Leucocytes or White Blood Cells (WBC) – Class 11 | Chapter – 18 | Biology Short Notes Series PDF

Leucocytes or White Blood Cells: White blood cells are also called leukocytes or leucocytes. They are cells of the immune system, which is mainly responsible for:

  • Protecting and fighting against invading pathogens
  • Stimulates the production of the progesterone hormone
  • Play a vital role in the human reproductive system by producing a network of blood vessels within the ovary

Types of White Blood Cells and their Function

There are three different types of White Blood Cells:

1. Granulocytes: 

Granulocytes are a type of immune cell that contains small particles called granules which also contain enzymes that are released when we have allergies, infections, and also asthma. They are subdivided into three types:


  • Neutrophil is 8.85 micrometer in diameter.
  • Neutrophils contain very fine cytoplasmic granules. They are called polymorphonuclear cells (PMN) because they have a variety of nuclei.
  • The nucleus is divided into two 3-5 lobes in neutrophils.
  • The neutrophils are the cells that contain neutral granules and the nucleus of cells can be stained by using acidic or basic dyes.
  • There are 3000-7000 (40-70% of WBCs) neutrophils per mm3 of blood.
  • The color of granules in neutrophils is fine violet-pink in colour.
  • Granules contain several enzymes such as protease, lysozymes, and myeloperoxidase. These enzymes act as a bactericidal, it breaks down bacterial cell walls.
  • If the neutrophil count exceeds in our body it is known as neutrophilia, which induces pneumonia, burn, and injuries.
  • Neutrophils circulate for 4-8 hrs in the body then they go in tissue where they are needed and live for 4-5 days.
  • The normal value of neutrophils is 40-70% of total WBC.


  • Eosinophils are 12-15 micrometers in diameter.
  • They have bright reddish-orange colour granules and a bilobed nucleus.
  • Eosinophils constitute 2-6% of circulating WBC.
  • The half-life of eosinophil is 8-18 hrs in the bloodstream. The tissue lifespan is estimated to be 2-5 days.
  • Eosinophil releases enzymes during degranulation. These enzymes destroy parasites which in return eosinophil cause allergic reactions. For example: If you inhale dust particles, in response to it your body shows some allergic reaction like sneezing and red eyes. This happens due to eosinophil which reacts to foreign pathogens.
  • There is 100-400 (1-4% of WBCs) eosinophil per mm3 of blood.


  • Basophils are 10-14 micrometers in diameter.
  • They are bluish in colour.
  • They are a granulated cell that contains histamine, heparin, and other inflammatory mediators.
  • Basophil travels for 70 hrs and survives for 2-3 months in our body.

Basophil matures in the bone marrow and circulates in the peripheral as mature cells.

  • Basophil nucleus is lobated.
  • They play an important role in preventing blood clots.
  • It provides a defense against parasitic infection.
  • There are 20-50 (0-1% of WBCs) basophils in mm3 of blood.

Difference Between Basophils, Eosinophils, and Neutrophils:






Helps to diagnose autoimmune disease or blood-related disorders

Helps to fight against allergies and diseases

Provide an immune response against any foreign particle attack


Multi-lobed nucleus

Bean-shaped nucleus

Two or bilobed nucleus

Life span

Life span is about 60–70 hours

Life span is about 8–12 hours

Life span is about 5– to 90 hours


Diameter is of 10–14 micrometres

Diameter is about 12–17 micrometres

Diametre is about 8.86 micrometres



Leukopenia (low level) and Eosinophilia (high level) 

Leukocytosis (high level) and Neutropenia (low level)

2. Lymphocytes: 

The lymphocytes are further divided into three types:

  1. B cells: The B cells are also referred to as B lymphocytes, which produce antibodies in the immune system.

  2. T Cells: The T cells are also referred to as T lymphocytes, which helps to recognize and remove the infection-causing pathogens.

  3. Natural Killer Cells: These cells are responsible for attacking and killing the pathogens, it also kills cancerous cells.

Difference Between B Cells and T Cells:


B Cells

T Cells

Site of maturation

It originates and matures in the bone marrow.

It originates in the bone marrow and matures in the thymus.

Also called

B lymphocytes.

T lymphocytes.


Present outside the lymph nodes.

Present inside the lymph nodes.

Life span

Shorter life span.

Larger life span compared to B cells.

3. Monocytes:

Monocytes make up around 2–8% of the WBC, which helps to fight against chronic infections.

White Blood Cells Normal Range:

The White Blood Cells normal range is based on age.


Normal Range (Per Cubic Millimetre)



2-week-old baby





Agranulocytes do not have granules in the cytoplasm. There are two subtypes of agranulocytes:


  • The size of lymphocytes is 8 to 10 micrometers.
  • The nucleus in lymphocytes is huge and there is very little place left for the cytoplasm.
  • Lymphocytes are of lymphoid origin.
  • They are natural cell killers.
  • Lymphocytes are a part of our immune system.
  • Antigens are recognized by antibodies of lymphocytes and they attach to the antigen which results in the destruction of pathogens.
  • There are two types of lymphocytes: In B- lymphocytes, the humoral immunity depends on B- cells; and in T- lymphocytes, cell immunity depends on T cells.
  • There are 1500-3000 (20-40% of WBCs) of lymphocytes per mm3 of blood.


  • The size of a monocyte cell is 12 to 10 micrometers.
  • They have a large kidney shaped nucleus as well as an extensive frosted glass cytoplasm.
  • The word mono refers to one nucleus that monocytes have.
  • This cell arises from myeloid lineage.
  • Monocytes are created by a type of cell in the bone marrow known as hematopoietic stem cells.
  • Monocytes move around in the bloodstream for one to three days.
  • Monocytes fight against certain infections and remove dead cells and fight against cancer cells.
  • They are 100-700 (4-8% of WBCs) monocytes per mm3 of blood.

What Happens When a Person has a High White Blood Cell Count?

If the WBC count increases in the body, then it may lead to the disease leukocytosis. Below are the medical conditions indicated dues to high white blood cell count:

  1. Asthma attack

  2. Heart attack

  3. Rheumatoid arthritis, inflammatory bowel diseases

  4. Leukaemia

What Happens When a Person has a Low White Blood Cell Count?

If the human body is producing lower numbers of  WBC, it leads to a disease called leukopenia.

Conditions for leukopenia are as follows:

  1. Bone marrow disorders

  2. Vitamin B-12 deficiency

  3. Autoimmune conditions, HIV

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By Team Learning Mantras