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One of the five main types of white blood cells which play a critical role in fighting infections they have many "roles" though.
The main role of white blood cells is to protect the body against infection and fight infection when they occur.
eosinophils
There are six main types of white blood cells neutrophils, eosinophils, basophils, bands, monocytes and lymphocytes. Neutrophils, which make up about 58% of the white blood cells in the body, are the body's main defense against bacteria. Monocytes also play a role in bacteria removal.
The blood coagulation which you are referring to has several stages and types. The main cells involved directly are thrombocytes, while endothelial cells etc may also play a role. There are also soluble factors of the blood involved. (see coagulation cascase) See http://en.wikipedia.org/wiki/Coagulation
its a protein that is the main part of and resides in your red blood cells. The protein contains iron that carries oxygen to the cells.
Red blood cells because that is what carries oxygen throughout the body
Blood cells
It depends on the type of cell we're talking about. I'm going to use the assumption that you mean blood cells when you ask about the circulatory system cells (as opposed to the cells that make up the heart, arteries, veins and capillaries). Blood is made up of 4 major components: Red Blood Cells, White Blood Cells, Platelets, and Plasma. Plasma is the vast majority of blood and it's basically salt water that your cells float in and that carry nutrients and wastes through the body. Platelets are generally referred to as "cell fragments" that release thread-like proteins to help stop bleeding. Red blood cells have one primary role: Carry oxygen. That's it. "White blood cells" are a collection of many different types of cells that collectively work towards fighting infections. Some (macrophages) actually engulf and eat invading cells! Plasma cells create antibodies for remembering how to fight certain infections. Killer T-cells chemically attack infected cells. There are actually quite a few more. In all, there are 7 main types of cells (T-cells, B-cells, memory cells and plasma cells are actually subsets of the lymphocyte group).
Neutrophils, monocytes, lymphocytes, eosinophils and basophils.
They transport oxygen to cells.
glycoproteins