White Blood Cells | Types and functions:

White Blood Cells are otherwise called as Leukocytes. The name leukocyte derived from Greek word leuk means ‘white’, cyte means ‘cell’. WBC main role is to protect the body against microbes. WBC surround microbes, engulf and destroy the foreign particles by phagocytosis mechanism.

White Blood Cells are contain nuclei and some have granules in their cytoplasm.

Types of white blood cells:

 Granulocytes (polymorphonuclear leukocytes)

 Agranulocytes

Granulocytes:

Granulocytes are otherwise known as polymorphonuclear leukocytes. These are originate from the bone marrow. Granulocyte contains granules in their cytoplasm. Neutrophils , eosinophils, basophils. In granulocytes, especially the basophils helps the body fight with bacterial infections.

Neutrophils:

Neutrophils are act as the first line defense system. These are about 60% present in whole WBC. They are attracted in large numbers to any area of infection by chemical substances, released by damaged cells called chemotaxins. Neutrophils passed through the capillary walls in the affected area by amoeboid movement. Neutrophils are engulf and kill the microbes by phagocytosis. Their granules are lysosomes that contain enzymes that digest the engulfed material.

Their normal range is between 2,500 to 7,500 neutrophils per microliter of blood.

Functions of neutrophils:

Phagocytosis: Neutrophils are highly efficient phagocytes, meaning they can engulf and digest microorganisms, such as bacteria and fungi. They use their flexible cell membranes to surround and engulf pathogens, forming a vesicle called a phagosome. Once the pathogen is inside the phagosome, it is broken down by enzymes and other antimicrobial substances.

Chemotaxis: Neutrophils can sense and move towards chemical signals produced by microorganisms or damaged tissues. This directional movement is called chemotaxis, and it helps neutrophils navigate towards sites of infection or inflammation.

Respiratory Burst: Neutrophils have a process known as the respiratory burst, where they generate reactive oxygen species (ROS) as part of their antimicrobial activity. ROS help kill ingested microorganisms by causing oxidative damage.

Extracellular Traps (NETs): Neutrophils can release web-like structures called neutrophil extracellular traps (NETs). These structures consist of DNA, histones, and antimicrobial proteins. NETs help trap and kill pathogens, preventing their spread.

Cytokine Production: Neutrophils can produce cytokines, which are signaling molecules that modulate the immune response. They contribute to the coordination and communication between different immune cells.

Eosinophils:

Eosinophils although capable for phagocytosis, are less active in this than neutrophils. Their specialized role is to eliminate the parasites such as worms, which are too big to be phagocytosed. These are present about 1 to 6% of total White Blood Cells. Its normal range is between 30 to 350 cells per microliter in the blood.

Functions:

Parasite Defense: Eosinophils are particularly effective in combating parasitic infections, such as helminths (worms) and certain protozoa.

Allergic Reactions: Eosinophils are involved in the regulation of allergic responses and asthma. In allergic conditions, eosinophils are recruited to sites of inflammation in response to the release of chemical signals (such as cytokines and chemokines) triggered by allergens. Eosinophils can release inflammatory mediators and granule proteins that contribute to tissue damage in allergic reactions.

Modulation of Inflammation: Eosinophils can produce and release cytokines, which are signaling molecules that influence the immune response. They contribute to the regulation of inflammation by interacting with other immune cells and modulating the activity of immune mediators.

Wound Healing: Eosinophils are involved in the resolution of inflammation and tissue repair after an immune response. They may play a role in the remodeling and repair of damaged tissues.

Basophils:

Basophils which are closely associated with allergic reactions, contain cytoplasmic granules packed with heparin, histamine and other substances that promote inflammation. Basophils are present in 0.5% to 1% of white blood cells.

Functions:

Release of Histamine: Basophils contain granules filled with substances, including histamine. Histamine is a potent vasodilator (widens blood vessels) and increases vascular permeability.

When basophils are activated, they release histamine, contributing to the immediate inflammatory response seen in allergic reactions.

Mediation of Allergic Responses: Basophils play a crucial role in the early stages of allergic reactions and hypersensitivity responses.

They are involved in the release of inflammatory mediators, such as histamine and other chemicals, which contribute to the symptoms of allergies, including itching, redness, and swelling.

Antiparasitic Defense: Basophils, along with eosinophils, contribute to the defense against parasites. They release substances that can be toxic to parasites and are involved in the immune response against parasitic infections.

