Lymphocytes

Lymphocytes are a type of white blood cell known for their consistent appearance yet diverse functions. They comprise T, B, and natural killer (NK) cells. While T and B cells are integral to adaptive immunity, NK cells lack recombined antigen receptors, placing them within the realm of innate immune lymphocytes. They are the main type of cell found in lymph, which forms the name "lymphocyte."

1.0What are Lymphocytes?

Lymphocytes, arising from a shared lymphoid precursor, differentiate into various types. B cells mature primarily within the avian bursa of Fabricius or mammalian-associated Associated Lymphoid Tissue (GALT) found in Peyer's patches of the intestine. Conversely, T cells mature in the thymus. Subsequently, mature lymphocytes enter circulation and populate peripheral lymphoid organs.

2.0Lymphocyte Diagram

3.0Lymphocyte Function

• Lymphocytes are essential immune system components and are divided into three primary forms: T lymphocytes, B lymphocytes, and NK cells. NK cells are renowned for eliminating virally infected cells and malignant tumor cells and for displaying rapid responses to threats.
• T lymphocytes, or T cells, are responsible for cell-mediated immunity and act as the body's vigilant defenders. They mobilize to infection sites, targeting and eliminating host cells compromised by invading pathogens or foreign entities.
• B lymphocytes, or B cells, perform antibody-mediated immunity, undergoing maturation into plasma cells that manufacture new antibodies. These antibodies precisely bind to specific antigens, either neutralizing or destroying them.
• Lymphocytes are abundant within lymphoid organs such as the spleen, lymph nodes, and tonsils.

4.0Lymphocyte Types

According to their structure and function, lymphocytes are categorized into three primary types:

• B Cells
• T Cells
• Natural Killer (NK) Cells

5.0B Lymphocytes

• During mid-fetal life, B lymphocytes in humans undergo initial processing in the liver, transitioning to the bone marrow during late fetal development and post-birth stages.
• Unlike T lymphocytes, B lymphocytes function by actively secreting antibodies instead of the entire cell developing reactivity against antigens.
• This results in a vast diversity of B-lymphocyte antibodies with various specific reactivities.
• Before encountering a specific antigen, B lymphocyte clones remain inactive within lymphoid tissue.
• When a foreign antigen enters, nearby macrophages engulf and present it to adjacent B lymphocytes.
• Those specific to the antigen quickly expand and transform into lymphoblasts, with some differentiating further into plasmablasts.
• Plasmablasts undergo rapid division, resulting in a significant population increase within days. Some activated lymphoblasts generate new B lymphocytes similar to the original clone.
• These cells, known as memory cells, disperse throughout the body's lymphoid tissue, remaining inactive until they encounter the same antigen again.

6.0T Lymphocytes

• T cells undergo crucial processing within the thymus gland during their journey from the bone marrow to the lymph tissues. This organ houses T cells, which undergo a series of transformations. They undergo multiple divisions and acquire specific surface proteins with unique receptor sites.
• The population of T cells undergoes a positive and negative selection process. In positive selection, T cells mature if they recognize self-antigens while binding with foreign antigens. In adverse selection, T cells that react to self-antigens undergo apoptosis, ensuring that only appropriate T cells survive.
• Immunologists estimate that over 90% of developing T cells are negatively selected. The remaining T cells, having passed through the selection process, differentiate and depart from the thymus to reside in other lymph tissues or initiate immune responses in infected tissues.
• The thymus gland's role is crucial in ensuring T cells can distinguish between the body's molecules and foreign antigens. Most T cells undergo differentiation just before birth and in the initial months of postnatal life.
• If the thymus is removed before this differentiation occurs, an organism fails to develop cell-mediated immunity. However, cell-mediated immunity is less severely affected if the thymus is removed after this critical period.
• T cells are characterized by their T-cell receptor (TCR), which enables them to recognize specific antigens. Within the thymus, two primary populations of T cells emerge T cytotoxic cells and T helper cells.
• T cytotoxic (TC) cells, also referred to as CD8 cells due to the presence of the glycoprotein CD8 on their surface, are commonly known as killer T cells. These cells identify and eliminate cells presenting foreign antigens, including virus-infected cells, cancer cells, and foreign tissue grafts.
• On the other hand, T helper (TH) cells, also recognized as CD4 cells, bear the surface glycoprotein CD4. TH cells serve as regulatory agents, secreting cytokines that activate B cells and macrophages. Following an infection, memory T cells, comprising both TC and helper cells, persist in the body, providing long-term immunity.

7.0Natural Killer Cells

• Natural killer (NK) cells are classified as Large Granular Lymphocytes (LGL). Unlike B and T cells, NK cells exhibit unique characteristics and functions.
• NK cells undergo differentiation and maturation primarily in various anatomical sites, including the bone marrow, lymph nodes, spleen, tonsils, and thymus.
• They enter the bloodstream from these locations, contributing to systemic immune surveillance.
• Natural killer (NK) cells serve various functions within the immune system. They trigger apoptosis in target cells through the release of cytolytic granules, thereby efficiently eradicating infected cells.
• NK cells contribute to antibody-dependent cell-mediated cytotoxicity. These cells detect and eliminate tumor cells, crucial for maintaining cellular homeostasis and preventing cancerous growth.
• NK cells play a vital role in regulating immune responses during pregnancy, ensuring the proper development and protection of the fetus.

8.0Condition and Disorders

LYMPHOCYTOSIS refers to the rise in the number of lymphocytes present in the bloodstream. When the number of lymphocytes in the blood exceeds 40% of the total white blood cell count, it is deemed abnormal.

Causes of Lymphocytosis

• Viral infections like influenza, chickenpox, rubella, varicella, herpes simplex, hepatitis, and cytomegalovirus.
• Bacterial infections such as tuberculosis, brucellosis, and whooping cough.

Lymphocytopaenia or Lymphopenia

It is characterized by an insufficient level of lymphocytes in the bloodstream and is typically diagnosed when a complete blood count indicates a lymphocyte count below the age-appropriate reference range.

Causes of lymphocytopenia

• Bacterial and fungal infections
• Viral infections such as influenza, feline leukemia, HIV, and infectious bursal disease
• Malnutrition
• Intense or prolonged physical exertion, severe stress, or pregnancy
• Rheumatoid arthritis