Humoral Immunity
In this text we will discuss one of the major components of acquired immunity.
Acquired immune responses are executed by two pathways.
These are as follows:
1. Humoral immunity
2. Cell mediated immunity
Basically, humours means body fluids; it involves production of antibodies against foreign antigens.
Antibodies are produced by B lymphocytes.
Antibodies are found in blood plasma, lymph, mucus etc and the surface of B cells & also mast cells and basophils.
Cells involved in humoral immune response includes:
CD4 cells
B lymphocytes
Plasma cells
Memory B cells
Criteria of humoral immunity:
Immunity by antibody production against exogenous antigens.
Then immunity against T dependent antigens which include antibody production with the help of T helper cells.
A macrophage ingests antigen and presents it to T helper cell.T cell stimulates B cells specific for antigen to become plasma cells.
In case of T independent antigens antibody production does not require assistance from T cells. These antigens are mainly polysaccharides or lipopolysaccharides with repeating subunits.
Exogenous antigens such as viruses are engulfed and placed in a phagosome of antigen-presenting cells through phagocytosis and degraded into a series of peptides, bind to grooves in MHC ll molecules.T4 lymphocytes are able to recognise peptide/MHC ll complexes by means of their T cell receptors and CD4 molecules.
There are 5 general steps in humoral immune response-
Contact of antigen with APCs
Clonal selection
Clonal expansion
Differentiation of selected clones of B cells into plasma cells
Production of memory cells
Figure: Development of humoral immune response.
When an antigen first encountered antibodies are detectable in serum after a longer lag period than occurs in the secondary response.
The lag period is typically 7-10 days but can be longer depending on the nature and dose of the antigen and route of the administration.
The serum antibody concentration continues to rise for several weeks, then declines and may drop to very low levels.
The first antibodies to appear are lgM followed by IgG, IgA.IgM levels decline earlier than IgG levels.
When there is a second encounter with the same antigen or a closely related one, months or year after the primary response, there is a rapid antibody response to higher levels than the primary response due to the persistence of antigen specific memory cells after the first contact.
These memory cells proliferate to form a large clone of B cells and plasma cells which mediate the secondary antibody response.
Figure: Kinetic of antibody production.
Acquired immune responses are executed by humoral and cell mediated pathways based on the components of the immune system involved.
Immunity by antibody production against exogenous antigens, T dependent antigens and T independent antigens.
In primary immune response predominantly IgM and secondary immune response IgM and IgG are produced. Humoral immunity plays a great role by producing antibody against specific antigens.