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Acquired Immunity: Cell-Mediated Immunity And Its Types

Acquired Immunity: Cell-Mediated Immunity And Its Types

What happens when an unknown person comes to our house? We will ask them, few questions.

Who are you? Where are you coming from? What is the purpose of your visit? etc. 

Right….. 

Just like that, whenever a foreign particle enters our body, our body will be asking similar types of questions!! Isn’t it interesting?

But our body might not be asking these questions directly on face. Instead it will be assigning some special cells like an army for this purpose. 

immunity

We have an army in our country for protection. Likewise some of the cells in our body are acting like an army and fight against the pathogens in our body. They also memorise them to protect the body from further attack. 

This ability of our body to fight against the disease causing microbes entering our body is called Immunity. Some of the immune responses will be there in our body by birth and it is called Innate immunity. But some are acquired by due course of time with the help of memory cells in the body and it is called Acquired immunity

Table of Contents

Types of Acquired Immunity

The third line of defence is the adaptive or acquired immunity. It generates this targeted response against the invader using its two main components. The first being cell-mediated immune response or cell-mediated immunity (CMI). The second type is called humoral immunity or antibody-mediated immune response (AMI). Humoral immunity is mainly taken care of by B lymphocytes and the CMI is mediated by T lymphocytes. Here we are going to discuss more about the CMI.

compoonents of adaptive immunity

Cell-mediated Immunity (CMI)

In cell-mediated immunity the immune responses are not done by the release of antibodies. It will be done by the activity of specific types of antigen specific cells and lymphocytes by releasing the cytokines in response to the antigens. So we can say that mediators of cell-mediated immunity are Dendritic cells, Lymphocytes and Macrophages.

Cells of Cell-mediated Immunity

Before discussing a movie we will be familiar with all the characters in that movie. Likewise, first we have to meet all the heroes of cell-mediated immunity. So here we are going to discuss more about the following cells of cell mediated immunity:

  • Lymphocytes
  • Antigen presenting cells

Lymphocytes 

Lymphocytes are the white blood cells with a non-granular cytoplasm and can provide immunity against specific foreign substances in the body. They are the major mediators of antigen specific acquired immunity that is developed in an individual when encounters various antigens (foreign bodies that are capable of triggering an immunological response) in his or her lifetime.

All lymphocytes are produced in the bone marrow (soft spongy tissue present in the cavity of long bones) from pluripotent stem cells which have the ability to divide and differentiate into any blood cell. 

After production, the lymphocytes migrate to primary lymphoid organs for maturation (able to differentiate between self and non-self antigens). Based on their site of maturation, lymphocytes have been categorised as follows: 

  • B lymphocytes that mature in the bone marrow
  • T lymphocytes that mature in the thymus (It is a lobed organ located near the heart and beneath the breastbone or sternum)

types of hymphocytes

T - Lymphocytes

T- lymphocytes or T cells are matured in the thymus. That is why it is called T - lymphocytes. If T- lymphocytes come in contact with an antigen, it divides rapidly to form a clone of T - cells. This clone comprises a group of cells which are similar in structure but perform different functions. There are several types of T lymphocytes as follows:

  • Helper T cells or CD4 or T4 cells
  • Killer or cytotoxic T cells or CD8 or T8 cells
  • Suppressor T Cells
  • T delayed hypersensitivity cells 
  • Memory T Cells

Let’s discuss more about them.

Helper T cells (CD4 cells or T4 Cells)

These are the most abundant T cells. These identify foreign bodies and get sensitised. Sensitised helper T - cells produce lymphokines to stimulate killer T cells and proliferate other T cells. Helper T cells can stimulate B lymphocytes to produce antibodies. They can also attract macrophages to the site of infection. 

helper t cell

Killer T cells (Cytotoxic T cells or CD8 Cells or T8 Cells)

These cells directly attack and destroy invading microbes, infected body cells and cancer cells. The cytotoxic T cells or Killer T cells are a group of lymphocytes. With the help of receptors on their body they bind to microorganisms. The immediate effect of binding is the swelling of the killer T cells and release of cytotoxic substances like lysosomal enzymes (perforins or cytolysins). These enzymes will create pores on the plasma membrane, through which water entering inside the target cells results in bursting of the cells. 

cytotoxic t cell

Suppressor T cells

These cells suppress the immune system and prevent it from attacking our own body cells. It prevents the over activity of the immune system.

T delayed hypersensitivity cells 

They help in delaying the hypersensitivity reactions (exaggerated immune response to a less potent antigen).

