HUMAN IMMUNE SYSTEM

Innate immunity is always ready to respond and works fast but is not very specific. 

The COMPLEMENT SYSTEM is a group of plasma proteins that work together to eliminate pathogens by rapturing plasma membranes. 

The activation of complement drives inflammation, signals and recruits other immune system components.

NEUTROPHILS are: 

• the most abundant cells in our bloodstream,

• are fast and efficient at detecting and destroying bacteria, 

• move quickly to the site of infection and engulf the invading bacteria to eliminate them,

• can also produce reactive oxygen species (ROS) and release its highly destructive granular content, such as myeloperoxidase, defensins or elastase,

• can also trap virions or bacteria through their neutrophil extracellular traps or NETs, 

• signal through release of cytokines and chemokines.

EOSINOPHILS live mainly in airways, intestines and genitourinary tracts. They fight fungi and parasites and they are also main cells implicated in allergies. 

BASOPHILS are loaded with histamine and are also implicated in allergic reactions but their importance in host defences is not very well studied. 

MAST CELLS are found in the skin and mucosal epithelia and they play an important role in defending against helminth parasites. They release histamine in response to parasitic infections.

MONOCYTES are circulating in the blood and can transform into macrophages when recruited to the site of infection or injury. 

MACROPHAGES are professional phagocytes that ingest microbes in the process called phagocytosis. 

• Monocytes, macrophages can also produce excessive ROS and produce large amounts of inflammatory mediators, such as cytokines and chemokines.

NATURAL KILLER CELLS provide early defense against viruses. 

• NK cell have powerful ability to recognize and eliminate virally infected cells and cancer cells. 

• NK cells can kill by direct cytolysis or indirectly through the production of cytokines and chemokines that activate other cellular mechanisms of destruction.

DENDRITIC CELLS are found in tissues and bloodstream. 

• DCs are essential for initiating the adaptive immune response. They capture microbial proteins and present them to T cells, which then activate the adaptive immune response.

The hardest to overcome, adaptive immunity is very specific and has memory meaning very efficient when your body encounters the same pathogen for the second third, fourth time...

T lymphocytes, the mediators of cellular immunity, arise from precursor cells in the bone marrow, which migrate to and mature in the thymus (T lymphocytes refer to thymus-derived lymphocytes). 

• Based on the expression of the cell surface proteins, CD8 and CD4, T cells can be divided into killer T cells (CD8+) and helper T cells (CD4+). 

• Killer T cells kill cells infected with viruses and other microbes that can live inside host cells, as well as cancer cells. 

• Helper T cells secrete cytokines and help other cells, including other T lymphocytes, B cells, and macrophages to help coordinate the adaptive immune response.

B lymphocytes, the cells that produce antibodies, were so named because in birds they were found to mature in an organ called the bursa of Fabricius. In mammals, no anatomic equivalent of the bursa exists, and the early stages of B cell maturation occur in the bone marrow. Thus, the name B lymphocytes now refers to bone marrow–derived lymphocytes. 

• B cells produce antibodies in response to specific recognition of the antigen through B cell receptor (BCR).

• A single B cell may, within a week, give rise to as many as 5000 antibody secreting cells.

ANTIBODIES are proteins that recognize and bind to foreign structures on germs to neutralize pathogens or amplify immune response through Fc receptors.