The Immune System Cells: The Basic Units of Our Body's Defense

Macrophage

The human body is constantly under attack by infectious agents, also called ‘pathogens’. Luckily our body has an intricate multi-systemic defense mechanism, that we call ‘the immune system’. Immunologists divide this immune system into two; innate immunity and acquired or adaptive immunity. The immune system has many different types of cells. These cells come from two main progenitors, or ancestor cells; myeloid progenitor cell and lymphoid progenitor cell.

 

Myeloid Progenitor Cells

Differentiation of cells of the myeloid ancestry gives rise to: monocyte, macrophage, dendritic cell, neutrophil, eosinophil, basophil, and mast cells.

Monocyte: Monocytes are phagocytic cells, that is they engulf foreign pathogens and debris in the blood. It differentiates into tissue macrophage after circulating in the blood for 1-2 days.

Macrophage: A macrophage is a mature monocyte, it functions to produce cytokines and performs phagocytosis which is the engulfing of foreign antigenic material.

Dendritic Cell:  This cell is characterized by its long cytoplasmic arms, which gives it its distinct appearance. This cell captures an antigen, transports it, and presents it to other cells of the immune system. The dendritic cell functions as a messenger between innate immunity and acquired immunity.

Neutrophil: Neutrophils are one of the most important cells of the innate immune system. Their function is phagocytosis and the activation of bactericidal mechanisms. It is essential for protection against bacterial invasions.

Eosinophil: These cells are characterized by large pink granules, they perform their function by killing antibody coated parasites and eliciting an allergic reaction. They are essential to fight against parasitic invasion of the body,

Basophils: Basophils are characterized by large blue granules in the cytoplasm of the cell. They play a key role in an allergic response. Pharmacological substances released by basophils increase the body’s response to invading allergens.

Mast cell: This cell is present in an allergic response and anaphylactic reactions. It releases granules containing histamines and heparin.

Natual killer cell attacking cancer cell; credit: Dr. Rupert Handgretinger, Univ. Hospital of Tuebingen

Lymphoid Progenitor Cells

The Lymphoid Progenitor Cell gives rise to: lymphocytes, natural killer lymphocyte, and plasma cells.

Lymphocyte: There are two types of lymphocytes; B lymphocyte and T lymphocyte. The B lymphocyte matures in the bone marrow, while the T lymphocyte matures in the thymus. The B and T lymphocytes play a critical part in distinct immune responses: Humoral immune response and cell-mediated immune response.

• Humoral Immune Response: In this immune response the B lymphocytes produce antibodies to help identify and eliminate invading pathogens. It is primarily mediated by antibodies made my plasma cells, which are mature B lymphocytes. It is a major defense mechanism against toxins and extracellular pathogens.
• Cell-mediated immunity: This immune response is mediated by T lymphocytes. This immune response is activated when the pathogenic or antigenic stimuli arises from within the body. It is a major defense mechanism against intracellular pathogens.

Natural Killer (NK) lymphocyte: These cells kill virus infected cells, tumor cells, and antibody coated target cells.

Plasma Cell: These are the mature B lymphocytes and produce antibodies. They play an essential part in the humoral immune response.