Expression of Immunoglobulin E (IgE) Receptors: Basophils express high-affinity receptors for Immunoglobulin E (IgE), an antibody class associated with allergic reactions. IgE antibodies bind to these receptors on the surface of basophils. Upon subsequent exposure to an allergen, the IgE-bound basophils become activated, leading to the release of inflammatory mediators.

Modulation of Immune Responses: Basophils can produce and release various cytokines (signaling molecules) that modulate the immune response. They interact with other immune cells and contribute to the coordination and regulation of immune reactions.

Agranulocytes:

Agranulocytes are otherwise called as mononuclear leukocytes. These are not having granules in their cytoplasm. Agranulocytes originate from lymph nodes. Agranulocytes consist of monocytes and lymphocytes.

Monocytes:

These are large mononuclear cells that originate in red bone marrow. Some circulate in the blood and are actively move and phagocytic while others migrate into the tissues where they develop into macrophages.

Functions of monocytes:

1) Immune Response:

Monocytes are involved in the initiation and coordination of immune responses. They can act as antigen-presenting cells (APCs) by capturing, processing, and presenting antigens to other immune cells, particularly T cells.

Antigen presentation by monocytes helps activate T cells, which are crucial for a specific and targeted immune response.

2) Inflammatory Response:

Monocytes play a role in the inflammatory response. When there is an infection or tissue damage, monocytes are recruited to the site of inflammation.

Once at the site, monocytes can differentiate into macrophages, which release inflammatory mediators and cytokines that help regulate and amplify the immune response.

3) Tissue Repair and Remodeling:

Monocytes, when differentiated into tissue macrophages, contribute to tissue repair and remodeling after inflammation or injury.

They participate in the clearance of cellular debris and the secretion of growth factors that support tissue healing.

4) Surveillance:

Monocytes circulate in the bloodstream and constantly survey the body for signs of infection, abnormal cells, or tissue damage.

They can quickly migrate to sites of infection or inflammation in response to chemical signals released by damaged tissues or invading pathogens.

5) Homeostasis:

Monocytes help maintain homeostasis by removing senescent (aged) cells, cellular debris, and other particles from the circulation.

Lymphocytes:

Lymphocytes are smaller than the monocytes and have large nuclei. They circulate in the blood and are present in great numbers in lymphatic tissue such as lymph nodes and the spleen. Lymphocytes are develop from the pluripotent stem cells in red bone marrow

Functions:

1) Recognition of Antigens:

B cells: B cells are responsible for recognizing antigens (foreign substances) such as bacteria, viruses, and toxins. Each B cell is specific to a particular antigen, and they have surface receptors that can bind to these antigens.

T cells: T cells can recognize antigens displayed on the surface of infected or abnormal cells. T cells do not directly recognize free-floating antigens; instead, they interact with antigen-presenting cells (APCs) like macrophages and dendritic cells.

2) Antibody Production:

B cells: Upon recognizing an antigen, B cells can differentiate into plasma cells, which are specialized cells that produce antibodies. Antibodies are proteins that circulate in the blood and other body fluids, binding to and neutralizing antigens, marking them for destruction or removal by other immune cells.

T cells: While T cells do not produce antibodies themselves, they help regulate the antibody response by assisting B cells and other immune cells.

3) Cell-Mediated Immunity:

T cells: T cells plays an important role in cell-mediated immunity. Cytotoxic T cells, also known as CD8+ T cells, can directly attack and destroy infected or abnormal cells. They recognize antigens presented on the surface of these cells and release toxic substances to induce apoptosis (programmed cell death).

4) Regulation of Immune Responses:

T cells: Regulatory T cells (Tregs) help control and regulate the immune response, preventing excessive activation and inflammation. They play a role in maintaining immune system balance and preventing autoimmune reactions.

5) Memory and Long-Term Immunity:

B cells and T cells: After an initial encounter with an antigen, some lymphocytes become memory cells. Memory B cells and memory T cells "remember" the specific antigen, providing a quicker and more robust immune response upon subsequent exposures. This memory function contributes to the development of long-term immunity.

6. Immune Surveillance:

T cells and B cells: Lymphocytes contribute to immune surveillance by patrolling the body and identifying and eliminating cells that display abnormal or foreign antigens, including those infected by pathogens or transformed (cancerous) cells.

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