Leucocytes, or white blood cells, fall into four main categories; granulocytes, lymphocytes, monocytes and thrombocytes, each with their own specific features and it is therefore difficult to provide a detailed description of leucocyte features without referring to each cell type individually.

In fish, the overall number of leucocytes varies in number; for example, the normal range of lymphocytes, the predominant type of leucocyte, in the salmonid, Rainbow Trout (Oncorhynchus mykiss), is between 7.8 and 20.9 x 103 cells per mm3.

White blood cells account for a small proportion of all circulating blood cells:

Sea Bass (Dicentrarchus labrax)                                 3.5%   of blood cells 

White Bream (Diplodus sargus)                                   3.5%   of blood cells

Saupe (Sarpa salpa)                                                        2%       of blood cells

In higher vertebrates such as mammals, formation of white blood cells is restricted to bone marrow, the spleen and the lymph nodes.  In fishes, organs such as the kidney, spleen and thymus take part in haematopoeisis.

Definitions

SPLEEN: Organ comprising a mass of lymphoid tissue in the mesentery; unlike lymph nodes it is interposed in the blood circulation.

THYMUS: A paired lymphoid gland situated dorso-laterally in the gill chamber.  The site of T-lymphocyte production, it is regulated by hormones produced by thymic epithelial cells.   In fish (and in all vertebrates) the thymus gradually atrophies after the onset of sexual maturity but does not completely disappear

HAEMATOPOIEISIS:  A general term referring to the formation of all types of blood cells, a process occurring in the haematopoeitic tissue.