In this article we will discuss about the humoral immunity of immune response.
The immune system protects the body from potentially harmful substances by recognizing and responding to antigens. Antigens are foreign particles, normally large or small molecules on the surface of cells, viruses, fungi or bacteria. Some non-living substances such as toxins, chemicals, drugs and foreign particles can also be antigens. Substances that contain these antigens are recognized and destroyed by the immune system.
One of the most important immune response is humoral effector response. The effector functions in humoral immunity are mainly mediated by secreted antibodies. It protects body from extra-cellular pathogenic agents by combining with them to form antigen-antibody complex and gradually leads towards their elimination.
Humoral immunity combats extracellular bacteria, fungi and even obligate intra-cellular microbes e.g. viruses; before they infect their target T-cells. Any defect in humoral immunity results in increased susceptibility to infection with bacteria and fungi.
Ways involved in Humoral Immunity:
Humoral effector functions facilitate effective elimination of foreign pathogens from a host animal in a variety of ways.
Antibodies play vital role in elimination of antigenic agents:
(i) The antibody can bind to the surface epitopes of the antigen making it more susceptible to phagocytosis—known as opsonization.
(ii) The antibody molecule can bind to the antigen forming an antigen-antibody complex, which then combines with the complement in a step-wise manner to initiate and facilitate phagocytosis of the antigen.
(iii) The antibody can bind to toxin molecules elaborated by microbes making them nontoxic.
(iv) Antibodies can inactivate free virus particles by combining with the epitopes on viral particles to make them incapable of attachment to host cell membranes.
(v) Binding to potential pathogens at mucous membrane surfaces, preventing colonization.
(vi) Binding to Fc (fragment crystalized) receptors on NK cells or macrophages in antibody dependent cell mediated cytotoxicity (ADCC), confirming specificity for antigen.
Humoral Immune Responses:
Most defenses that are mediated by antibody present in the plasma, lymph and tissue fluids are called humoral immune responses. It protects against extra-cellular bacteria and foreign macromolecules. Transfer of antibodies confers this type of immunity on the recipient. Humoral immune responses have an activation phase and an effector phase.
These phases occur as follows (Fig. 10.1):
1. The antigen is taken up by phagocytosis and degraded in a lysosome in an APC, such as a macrophage.
2. A T-cell receptor recognizes processed antigen bound to a class II MHC protein on the macrophage.
3. Cytokines released by the TH cell and IL-1 released by macrophage stimulate the TH cell to produce a clone of differentiated cells capable of interacting with B-cells.
Activation phase occurs in lymphatic tissue.
4. B-cells are also antigen presenting cells. Binding of antigen to a specific IgM receptor triggers receptor mediated endocytosis, degradation and display of the processed antigen on class II MHC proteins.
5. When a TH cell receptor binds to the displayed antigen—MHC II complex on the B cell, it releases cytokines.
6. These cytokines cause the B-cell to produce a clone of B-cells.
7. Now, these B-cells produce antibody secreting plasma cells.