Once I’ve completed Specific Immunity and the Lymphatic System, I’ll tie all this together using the example of a bacterial pathogen invading a cut. It will all make sense then.
In response to an emailed request, I’m going to end with a brief summary/recap similar to the one I did for non-specific immunity.
Cellular components
White blood cells are the cells involved in immune responses and are called leukocytes (leuko means white, cyto means ‘a hollow, receptacle or basket’ and refers to cells). There are many different types of leukocytes but, as general rule, all leukocytes are nucleated cells (have a nucleus, red blood cells don’t), originate in the bone marrow where they differentiate from stem cells, have a role in the body’s immune system and (this is pretty cool) are capable of independent movement. Yep, they creep around in a very similar manner to amoebas, through blood and tissue.
Some of these cells are termed phagocytes. Phage means ‘to eat or digest’, so a phagocyte is a cell that eats or digests stuff. This is a good little summary of how they work in general: phagocytes The two main phagocytic cells are neutrophils and macrophages, both of these I’ll get to later.
Leukocytes are broadly grouped into two main groups based on morphology. There are Granular Leukocytes, which have visible granules in their cytoplasm and a nucleus that is irregularly shaped, and Agranular Leukocytes, which as the name suggests, does not have granules in the cytoplasm. The agranular leukocytes also have a more typical round nucleus.
Granular Leukocytes
These are your ‘phils. Neutrophils, Eosinophils and Basophils. The ‘phils are named because the ‘like’ stuff; ie they are phillic for different things.
Neutrophils are the most numerous of the leukocytes comprising between 33 – 75% of all white blood cells in the blood depending on the species of mammal examined. Neutrophils are the first cells to respond to a pathogen, and after about 6 hours are gradually replaced by macrophages at the site of the infection until, after about 48 hours, they are completely replaced. The name neutrophil (neutro means neutral, phil means loving) comes from the fact that granules found within their cytoplasm are pH neutral. Neutrophils primarily hunt down invading bacteria and fungi and engulf these cells, attacking them with a diverse and powerful chemical arsenal contained within those granulated vesicles. The process they use to find these pathogens is called chemotaxis. Chemotaxis is the directed movement of a cell along a chemical concentration gradient. They sniff them out in a similar way that a male moth finds a female (read the Pheromones essay). They are primarily active in tissues, not the blood. Neutrophils grow from stem cells within the bone marrow and once mature do not differentiate again or divide. They have about a two week life span and most neutrophils don’t leave the bone marrow but are held back as an ‘in case of emergency’ supply. When they die within tissues at the end of their two week life span they release compounds that act as powerful fungicides.
Eosinophils
Eosin is a dye used to stain cells. Eosinophils respond well to this stain and are therefore named after their ‘love’ of this stain. Like all the cells discussed in this essay they differentiate from stem cells in the bone marrow then spend the rest of their ‘life’ in the blood or gut lining. Their primary role is to defend against parasitic organisms such as worms. These comprise about 1-3% of the body’s leukocytes. Again, it’s the chemicals contained within the granulated vesicles that are used to attack the parasites. When abnormal levels of eosinophils occur in the blood they can have an adverse effect on the body. Conditions such as asthma are triggered by eosinophils.
Basophils
Basophils are quite rare in the leukocyte population, comprising less than 1% of the total number. Basophils are transported by the circulatory system and are found in blood and tissue. They are poorly understood when compared to other leukocytes, however, we know they are responsible for the release of histamine and herapin, two chemicals I will get to later.
Agranular Leukocytes
There are two types of agranular leukocytes: monocytes and lymphocytes. Lymphocytes are primarily involved in the specific immune response so I will talk about them in the Specific Immunity essay. For the next little while, I’m going to rave about monocytes.
Monocytes aka Macrophages
Like the ‘phils monocytes originate in the bone marrow. They enter the blood stream and then migrate into different tissues where they differentiate into what we call macrophages. There are several different types of macrophages that do slightly different things, but primarily these are large phagocytes involved in the ingestion and destruction of pathogens. They are also involved in wound-cleaning (the attack of pathogens entering a wound), inflammation, have a regulatory role in insulin production. But for Yr12 and for non-specific immunity, you need to know that they are large phagocytes that are found in the blood and tissue.
As you can see, several of these cells have a role in both specific and non-specific immunity. Both specific and non-specific responses occur in reaction to a pathogen, usually simultaneously. I will revisit several of these leukocytes when discussing specific immune responses.
Non-cellular Components
Platelets
Platelets are cellular fragments that are found floating in the blood. They have two main roles: they aid in blood clotting at the site of a wound and they release histamine. When exposed to air at the site of a wound, platelets break apart releasing a series of chemicals. One of these helps form thread-like fibrin, which in turn forms a net-like mesh that inhibits blood flow. This hardens into a scab, preventing blood loss and creating a barrier.
Other chemicals released by the breakdown of platelets include histamine which I shall discuss now.
Histamine
Histamine is a protein based chemical that is involved in the inflammation response by causing vasodilation, increasing blood flow to the site of infection and causing the blood vessels to become permeable. This allows more leukocytes to reach the invading pathogens. Leukocytes are also attracted to the site because of the chemotaxic effect this chemical has. Histamine is released by many of the cells already mentioned and some you are yet to meet. Sometimes these cells attack things like pollen causing a massive release of histamine leading to hayfever. It is histamine that makes a mosquito bite red , swollen and itchy. We then take antihistamines. I’ll talk about this later.
Putting it all together
- · There are many types of leukocytes, some eat pathogens some, attack them with chemicals, some release chemicals such as histamine that increase blood flow and attract more leukocytes to the site of the infection.
- · Many leukocytes have a role in the non-specific and specific immune responses
- · There are chemical responses, such as histamine release, which also help fight infection but can itself become a problem
Hope this helps!
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