Leucocyte

Leucocytes or leukocytes, colloquially known as white blood cells (WBCs), are clear, colorless cells that circulate in the blood and lymph and are integral part of the immune system.

Leucocytes are divided into the types: neutrophils, eosinophils, basophils, lymphocytes and monocytes. Each of these types has further derivatives, for example Natural killer cells are derivative of lymphocytes.

There are two lymphoid tissue types: Secondary lymphoid tissues contain the mature leucocytes that react to infection, triggering an immune response.
 * Primary Lymphoid tissues, comprising the bone marrow and thymus where leucocytes are synthesized
 * Secondary lymphoid tissues, comprising the spleen, lymph nodes and mucosa associated lymphoid tissues (MALT)
 * Secondary lymphoid tissues, comprising the spleen, lymph nodes and mucosa associated lymphoid tissues (MALT)