The immune system is a complex network of cells, tissues and organs working together to defend the body against foreign invaders. The workhorse cells of the immune system are the white blood cells (WBC). They consist of both specific and non-specific defense cells that have the capability of recognizing self vs. non-self cells and microbes. The many different types of white blood cells can be found maturing in the lymph nodes and bone marrow, or traveling the bloodstream in search of potentially harmful outside organisms.
Neutrophils are non-specific immune cells and comprise approximately 55 to 70 percent of the total white blood cells. Neutrophils are the first line of defense against invading antigens and are first to arrive at the site of infection or injury. Chemical signals released by damaged cells attract neutrophils, which stick to blood vessel walls and engulf any foreign particles before they enter the bloodstream. Neutrophils are short lived and self-destruct after engulfing harmful antigens.
Monocytes comprise 2 to 8 percent of the total population of circulating white blood cells. Monocytes originate in the bone marrow and develop into large macrophages in the bloodstream. Macrophages are the largest of the white blood cells and are responsible for engulfing cell debris, waste and harmful bacteria. Macrophages attack microbes by extending pseudopodia (feet-like extensions) around the cells and then destroy the microbe by releasing enzymes from inside the macrophage.
Sometimes referred to as acidophils, eosinophils defend the body against multicellular parasites and moderate allergic reactions. Eosinophils develop in the bone marrow before migrating out into the bloodstream. Eosinophils combat foreign parasites and particles by releasing chemical mediators in a process called degranulation. During degranulation, small granules inside the eosinophils are released to destroy the foreign invaders. These harmful chemicals are reactive proteins such as peroxides, nucleases and lipases.
Comprising less than 1 percent of the total white blood cell count, basophils play an integral role in promoting blood flow and preventing coagulation. Basophils circulate the bloodstream and release two important chemicals at the tissue site: heparin and histamine. Heparin is an anti-coagulant that prevents blood cells from clotting too quickly and histamine is a vasodilator commonly released during allergic reactions to increase blood flow. These two molecules work together to quickly increase the availability of other immune system cells at the site of infection or inflammation.
Lymphocytes refer to a group of cells consisting of B cells, T cell and natural killer (NK) cells, which comprise 25 to 33 percent of the total white blood cell count. B cells and T cells are the major components of the body's adaptive immunity. The B cells are primarily responsible for producing antibodies against foreign particles, which remember and specifically bind to foreign particles more quickly to be presented to and destroyed by T cells. T cells serve many functions but primarily are involved in destroying cells identified by antibodies. NK cells are not as specific as T cells but also function in destroying cells by releasing granules, like eosinophils. All three cells work together too quickly and efficiently rid the body of harmful, invading particles but are also implicated in autoimmune disorders in which the immune cells attack cells of the human body.