Immune System

Human immune system is the defense line of body against pathogens. It includes innate and adaptive defense mechanisms linked to cytokines and hormones (2). Both innate and adaptive defense mechanisms can distinguish between self-cells and pathogens and the difference is in the way they do it.

Innate defense mechanism relies on detecting markers or receptors that are common among many pathogens and is very powerful against distinction of host cells and pathogens, but it cannot produce long-lasting immunity to the host. Innate defense mechanism can be found in all plants and animals and it is evolutionary older defines mechanism (10).

On the other hand, adaptive defense mechanism or acquired immunity is capable of fine distinction among pathogens. Also, they can produce memory cells that help the immune system to recognize the antigen faster in subsequent encounter. Cells of acquired immunity are lymphocyte B and lymphocyte T (10).

Lymphocyte B cells can produce memory B cells and Plasma cells. Plasma cells in turn can produce antibody. Lymphocyte B cells are the major cells that produce antibody. Antibodies are large Y-shaped proteins that are able to distinguish between host cell and pathogen. Each antibody can recognize a specific antigen and neutralize the target antigen only (10).

There are seven types of lymphocyte T cells: T helper cell, cytotoxic T cell, memory T cell, regulatory T cell, natural killer T cell, mucosal associated invariant T cells, and γδ T cells. T helper cells help other white blood cells in their immunological processes. Helper T cells become activated when they are presented by MHC class II molecules. Cytotoxic T cells destroy infected cells, but they need MHC class I to recognize their targets. Memory T cells help the immune system for long-term immunity. Regulatory T cells are important for the maintenance of immunological tolerance. Natural killer T cells link between innate and adaptive immune system. Natural killer T cells can produce cytokine and recognize tumor cells. γδ T cells are a small group of T cells that are mostly found in mucosa and respond to phosphoantigens (9).

Figure 2

Figure 2 Early, intermediate, and late human immune system to pathogens.