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White Blood Cells  (Leucocytes )

They are larger than red blood cells  and have nucleus . They defend the body  against diseases.  They are capable of amoeboid movement , these enables them  to squeeze themselves  through the tiny pores  in the capillary wall  so as to reach  the tissue and the site of infection

NB white blood cells are the  major  immunologically active cells  of the immune system ( others are  plasma cells , platelets  and mast cells)

Types of White Blood Cells

There are five different types of White blood cells and are classified mainly based on the presence and absence of granules.

1. Granulocytes
2. Agranulocytes

Granulocytes

They are leukocytes, with the presence of granules in their cytoplasm. The granulated cells include- eosinophil, basophil, and neutrophil.

Eosinophils

• They are the cells of leukocytes, which are present in the immune system.
• These cells are responsible for combating infections in parasites of vertebrates and for controlling mechanisms associated with allergy and asthma.
• Eosinophil cells are small granulocyte, which are produced in the bone marrow and makes 2 to 3 per cent of whole WBCs. These cells are present in high concentrations in the digestive tract.

Basophils

• They are the least common of the granulocytes, ranging from 0.5 to 1 per cent of WBCs.
• They contain large cytoplasmic granules, which plays a vital role in mounting a non-specific immune response to pathogens, allergic reactions by releasing histamine and dilates the blood vessels.
• These white blood cells have the ability to be stained when exposed to basic dyes, hence referred to as basophil.
• These cells are best known for their role in asthma and their result in inflammation and bronchoconstriction in the airways.
• They secrete serotonin, histamine and heparin.

1. Neutrophils

• They are normally found in the bloodstream.
• They are predominant cells, which are present in pus.
• Around 60 to 65 per cent of WBCs are neutrophils with a diameter of 10 to 12 micrometres.
• The nucleus is 2 to 5 lobed and cytoplasm has very fine granules.
• Neutrophil helps in the destruction of bacteria with lysosomes, and it acts as a strong oxidant.
• Neutrophils are stained only using neutral dyes. Hence, they are called so.
• Neutrophils are also the first cells of the immune system to respond to an invader such as a bacteria or a virus.
• The lifespan of these WBCs extend for up to eight hours and are produced every day in the bone marrow.

1. Agranulocytes

They are leukocytes, with the absence of granules in their cytoplasm. Agranulocytes are further classified into monocytes and lymphocytes.

1. Monocytes

• These cells usually have a large bilobed nucleus, with a diameter of 12 to 20 micrometres.
• The nucleus is generally of half-moon shaped or kidney-shaped and it occupies 6 to 8 per cent of WBCs.
• They are the garbage trucks of the immune system.
• The most important functions of monocytes are to migrate into tissues and clean up dead cells, protect against the bloodborne pathogens and they move very quickly to the sites of infections in the tissues.
• These white blood cells have a single bean-shaped nucleus, hence referred to as Monocytes.

1. Lymphocytes

• They play a vital role in producing antibodies.
• Their size ranges from 8 to 10 micrometres.
• They are commonly known as natural killer cells.
• They play an important role in body defence.
• These white blood cells are colourless cells formed in lymphoid tissue, hence referred to as lymphocytes.
• There are two main types of lymphocytes – B lymphocytes and T lymphocytes.
• These cells are very important in the immune systems and are responsible for humoral and cell-mediated immunity.