Cells of the Innate Immune System - labsstudies

Cells of the Innate Immune System

Cells of the Innate Immune System

Cells of the Innate Immune System

Cells of the Innate Immune System are white blood cells (leukocytes) that helps our body’s defense mechanism against pathogens.

White blood cells (WBCs) or leukocytes in the peripheral blood play an important role in both innate and adaptive immunity.

White blood cells, or leukocytes, are divided into two types:

  •  Granulocytes and
  • Agranulocytes.
  1. Granulocytes

GranulocytesGranulocytes are  :

  1. Neutrophils
  2. Eosinophils and 
  3. Basophils.

Granules are found in the cytoplasm of granulocytes,

Neutrophils, eosinophils, and basophils all have a multilobed nucleus. As a result, they are also known as polymorphonuclear leukocytes, or “polys.” Because neutrophil nuclei appear to be segmented, they are also known as segmented neutrophils or “segs.”

a. Neutrophils

In adults, the neutrophil, also known as the polymorph nuclear neutrophilic (PMN) leukocyte, accounts for approximately 50% to 75% of total peripheral WBCs. These are 10 to 15 m in diameter and have a nucleus with two to five lobes.

As a result, they are frequently referred to as segmented neutrophils, or “segs.” When stained with Wright stain, they contain a large number of neutral staining granules, two-thirds of which are specific granules and the remaining one-third are azurophilic granules.

The primary function of neutrophils is phagocytosis, which results in the destruction of foreign particles.

Normally, half of the total neutrophil population in peripheral blood is found in a marginating pool adhering to blood vessel walls, while the other half circulates freely for 6 to 8 hours.

However, there is a constant exchange between the marginating and circulating pools. Margination occurs to allow neutrophils to move from the circulating blood to the tissues via diapedesis, or movement through blood vessel walls.

Chemotactic factors draw them to a specific area.

Chemotaxins are chemical messengers that cause cells to migrate in a specific direction.

Neutrophils can survive in the tissues for several days.

To maintain a steady state, the influx of neutrophils from the bone marrow should equal the output of blood to the tissues. However, in the case of an acute infection, an increase in neutrophils in the circulating blood can occur almost immediately.

Neutrophils they are the body’s first line of defense against bacterial infection. Normally, the majority of neutrophils in the bloodstream are mature, with the nucleus of the cell divided or segmented.

Less mature neutrophils have a band or rod-like nucleus rather than a segmented nucleus.

Increased neutrophilia (neutrophilia)

When there is a greater need for neutrophils, such as during an acute bacterial infection, both the total number of mature neutrophils and the less mature bands or stabs increase to respond to the infection. When determining whether a patient has an inflammatory process such as acute appendicitis or cholecystitis, the term “shift to the left” is frequently used.

Neutrophil counts are increased in:

  • Many inflammatory processes
  • Bacterial infections.
  • Tissue necrosis that may occur following a severe burn
  • Under physical stress
  • Granulocytic leukemia 
  • Acute myocardial infarction


Reduced neutrophil count (neutropenia)

This occurs in the suffering from:

  • Typhoid fever
  • Brucelosis
  • Hepatitis
  • Influenza
  • Rubella, and mumps are examples of viral diseases.
  • A severe infection can also deplete neutrophil bone marrow, resulting in neutropenia.
  • Many cancer drugs cause bone marrow depression and can significantly reduce neutrophil counts.


b. Eosinophils

Eosinophils are about 12 to 15 m in diameter and make up between 1% and 3% of the circulating WBCs in a healthy person. Their population grows in response to an allergic reaction or certain parasitic infections.

The nucleus is usually bilobed or ellipsoidal, and it is frequently eccentrically located (Eosinophils absorb acid eosin dye, and the cytoplasm is filled with large orange to reddish-orange granules). Eosinophil granules, which are spherical and evenly distributed throughout the cell, contain a large number of previously identified proteins . Catalase, lysozyme, cytokines (chemical messengers), growth factors, and cationic proteins are all examples of proteins that are synthesized.

Eosinophils can phagocytose but are much less efficient than neutrophils due to their lower numbers and lack of digestive enzymes. Basophil and mast cell products can be neutralized by eosinophils. Furthermore, cationic proteins can be used to damage cell membranes and kill larger parasites that cannot be phagocytized. (For more information, see Chapter 22.) However, the most important role of eosinophils is immune response regulation, including mast cell function regulation.

IgE antigen-antibody reactions are associated with eosinophils.

