Immune system is a network of molecules, cells, tissues
and organs that work together to protect your body. 

Three types of barrers exist to protect us from invading pathogens:


The first layer of protection, very hostile for microbes but relatively easy to overcome.


Always ready to respond and works fast but is not very specific.


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


Intrinsic defenses are very hostile for microbes but relatively easy to overcome. This is your skin and its low pH, tears, saliva, ear wax, mucus.


A protective layer of thick mucus coats all mucosal surfaces and thus constitutes the first line of anti-pathogen defense. The major function of mucus is the provide a physical barrier and to trap pathogens to prevent them from reaching the underlying epithelial cells. Saliva contains anti-bacterial and anti-viral proteins such as cathelcidin, lactoferrin, lysozyme mucins peroxidase, among others.


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

NEUTROPHILS are the most abundant cells in our bloodstream. Neutrophils are fast and efficient at detecting and destroying bacteria, making them essential for fighting infections. Neutrophils move quickly to the site of infection and engulf the invading bacteria to eliminate them. Neutrophils can also produce reactive oxygen species (ROS) and release its highly destructive granular content, such as myeloperoxidase, defensins or elastase. Neutrophils can also trap virions or bacteria through their neutrophil extracellular traps or NETs, and 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 who 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. Natural killer 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 the bloodstream. These cells 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 harders 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. T cells recognize and eliminate infected cells and cancer cells, as well as 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 or 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. B cells produce large quantities of antibodies that can neutralize pathogens.


The organs of the immune system include:


10th edition Cellular and Molecular Immunology by Abdul K. Abbas, Andrew H. Lichtman, Shiv Pillai