The specific immunity facilitates a targeted response against a specific pathogen. Together, innate immunity and adaptive immunity are responsible for immune responses that protect the body from dangerous cells (cancerous cells), allergens, foreign bodies, pathogens (bacteria, viruses, fungi, parasites), etc. The adaptive immune system includes immune responses called 'cell-mediated immunity' and 'humoral immunity', which recognize specific antigens. The term 'antigens' is derived from the term 'antibody generator', and refers to substances which trigger an immune response or the production of antibodies by the immune system. An interesting feature of the adaptive immunity is that after neutralizing the foreign invader or pathogen, a few B cells or T cells stay in the body as memory cells. If the same pathogen attacks in the future, these recognize the pathogen, and mount an attack.
The use of the term 'humoral' is also due to the fact that the antibodies which bind to the antigen and trigger a response are dissolved in the humor (bodily fluids such as blood, lymph). On the other hand, the cell-mediated immunity involves the destruction of cells that are damaged by mutations or infected by viruses with the help of Helper T cells (CD4+) and cytotoxic T cells (CD8+). The T cells bind to the surface of other antigen presenting cells (cells that display the antigen), thereby triggering a response. Other types of T cells that are present include NK (Natural Killer) cells, regulatory T cells, etc.
As mentioned earlier, Major Histocompatibility Complex is the specific marker that helps the immune system differentiate between the invader and body's cells. While MHC-I markers are on every cell, MHC-II are present only on the antigen-presenting cells (APCs) and lymphocytes. In short, the steps of cell-mediated response include:
➠ Self-cells or APCs that present antigens bind to T cells.
➠ Cytokines or interleukins (secreted by APCs or helper T cells) trigger the activation of T cells.
➠ If MHC‐I and endogenous antigens are displayed on the plasma membrane, T cells proliferate, producing cytotoxic T cells. Cytotoxic T cells destroy cells that display the antigens.
➠ If MHC‐II and exogenous antigens are displayed on the plasma membrane, T cells proliferate, thereby stimulating the production of Helper T cells. Helper T cells release interleukins (and other cytokines), which in turn stimulate B cells to produce antibodies that bind to the antigens. These also induce NK cells and macrophages to destroy the antigens.
In short, the steps of antibody-mediated immune response include:
➠ Antigens bind to B cells.
➠ Helper T cells or interleukins activate B cells. In most cases, both an antigen and a co-stimulator are required to activate a B cell and induce B cell proliferation.
➠ B cells replicate, thereby producing effector cells. The plasma cells bear antibodies that are specific to that antigen on the activated B cells. This is followed by the release of antibodies, which then bind to the antigens.
➠ B cells produce memory cells, which help activate the immune system if the same pathogen attacks in the future.