Overview of Immune Cells

Immune cells, also known as leukocytes, are an integral part of the immune system and play a crucial role in defending the body against both infectious diseases and foreign invaders. These cells originate from multipotent stem cells in the bone marrow and circulate throughout the body in the blood and lymphatic system.

Types of Immune Cells

Granulocytes

Granulocytes are a type of immune cell characterized by the presence of granules in their cytoplasm. They include basophils, eosinophils, and neutrophils.

Basophils and eosinophils are primarily involved in the body's defense against parasites and also play a role in allergic reactions.
Neutrophils are the most abundant type of innate immune cells and are the first responders to microbial infection, acting by engulfing and digesting pathogens in a process known as phagocytosis.

Mast Cells

Mast cells are found in tissues and are best known for their role in mediating allergic reactions. They release inflammatory chemicals, such as histamine, which contribute to the body's defense mechanisms during an allergic response.

Monocytes and Macrophages

Monocytes are a type of white blood cell that circulates in the bloodstream and can differentiate into macrophages upon migrating into tissues. Macrophages are known as "big eaters" due to their ability to ingest and degrade bacteria and other harmful particles. They play a pivotal role in coordinating the immune response by acting as antigen-presenting cells and notifying other immune cells of invaders.

Lymphocytes

Lymphocytes are central to the adaptive immune response and include several subtypes:

T cells: These are critical for cell-mediated immunity and include various subtypes such as helper T cells and cytotoxic T cells. [Helper T cells] are important for orchestrating the immune response, while cytotoxic T cells destroy infected cells.
B cells: They are responsible for producing antibodies, which are necessary for the humoral immune response.
Natural killer cells: These cells play a role in the innate immune system by targeting and destroying compromised host cells, such as tumor cells or virus-infected cells.

Function in the Immune System

The immune system is broadly categorized into the innate immune system and the adaptive immune system.

Innate immune system: This is the body’s first line of defense and includes non-specific immune cells such as neutrophils and natural killer cells. These cells are always present and ready to respond to threats.
Adaptive immune system: This system mounts a more targeted response to specific pathogens, primarily mediated by lymphocytes. The adaptive immune response is highly specialized and includes immunological memory, allowing for faster and more efficient reactions upon subsequent exposures to the same pathogen.

Immune Cells