Immune Cells - Types and Functions

What Is Immune System?

The immune system comprises numerous components that cooperate to protect the body from invaders. The thymus and bone marrow are the two main immune system components. All body's blood cells originate in the bone marrow, making it a vital immune system component. T cells move to the thymus, while B lymphocytes continue to develop in the bone marrow.

What Are the Cells of the Immune System?

Lymphocytes (T-lymphocytes, B-lymphocytes, and natural killer cells), neutrophils, and monocytes or macrophages are the immune system's most prevalent cell types. White blood cells come in a variety of forms. The main proteins of the immune system are mostly antibodies (immunoglobulins), cytokines (a type of protein responsible for cell signaling between immune system cells), and complement proteins. The main cellular components of the immune system are:

• Antigen Presenting Cells (APCs) - Although some antigens (Ag) can directly elicit an immune response, most T cell-dependent acquired immunity responses necessitate the presentation of antigen-derived peptides within major histocompatibility complex (MHC) molecules by antigen-presenting cells (APCs). The antigen-presenting cells are:

  • Dendritic Cells- To activate B and T lymphocytes, the antigen combines with a major histocompatibility complex (MHC1) and is carried by the dendritic cells. Dendritic cells are seen in the skin, lymph nodes, and body tissues. In the skin, the dendritic cells are known as Langerhans cells.

  • Monocytes - The tissue macrophages' precursors are the circulating monocytes present in the blood. Monocytes invade the injured or infected tissues and change to macrophages. The entire transformation is governed by macrophage colony-stimulating factor (M-CSF), which is secreted by cells like endothelial cells and fibroblasts. Activated T cells release cytokines (such interferon-gamma [IFN-gamma]) at the sites of infection that trigger the creation of macrophage migratory inhibitory factor, which prevents macrophages from escaping. Depending on the situation, they can develop into macrophages and dendritic cells.

  • Macrophages - These cells help activate the immune system and engulf microbial agents and other foreign substances. Several microbial components and cytokines, such as IFN-gamma, interleukin (IL)-4, and IL-13, stimulate macrophages (lipopolysaccharide). Activated macrophages release cytokines, including tumor necrosis factor-alpha (TNF-alpha) and IL-10, and kill intracellular organisms.

  • B-cells - They grow and mature into plasma cells which produce antibodies.

• Polymorphonuclear Leukocytes - They are also known as granulocytes and are a part of white blood cells which exhibit an immune response.

  • Neutrophils - They constitute the first line of defense against infections and form about 40 to 45 percent of white blood cells. When infection or other acute inflammatory response occurs, neutrophils leave the bloodstream and enter tissues. The movement of neutrophils is stimulated by chemotactic factors and alerted by the expression of adhesion molecules on the blood vessel endothelium. They engulf and digest the microbial agents. The phagocytic activity of the neutrophils releases lytic enzymes along with DNA and histones. These combine with the granular content, such as elastase, and form fibrous components in the nearby tissues called neutrophil extracellular traps, as a result of which the enzymatic activity kills bacteria.

  • Eosinophils - They form five percent of the total white blood cells. Eosinophils act on pathogens that are large and cannot be engulfed. They act on them by releasing toxic substances and proteins that kill these microbes. They are also sources of mediators of inflammation, like cytokines and prostaglandins.

  • Basophils - They constitute less than five percent of the total white blood cells. They have a strong affinity for IgE receptors. Upon encountering an antigen, they start the immune response, cause cell degranulation, and release inflammatory mediators like histamine, prostaglandins, and thromboxanes.

• Mast Cells - They act similarly to the basophil and cause acute inflammatory responses due to the release of chemical mediators. In addition, they cause type 1 hypersensitivity reactions.

• Natural Killer Cells - Natural killer cells recognize and kill the virus-infected and tumor cells. They have granules present (intracellular compartments) in them, which contain proteins. These proteins create holes in the target cells and cause apoptosis and hence can eliminate the infected cells without evoking great immune response and inflammation.

• Lymphocytes:

  • T-Lypmhocyte - Bone marrow stem cells give rise to T lymphocytes, which undergo stringent selection in the thymus. Four primary categories of T cells exist:

    • Helper T-Cells - Helper T cells release cytokines. Cytokines help in the differentiation of plasma cells into B cells. These cells also aid in the activation of macrophages and cytotoxic T cells.

    • Regulatory or Suppressor T-Cells - Regulatory cells (Tregs) help suppress the immune system.

    • Cytotoxic T-Cells - The primary effector cells of adaptive immunity are cytotoxic T-cells. Cytotoxic T cells that have been activated can travel through non-lymphoid tissues and blood vessel walls. They can pass across the blood–brain barrier as well. Cytokines trigger the activation of cytotoxic T lymphocytes. They can bind and kill cancer cells.

    • Memory T cells - They develop from activated cytotoxic T cells and are durable and antigen-experienced. A single memory T-cell can give rise to many cytotoxic T-cells.

  • B-Lymphocytes - They are named CD19 or CD20 cells in the lab reports. These specialized immune system cells, whose main job is to create antibodies (also known as immunoglobulins). In the bone marrow, stem cells give rise to B cells. B cells are trained as part of their regular maturation in the bone marrow not to make antibodies against healthy tissues. Fully developed, B cells are located in the bloodstream, lymph nodes, spleen, some regions of the gut, and bone marrow. B-lymphocytes are of two types:

    • Plasma B-Cells - These activated B-cells produce antibodies upon encounter with invading pathogens (antigens). These cells are present in the spleen and the lymph nodes.

    • Memory B-Cells - B cells can develop into memory cells, enabling a quick response if the same infection recurs.

Conclusion

The immune system of the body is important for survival. The body attacks to eliminate infections when encountering dangerous intruders, such as microbial agents or a splinter in the finger. Human beings are born with immunity (innate immunity) that starts protecting our body from the time of birth. However, immunity is boosted when the body is exposed to diseases and vaccines. Therefore, the immune cells play a major role in protecting our body from infections.