Types of White Blood Cells (Leukocytes) With Their Structure and Functions

Based on structure, all white blood cells are broadly classified into five types, which are further grouped into two major types, based on the presence of granules in their cytoplasm as follows:

Granulocyte: WBCs with granules in their cytoplasm. Granulocytes are of three types: 1) neutrophil, 2) eosinophil, and 3) basophil.

Agranulocytes: WBCs with no distinct granules in their cytoplasm. Agranulocytes are of two types: 4) monocyte and 5) lymphocyte.

Also known as a polymorphonuclear (PMN) leukocyte, it has a multi-lobed nucleus (generally consisting of three to five lobes) with each lobe being connected by thin strands, making it look multinucleated. Inactivated neutrophils are spherical, but once activated, they take on an amoeba-like appearance. They have an average diameter of 12-15 µm in the peripheral blood, while in suspension, they have a diameter of 8.85 µm.

Abundance: Constitutes 62% of the total leukocyte count.

Composition: The cytoplasm of a neutrophil looks transparent because of the presence of fine granules. Other organelles like the Golgi apparatus, mitochondria, and ribosomes are also present.

Lifespan: It can vary widely between 5 to 135 hours.

Functions

• Engulfing pathogens like bacteria by a process called phagocytosis
• Consuming and digesting pathogens by releasing superoxide radicals to present them in the form of pus

It has a bi-lobed nucleus with the two lobes being connected by a thin-strand. They are about 12-17 µm in size with almost 200 granules in their cytoplasm. 

Abundance: Comprises about 2-4% of the total WBCs. Their number is found to increase in response to a parasitic infection.

Composition: Acidic granules in the cytoplasm, which contains enzymes and proteins that take up pink-orange color when stained with eosin.

Lifespan: Tissue life of about 2 to 6 days.

Functions

• Destroying large parasites such as hookworms and tapeworms, by secreting specific chemicals
• Found to cause allergic reactions such as asthma, hay fever, and infections of hives

It has a bi or tri-lobed nucleus, with the presence of large cytoplasmic granules. They are generally round but can change shapes as they migrate.

Abundance: Accounts for only 0.5% of the total white blood cell count.

Composition: Sharing similar physicochemical properties with other white blood cells.

Lifespan: About 60–70 hours

Functions

• Producing inflammatory reactions during an immune response by releasing immune-complexes like histamine, serotonin, and heparin
• Producing the body’s response to allergic reactions during anaphylaxis, asthma, and hay fever

They are immune cells with a diameter of 12-15 μm that are broadly classified into three types:

a) B cells: Also called B Lymphocytes, it is identified by the presence of B cell receptor or BCR 

b) T cells: Also called T Lymphocytes, it identified by the presence of T cell receptor or TCR 

c) Natural killer cells (NK cells): Identified by the presence of CD3 receptor

Abundance: Comprising 30% of the total white blood cells. They are much more abundant in the lymph nodes of the lymphatic system compared to blood.

Lifespan: Mostly have a short life span ranging from a week to a few months, but a few even live for years, thus providing immunity to certain infections.

Functions

• Providing body defense against infections by recognizing antigens, producing antibodies, and killing infected cells
• B cells produce antibodies that bind and attack pathogens and helps in their destruction
• T cells provide immunity from foreign pathogens through an antibody-mediated response called adaptive immunity
• NK cells provide the first line of defense in our body called innate immunity that helps to destroy virus-infected cells and other harmful organisms. They also play a role in destroying cancer cells

They are the largest of all WBCs, resembling amoeba in appearance with unilobar nuclei and non-granulated cytoplasm. The nucleus of monocyte is typically bean or kidney-shaped.

Abundance: Consists of about 2 to 10% of the total circulating white blood cells, that may increase in case of infections.

Composition: Contains numerous fine granules that are most abundant near the cell membrane.

Lifespan: About 24 hours.

Functions

• Having a similar role to neutrophils that work by phagocytosis.
• Helping in the production of antibodies
• Producing cytokine, a specific group of small chemicals that help to kill pathogens.
• Helping in antigen presentation using specific receptors.