Immune Cells

by Meghana Matur

Immune cells, commonly known as white blood cells (WBCs), are components of blood that are essential for the protection of an organism from infectious and other diseases. The immune cells, which include neutrophils, eosinophils, basophils, mast cells, monocytes, macrophages, dendritic cells, natural killer cells, and lymphocytes (B cells and T cells), develop from undifferentiated hematopoietic stem cells (HSCs) in the bone marrow. Many of these cells migrate to distal organs and tissues e.g, spleen, thymus, and lymphatic nodes, where they mature into specific, functional immune cells capable of fighting infections. Therefore, the immune system, comprised of white blood cells, hematopoietic and lymphatic organs, and other substances, represents a complex network of defense against foreign agents (bacteria, viruses, and other pathogens), and is necessary for adaptation and longevity. Traditionally, the immune system of mammals is divided into innate and adaptive immune systems based on immunological memory and the specificity of immunologic responses.

Immune responses are a quick form of immune reactions against a broad range of pathogens, co-called nonspecific immune reactions. Skin, mucus, and tears are examples of organs, tissues, and substances that convey physical barriers to those foreign agents. White blood cells that are a part of innate immunity e.g, eosinophil, basophil, mast cells, macrophages, neutrophils, and natural killer (NK) cells, are ready to mobilize and activate immune response at the cites of infection. These cells produce chemical substances such as histamines, reactive oxygen species, cytokines, etc. that either have toxic effects on pathogens or have a signaling function to recruit other immune cells at the site of infection. Macrophages and neutrophils engulf pathogens through a process called phagocytosis. NK cells are the key effector cells that play a very important role in immunosurveillance. NK cells are known for their rapid response toward infected host cells through the release of cytolytic or inflammatory granules.

The second type of immune response that relies on specialized T and B cells to provide antigen-specific effects is known as adaptive immunity. The adaptive response typically sets in within 6 days after exposure, since, lymphatic T and B cells are tailored and expanded in real-time to recognize and eliminate the pathogens. T-cells evolved during the process are categorized as effector T-cells, which include cytotoxic T-cells (T_C) and helper T-cells (T_H); and memory T-cells. T_C induces the death of infected host cells while T_H mediates adaptive immune response. Post resolution, memory T-cells survive to provide robust responses towards subsequent invasions by the same pathogen. B-cells are trained to produce circulating antibodies that play a key role in humoral immunity.

Although innate immunity is said to be the body’s first line of defense and adaptive immunity is associated with immunological memory, both forms of responses collaborate to protect hosts from pathogens. The adaptive immune cells require innate immune cells for their activation and the innate system relies on the longevity of the specific recognition systems during secondary infections. Altered interactions between adaptive and innate immune cells often lead to chronic inflammatory disorders. Importantly, both innate and adaptive immune cells play an important and sometimes paradoxical role in cancer development. For example, macrophages phagocytose cancerous and other abnormal cells in the body. It is believed that strong innate immune responses can eliminate cancerous cells at the very early stages of cancer development. However, in developed malignant tumors, an abundance of infiltrating innate immune cells, such as macrophages, mast cells, and neutrophils, often correlates with increased angiogenesis and poor prognosis. An abundance of infiltrating lymphocytes in cancer, conversely, correlates with a favorable prognosis. Cancer immunology is a fast-developing and promising biomedical field that investigates the antigens that cancer cells express and the recognition of cancerous cells by the innate and adaptive immune cells.

References:

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