The immune system faces numerous challenges in protecting us against attacks by microorganisms.
It must respond quickly and locate and destroy microbes that can enter any body part.
It has several protective cells of variable nature and functions.
Read on to find out what these immune system cells are and what each one does.
Cells of the immune system
Almost all immune system cells are derived from hematopoietic cells in the bone marrow. Then they differentiate and generate different populations.
A summary of the different types of immune system cells with their respective function is described below.
- Monocytes and macrophages
Monocytes are cells that makeup 5 to 10 percent of white blood cells. They are found lining the walls of blood vessels in organs such as the liver and spleen. Here they capture microorganisms in the blood as they circulate.
When monocytes leave the bloodstream and enter tissues, they change shape and size and become macrophages.
Macrophages are cells whose primary function is to ingest microbes through the process of phagocytosis and then kill them.
To kill, macrophages can form cytoplasmic organelle and fuse them with lysosomes. Lysosomes contain reactive nitrogen and oxygen species that are toxic to microbes.
Together with the activity of proteolytic enzymes, they constitute a fundamental mechanism for eliminating pathogens.
The macrophages are activated by microbial substances and recruit other immune cells to the site of infection. In this way, they amplify the immune response. For example, they serve as antigen-presenting cells to activate T lymphocytes.
In addition, macrophages can ingest necrotic cells of their own and other cells of the immune system, just like neutrophils. This is part of the cleaning process that occurs after an infection.
The neutrophils are a type of cell found in the bloodstream that can quickly ingest and kill microorganisms.
Neutrophils are the most abundant circulatory immune cell population and play a significant role in innate inflammatory reactions.
Once inflammation occurs, neutrophils rapidly travel to the site of infection. This is where they perform their primary function – phagocytosis, particularly those microbes that have undergone the opsonization process.
In addition to phagocytosis, neutrophils can attack pathogens in other ways. For example, they can release granules filled with enzymes and aggressive substances, such as defensins, just as they can export traps in the form of networks (NET) to the extracellular medium.
- Dendritic cells
Dendritic cells (DC) are immune cells that fulfill a unique role of communication between the response of the innate and adaptive immune systems.
On the one hand, their main functions are to act as sentinels, detecting the presence of microbes and initiating innate defense reactions.
On the other, activate adaptive responses by capturing and presenting microbial peptides to T lymphocytes.
Dendritic cells can fulfill these dual functions because they have several types of receptors.
For example, TLRs respond to microbial molecules. When these receptors bind, cytokines are released, and other cells of the immune system are rapidly recruited to the site of infection.
Furthermore, all dendritic cells express MHC molecules of the I and II classes, which explains their ability to mediate with the adaptive immune system through binding to T lymphocytes.
The lymphocytes are the primary immune cells of the adaptive response. All lymphocytes are similar in shape, and their appearance does not reflect their variety of functions.
It is these cells that are responsible for generating antibodies and ensuring memory function, which is why they play a unique role in the transfer of immunity.
A fascinating feature of lymphocytes is their diversity of receptors with different specificities. In other words, there are millions of lymphocyte clones in the body and each one is specific against a certain antigen.
This diversity of recognition is achieved through a process known as clonal expansion.
There are two main classes of lymphocytes, B and T cells.
Type B lymphocytes recognize many different antigens and evolve into antibody-secreting cells. Its function is essential to neutralize the microbe, activate the complement, and engulf it.
T lymphocytes can have various functions and subtypes. For example, they can act as helpers, recognizing antigens on presenting cells and marking them to stimulate other immune system responses.
They can also evolve into cytotoxic T lymphocytes that recognize antigens on infected cells and kill microbes directly, as well as they can fulfill the regulatory function and avoid an immune response to their cells.
- NK cells
Natural killer (NK) cells are a subtype of lymphocytes that play fundamental roles in the innate immune system.
They are so named because they readily kill virus-infected cells and do not require the same thymic education that T cells require.
NK cells are cytotoxic; they contain small granules in their cytoplasm with particular proteins like perforin and proteases known as granzymes.
NK cells are derived from the bone marrow and are present in relatively low amounts in the bloodstream and tissues. They are essential to defend against viruses and prevent cancer.
- Eosinophils, basophils, and mast cells
Basophilic eosinophils and mast cells are three additional cell types of the immune system that share the property of having cytoplasmic granules filled with inflammatory and antimicrobial molecules.
These immune cells play essential roles in fighting parasites and allergic diseases.
Mast cells are derived from the bone marrow and are found in the skin and mucous epithelia. They have granules filled with histamine, and when activated, they promote inflammation.
Basophils are rare cells and are generally found circulating in the blood (they represent 1% of circulating immune cells). Although their function is not entirely clear, they are known to have mast cell-like granules and become activated upon binding of the IgE antigen.
Eosinophils are granulocytes that contain enzymes capable of damaging the cell walls of parasites. They are found in the blood and mucous membranes, where they perform essential defense functions in the digestive and respiratory systems.
Cells of the immune system can be classified as lymphocytes (T cells, B cells, and NK cells), granulocytes (neutrophils, basophils, etc.), and monocytes/macrophages. These are all types of white blood cells.
Each type of cell has different functions, and they work in conjunction with other elements such as signaling proteins (cytokines), antibodies, and complement proteins.
The cells of the immune system act in a coordinated way to elicit rapid immune responses (innate and adaptive)