination of the antibody and antigen from the epitope recognized by the immune system, specifically by antibodies, B cells, or T cells. These antibodies will encounter antigens and bind with them. This will either interfere with the chemical interaction between host and foreign cells, or they may form bridges between their antigenic sites hindering their proper functioning. Their presence might also attract macrophages or killer cells to attack and phagocytose them.