Without soldiers or police officers, there would be no army or police force. Without cells, there would be no immune system. And, as this is the immune system and immunologists are never happier than when they're naming things, there are a lot of cells. Which is why this post is dedicated to understanding what they are, what they do, and why they're important.
Please bear with me here! I know it's a lot of info - you can skip to the end for a quick overview of the major immune cells (but you'll have to forgive my drawing).
Before we go any further, we need to discuss the blood cells. As everyone knows, there are two overarching types: the red blood cells and the white blood cells. The red blood cells, erythrocytes, are essential for carrying oxygen around your body but have no function in immunity. The white blood cells, the leukocytes, include all the different types of immune cell.
So, first up (because that's when they arrive), we have the cells of the innate system.
Neutrophils - these are the most common of your innate immune cells and are generally the first responders to any crime scene. They produce mass amounts of signalling molecules that initiate further immune responses (a radio call to arms, if you will). They produce chemicals that are toxic to cells (cytotoxic) to destroy infected cells and will affect a large area. They are also phagocytic , meaning they 'eat' anything that looks good (bacteria, dead/dying cells, etc.)
Eosinophils - these are generally found along the respiratory, gastrointestinal (stomach and intestines) and urinary tracts (basically wherever your insides contact the outside world). They are mainly involved in defence against parasites. They are also big players in allergy (so blame them for your hay fever).
Monocytes/macrophages - in simple terms, monocytes are the early stage of the macrophage's life cycle (this will be discussed further in a separate post). When they receive activating signals, monocytes become macrophages. Like neutrophils, they can produce cytotoxic chemicals and are phagocytic. They also share some similarity to dendritic cells (both in looks and function) and can present parts of destroyed bacteria, etc., to the T cells (although this function is limited). They are also important in basic maintenance of your tissues (they have roles outside of the immune system and are the reason you don't have webbed fingers when you're born).
Mast cells and basophils - these belong to the same family as neutrophils and eosinophils (granulocytes) but are much less numerous. Both produce effector chemicals from their granules that drive further immune responses. Both also play a role in allergy.
Dendritic cells - these are often thought of as the bridge between innate and adaptive immunity. They have a role in bacterial defence, but it's their ability to present bits of bacteria, viruses etc to the cells of the adaptive immunity that makes them so important. Dendritic cells are critical for ensuring T and B cells 'see' the bit of a pathogen that matches them, which allows the B/T cell to become activated and do its job. Dendritic cells are also important during B/T cell maturation - they make sure B and T cells do not recognise bits of your own cells. This process, called immune tolerance, is essential to stop autoimmunity and will be discussed later.
The adaptive immune cells are unique in their ability for specificity. While innate immune cells essentially see 'bad thing here, attack everything nearby,' B and T cells recognise the bad thing and will only attack it. T and B cells are also referred to as being lymphocytes, because of their ability to enter and reside in the lymph nodes.
T cells - whether helping B cells or killing infected/damaged cells, these cells are integral to the adaptive immune system. They are responsible for the cell-mediated response arm of the adaptive system. They mature in your thymus (hence the T).
B cells/Plasma Cells - if you want antibodies, these are your guys. B cells differentiate (turn into) plasma cells when stimulated. Plasma cells mass produce antibodies which target the pathogen, immobilising/inactivating them and making it easier for them to get 'eaten' by phagocytic cells (e.g. neutrophils, macrophages or dendritic cells). These cells are almost always the targets of vaccination (discussed more later).
The list of immune cells goes much further, but these are the main types that are likely to be encountered. If others appear, they will be described later.
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