What are the body’s external barriers?
The immune system stands guard at all times and protects us from harm due to dangerous organisms. There are not only cells within our body that perform this valuable duty, but there are other parts of the body that provide a defensive function:
- The skin itself is a defensive barrier: not only in its structure, but because of the bacteria
that reside there locally: foreign bacteria would have to compete. This only works though if the skin is intact: it is far easier for foreign bacteria to get into open wounds.
- Your stomach acid is so acidic that it kills most bacteria: only those that can survive very low pH levels such as Helicobacter pylori can survive into the intestines.
- Your saliva has anti-bacterial properties.
- The top layers of your respiratory, gastrointestinal and genitourinary tracts are lined with mucous membranes, which are layers of cells that are constantly being shed as phlegm.
- The top sections of your trachea also have cilia, as shown in the picture above, which are tiny hair-like structures that push up unwanted material towards the throat and create the coughing reflex.
All these elements together make up the innate (born-with) immune system.
What are the cells within the immune system?
The cells of the immune system follow a hierarchy. They are derived from stem cells, which are the building blocks for the rest of the cells. There are two groups of immune cells: lymphocytes and myeloid cells. The hierarchy can be seen in the Smart Art Graphic below:
In regards to the functions, I’ll only be referring to the cells listed above in purple.
What do these cells look like and what functions do they serve in the immune system?
Firstly I’ll tackle myelocytes and then I’ll tackle lymphocytes. For myelocytes, I’ll focus on the granulocytes and then the monocytes.
Functions: Neutrophils are the primary line of defense in the acute inflammatory response as well as bacterial invasion (as they don’t require ‘priming’) and they are also eating machines.
Appearance: Under the microscope, they have multiple nuclei (from two to five) and have a granulated, pale purple cytoplasm. If the levels of neutrophils are increased within a blood sample, it can indicate either an inflammatory process or infection.
Functions: Eosinophils are the specialists when it comes to fighting off parasitic infections, as discussed in Intro to Parasites, Life Cycle of Parasites and Parasite and Prion Infections. They do this by attaching themselves and secreting killing agents at the parasite rather than eating it whole. They also play a role in the allergy response. When the levels of eosinophils are raised, it can indicate the presence of a parasite in the system.
Appearance: Eosinophils are easy to spot under the microscope as they are bigger in size than the surrounding erythrocytes (RBCs or red blood cells) and they are both red and intensely granulated. They also have two purple nuclei.
Basophils and Mast Cells
Function: Both basophils and mast cells create and release heparin and histamine. Heparin is famous for its blood thinning qualities, but it can also help the cleanup process after eating a meal high in fat. Histamine is the stuff which is responsible for you getting the runny nose and watery eyes associated with hayfever.
Appearance: Basophils are a rare find within the blood: if you do a manual count of blood cells under the microscope, you’d be lucky to be find even one of these when screening several samples. Mast cells are never seen within the blood as they tend to be located within the tissues. They are intensely blue-purple in colour due to the high amount of granules and they have two nuclei.
Monocytes and Macrophages
Functions: Monocytes and their mature forms, macrophages, are professional eating machines. Monocytes are produced in the bone marrow, and then circulate within the blood for a couple of days before settling into tissues, becoming bigger and turning into macrophages. These macrophages are capable of living for years, but can be killed off sooner by either eating too much foreign material (i.e. succumbing to gluttony) or by an infectious agent destroying them whilst the macrophage is trying to eat it.
Appearance: Monocytes have a bluish-grey cytoplasm and often has a large sausage-like nucleus. Other defining features of monocytes include: vacuoles or white circles within the cell, and hair-like extensions on the edge of the cell.
Introduction to the Acquired Immune System
Both B lymphocytes and T lymphocytes look similar and can only be separated by other tests such as immune assays. They both play a part of the acquired immune system. Unlike the innate immune system which I described above, the acquired immune system requires ‘priming’ which can take up to a week to do. Once this is done, however, the lymphocytes produce ‘memory cells’ which survey the area and remain within the circulation once the infectious agent has left: the innate immune system does not do this.
Appearance of B and T cells: With both B and T cells, they have large singular nuclei and a small rim of cytoplasm.
Functions of B lymphocytes and Plasma Cells: B lymphocytes are part of the humoral immune system. Once an infectious agent comes into the area, it transforms into a plasma cell, and these release antibodies to fight the infection. These antibodies tag the foreign material to be destroyed.
Appearance of Plasma Cells: Plasma cells, as the picture alongside shows, appear different to lymphocytes: the cytoplasm becomes more egg-shaped and is stained deep purple. The nucleus become more circular and looks mottled in appearance. These are not often seen within a blood smear.
Function of T lymphocytes:
Unlike the humoral immune system which involves antibodies, T lymphocytes are directly involved in the destruction of foreign cells. Such foreign cells include cells of the body which have become invaded by viruses (as discussed in Intro to Viruses and Viral Infections) and cancer cells. There are two separate types of T lymphocytes: T helper cells and cytotoxic T cells. T helper cells are like the managers of a business: they coordinate everything and if this line of cells is compromised, it can spell massive trouble (as HIV/AIDS shows: this is the cell that that particular virus attacks). Cytotoxic T cells are the assassins of the immune system. They release chemicals which punch holes within the target cell, causing them to die.