Read Also: Antibody

Increase in the eosinophil count are:

  • Allergic reactions such as hay fever, asthma, or drug hypersensitivity are the most common causes of an increase in the eosinophil count.
  • Infection with parasites
  • Eosinophilic lymphoma


c. Basophils

Basophils are the smallest WBCs found in peripheral blood, accounting for less than 1% of all circulating WBCs.

Basophils, the smallest granulocytes, are slightly larger than RBCs (between 10 and 15 m in diameter) and contain coarse, densely stained deep-bluish-purple granules that frequently obscure the nucleus.

These granules contain histamine, cytokines, growth factors, and a trace of heparin, all of which play important roles in inducing and maintaining allergic reactions.

The function of basophils is not fully understood.

Heparin, histamines, and serotonin are all found in basophils, which are phagocytes.

Mast cells are another name for tissue basophils. They produce and store heparin, histamine, and serotonin in the same way that blood basophils do.

Allergic reactions are studied using basophile counts.

An increase in basophils may result from a change in bone marrow function, such as leukemia or Hodgkin’s disease, or from an allergic reaction.


  1. Agranulocytes


Agranulocytes are lymphocytes, and monocytes that lack granules and lobular nuclei. They are also known as mononuclear leukocytes.

a. Monocytes

Monocytes are the largest cells in the peripheral blood, with diameters ranging from 12 to 22 m (the average being 18 m).

 One distinguishing feature is an irregularly folded or horseshoe-shaped nucleus that takes up nearly half of the cell. The abundant cytoplasm stains a dull grayish blue and has a ground-glass appearance due to the presence of fine dust-like granules. Monocytes constitute between 4% and 10% of total circulating WBCs; however, they do not remain in the circulation for long.

Monocytes they can stay in the peripheral blood for up to 30 hours before migrating to the tissues and becoming known as macrophages.

Monocytes are the largest cells in normal blood. They function as phagocytes in some inflammatory diseases and are the body’s second line of defense against infection.

Increased of Monocytes(monocytosis) are:

  • Tuberculosis
  • Malaria
  • Brucellosis
  • Monocytic leukemia and
  • Chronic Ulcerative Colitis 


b. Lymphocytes

Lymphocytes are classified as small, intermediate, or large.

Small lymphocytes are typically round with smooth margins.

Cytoplasm: thin rim around nucleus, moderate to dark blue

The nucleus is round or oval in shape and may be slightly indented. There are no nucleoli visible.

Lymphocytes are the primary components of the body’s immune system. They are the source of serum immunoglobulins as well as the cellular immune response.

As a result, they play an important role in immunologic reactions.

All lymphocytes are produced in the bone marrow. The B-cell lymphocyte matures in the bone marrow and controls the antigen-antibody response that is specific to an offending antigen; the T-cell lymphocyte matures in the thymus gland; T cells are the master immune cells of the body, consisting of T-4 helper cells, killer cells, cytotoxic cells, and suppressor T-8 cells.

Because lymphocytes are the second most common WBC type in adults after neutrophils, lymphocytosis is usually associated with neutropenia and lymphopenia with neutropenia. Lymphocytes outnumber neutrophils in children under the age of eight.

Increase in lymphocytes (lymphocytosis) are:

  • Many viral infections 
  • Tuberculosis.
  • Typhoid fever
  • Lymphocytic Leukemia 

A patient with a lymphocyte (lymphopenia) count of less than 500 is at very high risk of infection, particularly viral infections.

Things to notice in Cells of the Innate Immune System

Neutrophil, Eosinophils, Basophils, and Monocytes  are all associated with innate immunity.

Lymphocytes they are considered as a part of adaptive immunity.

Several cell lines found in tissues, including mast cells, macrophages, and dendritic cells, they all also contribute to the immune process.

All blood cells develop from a type of cell known as a hematopoietic stem cell (HSC).

The HSC generates two types of precursor cells to form WBCs:

  • Common myeloid precursors (CMP) and
  • Common lymphoid precursors (CLP) (CLP).

CMPs give rise to the myeloid line of WBCs, which participate in phagocytosis.

Phagocytic cells are essential for innate immunity, but they are also involved in antigen processing for the adaptive response.

Lymphocytes develop from CLPs and serve as the foundation of the adaptive immune response.

Mature lymphocytes can be found in both tissues and peripheral blood.


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  1. September 8, 2022

    […] Also read: Cells of the Innate Immune System […]

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