White Blood Cells (Leukocytes):

These blood cells are colorless, as they do not contain pigments. They are not confined to the bloodstream, as they also migrate out into the tissue fluid.

Amount of White Blood Cells:

There are 1 or 2 leukocytes for every 1000 RBCs. One cubic millimeter of blood contains 7000 to 8000 of them. They are much larger (two to three times) than the red blood cells.

The average life span of a white blood cell:

They have a life span of months or even years, but this depends on the body's needs.

Types of leukocytes:

There are 5 types of leukocytes which can be divided into 2 main types.

1. Granulocytes:

   Granulocytes are the leukocytes with granular cytoplasm. These include neutrophils, eosinophils, and basophils. They are formed in the red bone marrow.

   1. Neutrophils:

      Their nucleus is divided into 2-5 lobes. They destroy small particles by phagocytosis. They are 62% of the leukocytes and their average lifespan is 07 hours.

   2. Eosinophils:

      Their nucleus is bilobed and they provide defense against parasites: They are 2% of the leukocytes and their life span depends upon needs.

   3. Basophils:

      Their nucleus is bilobed and they prevent blood clotting by releasing an anticoagulant, heparin. They are also responsible for developing inflammatory responses when they rupture and release the inflammatory agent, the histamines. They are less than 1% of leukocytes and their life span depends upon needs.

2. Agranulocytes:

   Agranulocytes are leukocytes with clear cytoplasm. These include monocytes and lymphocytes (B & T lymphocytes). They are formed in the lymphoid tissue of the lymphatic system i.e. in lymph nodes, spleen, tonsils, adenoids, and thymus.

   1. Monocytes:

      Their nucleus may be rounded or lobe. They are 3% of leukocytes with a life span of 3 days.

   2. Lymphocytes:

      They have a large nucleus and are are responsible for the production of antibodies which target specific antigens present on pathogens. They are 32% of the Leukocytes and are the major component of the immune system. Lymphocytes include B cells (which become antibody-secreting plasma cells) and T cells (which mediate B cell activity)