Blood

Blood is composed of plasma, white blood cells, red blood cells and platelets. Plasma makes up more than half of blood's volume. It is the fluid part of blood, which is mostly water. Plasma also contains nutrients, waste products and protein molecules that give plasma its yellow color. Red and white blood cells make up a little less than half of blood's volume.

One type of blood cell is the leukocyte, or white blood cell. Leukocytes are important defense system cells. They are subdivided into cells with or without granules.

White blood cells with granules include neutrophils, eosinophils and basophils. Neutrophils, the most common leukocyte, are the most important first line of defense against infections. Eosinophils provide important defense against parasites. Basophils are the least numerous type of leukocyte, but are important in inflammation responses.

White blood cells without granules include lymphocytes and monocytes. Lymphocytes are very important cells of the body’s immune system. Monocytes help engulf foreign particles such as bacteria.

Erythrocytes, called red blood cells, are not really cells because they do not contain a nucleus. They are highly flexible and contain a very large number of hemoglobin molecules that bind and transport oxygen. Platelets are small, simple cell fragments with a cell membrane around them that are important in the formation of blood clots that help to reduce bleeding.

Very specific markers known as blood antigens exist on the surface of red blood cells. Antigens carry inherited characteristics and give rise to a person’s specific blood type. Based on the presence or absence of these antigens, an individual’s blood type is defined as A (A antigens present), B (B antigens present), AB (both A and B antigens present), or O (neither A nor B antigens present).

Your blood type, as well as other antigen typing, is very important if you ever have to receive a blood transfusion or wish to donate blood. The blood types of donor and receiver must match or the body’s organs might reject the blood and create serious medical problems.

Some of the fluid in capillaries moves into the surrounding tissues. The lymphatic system of vessels helps drain this fluid and return it back to the bloodstream. The lymph moves slowly in one direction, toward the heart. Along the way, lymphatic vessels are interrupted by specialized filtration units known as lymph nodes.

Lymph nodes are rich in lymphocytes designed to filter out foreign material such as bacteria. When you have a sore throat, the lymph nodes below your jaw become enlarged. The lymphocytes are responding to viruses or bacteria being filtered from the fluid draining from your throat into these nodes.