The cells of the immune system can be divided into three main categories; phagocytes, lymphocytes, and granulocytes. Each type of immune cell is highly specialized to perform a specific function within the innate or adaptive immune system and, together, they defend the body against illness and infection.
What Are Immune Cells?
Immune cells are cells that belong to the human immune system. They all develop from stem cells in the bone marrow but differentiate to form different types of mature immune cells with different functions. Immune cells are present throughout the body and are found in the blood, lymphatic system, and other body tissues.
Categories of Immune Cells
There are three main categories of immune cells in the human immune system. These are phagocytes, lymphocytes, and granulocytes.
Phagocytes are a central component of the innate immune system. There are several different types of phagocytes in the human body, but all engage in phagocytosis (the process of engulfing and digesting foreign agents).
Most phagocytes develop from stem cells in the bone marrow, and the main types are monocytes, macrophages, neutrophils, mast cells, and dendritic cells.
Types of Phagocytes
Monocytes originate in the bone marrow and mature in the blood. Once mature, they circulate in the bloodstream on the lookout for foreign agents, which they engulf and digest.
Monocytes function as antigen-presenting cells (APCs) and present protein fragments of digested pathogens on their surfaces as part of their MHC II complex. These protein fragments (or antigens) are recognized by naïve lymphocytes, which bind to the MHC II complex and are stimulated to proliferate and differentiate.
Monocytes typically circulate in the bloodstream for up to 40 hours before traveling to other tissues and organs. When they reach the tissues, they differentiate into either macrophages or dendritic cells.
Macrophages may develop from monocytes, granulocyte stem cells, or from the cell division of other mature macrophages. They are present in tissues and organs throughout the body, where they promote inflammation by releasing proteins called cytokines.
As phagocytes, macrophages also engulf and destroy pathogens, dead or dysfunctional body cells, and other potentially harmful particles in various organs and tissues of the body. They also function as antigen-presenting cells and stimulate the proliferation of T cells to initiate the adaptive immune response.
Neutrophils are the most abundant type of phagocyte and play a key role in the innate immune response. They are rapidly recruited to sites of infection, where they engulf and destroy antibody-tagged pathogens, dead or dysfunctional body cells, and cell debris. Unlike APCs, neutrophils die after phagocytosis and accumulate at the site of infection as pus.
Neutrophils also have cytotoxic functions and release perforin, granzymes, proteases, and other toxic agents to attack surrounding pathogens.
Dendritic cells are antigen-presenting cells with specialized structures called dendrites, which help them to capture and engulf infectious agents. They then digest the invading microbe and display its antigens as part of the MHC II molecule, which is then used to activate naïve T and B cells in lymphoid tissues. Therefore, dendritic cells have a critical function in the initiation of the adaptive immune response.
Mast cells are found in connective tissues throughout the body and play a major role in initiating inflammatory responses. These phagocytic cells originate in the bone marrow before migrating into the blood but do not reach maturity until they enter the tissues.
Mast cells produce cytokines (such as histamine), which induce inflammation in response to infection. Sometimes, mast cells become sensitized to allergens that would not usually trigger a reaction, such as pollen. In these cases, the inflammatory response is triggered by exposure to the allergen, as is the case for people with hay fever.
The many functions of mast cells are still not fully understood, but they are known to phagocytize antigens. Mast cells also have MHC II complexes on their surface and are thought to participate in antigen presentation.
Lymphocytes (AKA white blood cells) are effector cells of the adaptive immune system. The two main types of lymphocytes are T cells and B cells, both of which originate from stem cells in the bone marrow. Some lymphocytes remain in the bone marrow, where they mature and become B cells, while others migrate to the thymus where they mature into T cells. Once activated, they become effector cells that target and destroy invading pathogens and infected cells in the body.
A third type of lymphocyte, the Natural Killer (NK) cell, develops and matures in the bone marrow, the spleen, the tonsils, or the lymph nodes. Like T cells and B cells, NK cells find and kill infected cells in the body. However, unlike other types of lymphocytes, NK cells are part of the innate immune system.
Types of Lymphocytes
Lymphocytes that mature in the thymus become T cells, which are then activated to become either helper T cells or cytotoxic (AKA ‘killer’) T cells.
Helper T cells ‘help’ to coordinate the adaptive immune response against foreign agents by secreting cytokines to activate other immune cells. B cells, macrophages, leukocytes, and other T cells are all stimulated to proliferate and differentiate by helper T cells.
Cytotoxic T cells are also known as killer T cells because they directly attack and destroy pathogens, infected cells, and tumor cells. Once they find an abnormal cell, killer T cells release cytotoxic granules containing perforin and granzymes. Perforin creates pores in the cell membrane of the target cell, and granzymes enter through these holes to trigger apoptosis of the infected cell.
B cells mature in the bone marrow before migrating to the lymphatic system, where they circulate in the body as naïve B cells. Once they encounter their specific antigen, the naïve B cells are activated to proliferate and differentiate into either plasma B cells or memory B cells.
Plasma B cells pump out large quantities of proteins called antibodies, which are specific to the same antigens as their parent cell. These antibodies circulate in the bloodstream seeking out foreign invaders and, once they locate their target, will bind to its surface. Some antibodies are neutralizing antibodies that deactivate the pathogen by blocking surface sites that are key for infectivity. Other antibodies are binding antibodies, which ‘tag’ the target pathogen for destruction by other immune cells.
Natural Killer Cells
Natural killer cells attack pathogens in the same way as cytotoxic T cells do; by releasing cytotoxic granules that destroy the target cell.
The key difference between cytotoxic T cells and NK cells is that the latter belong to the innate immune system, and do not require antigen exposure to become activated. They are ‘naturally’ cytotoxic, and primarily destroy virus-infected host cells to limit the spread of infection in the body.
Granulocytes are white blood cells with lobed nuclei and granules in their cytoplasm.
They have roles in both innate and adaptive immunity but are typically the first immune cells on the scene in cases of injury or infection. This makes them the body’s first line of defense against viral, bacterial, and parasitic pathogens.
Granulocytes are produced from stem cells in the bone marrow and circulate in the blood when they reach maturity. The three types of granulocytes are basophils, eosinophils, and neutrophils, and each is specialized to perform a specific function.
Types of Granulocytes
Neutrophils are the smallest and most abundant type of granulocyte in the immune system. Their specific function is to fight bacterial infections and circulate in the bloodstream on the hunt for pathogens. Neutrophils are also a type of phagocyte and will engulf and degrade any bacteria they find. These granulocytes are crucial for clearing infectious agents from the body, and patients with low neutrophil counts are at greater risk of developing serious infections.
The granules in basophils contain histamine, which they release in response to allergen exposure. Therefore, one of the key functions of basophils is to mediate allergic reactions. Basophils also directly attack and destroy infectious agents such as bacteria, parasites, fungi, and viruses in the body. They destroy these pathogens in the same way as neutrophils do; by phagocytosis.
The primary function of eosinophils is to defend their host from parasitic infections, such as nematodes. They can also be activated as part of an inflammatory response to allergens, which can have a damaging effect. For example, eosinophils are responsible for causing the symptoms associated with allergic asthma.