Types of Immune Cells | Their Role in Body Defense
There are different types of immune cells in the body like
- Monocytes and macrophages
Some of the cells work by neutralization of foreign material. While others eat and destroy the disease-causing microbes.
But few of them help to keep a memory of the previous attack. This is done so as to launch a quick attack in-case of subsequent infection. This principle is exploited in preparation and use of vaccines.
Details on different types of immune cells
All the cells of the immune system are white blood cells. These white blood cells are of five types, and all of them have a role in the immune system.
- Monocytes and macrophages
The lymphocytes are the master cells of the immune system. They are differentiated into three sub-cells as
a) T lymphocytes
b) B lymphocytes and
c) Natural killer cells.
These cells are processed in the thymus gland. The hormone thymosin released from the thymus promotes the processing. This leads to the formation of well differentiated, mature and functional T-lymphocytes.
The T-lymphocytes are programmed to recognize only one specific antigen. Hence, they do not attack antigens, other foreign agents. For example, T-lymphocytes directed against cancer cells only recognize cancer cell and attack them. But they ignore other bacteria and virus particles. So, for every pathogen or foreign agent, a specific T-lymphocyte is generated.
There are four different types of T-lymphocytes produced like
1. Memory T-cells
2. Cytotoxic T-cells
3. Helper T-cells
4. Suppressor T-cells
Memory T-cells: These are immune cells which live a long life even after the infection or threat is neutralized. This is necessary because they are required to produce a rapid immune response to any subsequent attack by the same pathogen or the antigen.
Cytotoxic T-Cells: These cells attack and inactivate body cells containing foreign antigens on their surface. Cytotoxic T-cells attach themselves to the target cells and release toxins like perforin. Since the cells are attached, the action is quite effective. The infected body cells and cancer cells are the main targets of these cytotoxic cells.
Helper T-cells: These are the most common T-lymphocytes. They are involved in both cell-mediated and humoral immunity. HIV attacks these cells and when their numbers fall, the body’s immune system is compromised. They are involved in immune defense by two methods.
a) They produce cytokines like interleukins and interferons which stimulate cytotoxic T-cells and macrophages.
b) They stimulate B-lymphocytes to produce antibodies against the specific antigen.
These cells act as inhibitors or brakes so as to turn off the activated T and B cells. This way, they limit the extent of damage only onto foreign bodies. Thus, these inhibit any chances of potential damage to the body by the immune system.
These cells play an essential role in the humoral immunity of the body. They produce immune defenses by releasing the antibodies i.e. immunoglobulins. These antibodies bind and destroy the antigen. These B cells comprise about 10-15% of the total lymphocytes count. They are less mobile than T-cells and just release their antigens into the bloodstream. Hence, they stay in lymph tissue for a quite long time.
Similar to T-lymphocytes, B-lymphocytes produce antibodies which are specific against one antigen. On getting in contact with the antigen, the B-cells are activated, proliferate and transform into from into plasma cells.
There are two types of B-cells
1. Memory B-cells
2. Plasma cells.
Memory-B cells: These are similar to memory-T cells in function. They remain in the body for a long time after initial attack is neutralized. When there is a subsequent attack, they respond immediately in stimulating plasma cell. When stimulated, the plasma cells release antibodies against the antigen.
Plasma cells: These cells produce large amounts of antibodies into the blood. These are carried to almost all the tissues. They live for only one day at most and secrete millions of antibodies. These antibodies are directed against a specific antigen.
Natural killer cells
These cells comprise about 10-15% of circulating lymphocytes. The cells do not have B or T cells markers. The natural killer cells are part of the natural or innate immunity. These cells recognize antibody-coated target cells and bring about the killing of the target directly. This mechanism is mainly operative involved in viruses and tumor cells.
Monocytes and macrophages:
The role of macrophages in inflammation consisting of circulating monocytes and organ-specific macrophages. They remain in circulation for about three days before they enter tissues to become macrophages. The macrophages sub-population like dendritic cells found in the lymphoid tissue and Langerhans cells. The important immune function macrophages are as follows.
- Antigen recognition
- Secretory function
- Antigen presentation.
Neutrophils: These are granulocytic white blood cells. They are very high in some all the W.B.C combined. They are an important component of innate immunity. They are highly phagocytic and they attack and engulf any microorganism present in the bloodstream. Once the microbe is engulfed, it destroyed by the release of lysosomal enzymes. During an infection, they are the first to react and move towards the point of inflammation.
Eosinophils: These cells are also called as acidophils. They combat parasitic infection predominantly than the microbial infections.
Basophils: These cells are also involved in immune reactions. They can perform phagocytosis, release histamine and serotonin. They are found in large number where there is an ectoparasitic infection due to ticks. They are very low in concentration in comparison to other white blood cells.