These are lymphocytes, a type of leukocytes or white blood cells (WBCs). Mature, normal lymphocytes are usually only slightly larger than red blood cells with round, dense nuclei of homogenous chromatin, encircled by a very small rim of light blue cytoplasm (more variable in appearance in ruminants).
WBCs defend the body from damage by organisms and foreign substances, and help heal wounds. WBCs include phagocytes and immune cells or lymphocytes.
Phagocytic cells are the « first defenders » – monocytes and granulocytes (neutrophils, eosinophils, basophils).
Basophils are included as they contain colored granules, but phagocytosis is not their primary role.
Granulocytic phagocytes have segmented nuclei and cytoplasmic granules – orange/pink in eosinophils; purple/dark blue in basophils; pale pink in neutrophils.
Lymphocytes are produced in the bone marrow and are responsible for humoral (B cells) and cellular immunity (T cells w/ help of B cells).
Image courtesy of Echinaceapallida.