Memory T cells

These cells are previously sensitised and retain the sensitisation for the future. They store information about microbes. They mediate the faster and more potent response upon the subsequent encounter with the same antigen. These cells are long-lived.

memory t cell

B - Lymphocytes

B - lymphocyte’s name is derived from the bursa of Fabricius of birds. It is a primary lymphoid organ in birds. The mammalian equivalent of the bursa of Fabricius is considered to be bone marrow. So the site of development of B lymphocytes is bone marrow. There are several types of B - Lymphocytes. 

Here we are going to discuss two types of B - lymphocytes. 

  • Plasma or effector cells 
  • Memory cells

Plasma cells

They are short lived. They produce soluble protein molecules called antibodies against antigen. These soluble molecules identify and tag the antigen-infected cells and elicit an immune response against it. These antibodies have specific sites for binding to particular antigens. Once the antibodies bind to the antigens, they are eliminated from the body by masking their toxic sites, clumping them together, engulfing and digesting them by phagocytic cells or lysing the cells.

Memory cells 

They are long lived cells. They form memories which elicit faster immune responses for similar infections in future. They remain in the body for a long time. In the subsequent encounters with the same antigens, these cells start dividing and differentiating into plasma cells. They then release antibodies against the antigen. 

Difference between T lymphocytes and B lymphocytes

T lymphocytes

B lymphocytes

Matures at thymus

Origin and matures at bone marrow

Seen inside lymph nodes

Seen outside lymph nodes

Surface antibodies are absent

Surface antibodies are present 

They could connect only to the virus antigen on the outside

They could connect to the surface of invading bacteria and virus

Membrane receptor for antigen is TCR

Membrane receptor for antigen is BCR

Long life span

Short life span

They secrete lymphokines

They secrete antibodies

APCs or Antigen presenting cells

Out of all the cells of the immune response, macrophages, dendritic cells and B cells are together called the Antigen presenting cells (APCs). 

antigem presenting cells

Dendritic cells 

They are those cells which are found in tissues like skin. They have the ability to engulf the antigen and to show them on their surface.

Macrophages 

They are also similar to dendritic cells. They are white blood cells which have the ability to engulf and kill the antigens. They also do the antigen presentation.

Major Histocompatibility Complex or MHC

All the antigen presenting cells do the antigen presentation with the help of a protein complex called Major Histocompatibility Complex or MHC. MHC is a protein complex present on all cells of our body. 

cell with mhc

When the T cells come close to other cells they check whether the peptide is self or non-self and based on this identification method they decide whether it is a self or nonself cell. When an antigen enters the body, T cells use MHC to differentiate between self and non-self cells. 
Now we know that various cells are the real heroes of cell-mediated immunity. So let’s meet them in depth.

Action of cell-mediated immunity

APCs like macrophages engulf pathogens and present a part of the pathogen or antigen on its MHC.

macrophage engulfing the pathogen

The dendritic cells also engulf pathogens and present the antigens on their MHCs.

dendritic ceil engulfing the pathogen

These APCs move to the closest lymph node. 

lymph nodes

Once reached inside the lymph node, the naive T cells recognise the antigen as non self being presented by the APCs. 

naive t cells recogmizes apc

The naive T cells get activated and the activated naive cells then duplicate and proliferate into cytotoxic T cells, memory T cells and helper T cells. They are capable of killing both pathogens and the cells that are infected with pathogens using different mechanisms. 

duplication of nauve t cells

They do so by releasing pore or hole forming proteins called the Perforins that punch holes in the cells and kill them. Helper T cells cannot kill the infected cells or pathogens. When helper T cells get activated and secrete cytokines, that can in turn activate cytotoxic T cells and B cells. When helper cells activate the B cell, the second component of acquired immunity takes place.

Practice Problems of Acquired Immunity

Question 1. Antigen presenting cells _____________________.

a. activates T cells
b. fights and kills the pathogen
c. become reservoirs of pathogen
d. carry pathogens to blood

Solution: Macrophages, dendritic cells and lymphocytes are together called antigen-presenting cells (APCs). APCs activate T cells to evoke an immune response, but are not involved in directly fighting and killing the pathogen. Hence the correct option is a.

Question 2. Read the following statements (I - III) and mark them as true or false.

1. In acquired immunity, pathogen-specific receptors are ‘acquired’ during the lifetime of the organism.
2. Antigen is any portion of a pathogen that the body recognizes as foreign or harmful to the body resulting in an immune response.
3. Acquired immunity is the first line of defence.

a. I - T, II - T, III - T
b. I - T, II - F, III - T
c. I - T, II - T, III - F
d. I - F, II - F, III - T

Solution: The immunity obtained during a lifetime of an individual is called acquired immunity. It is also called adaptive immunity. This type of immunity is pathogen-specific. Antigen is any portion of a pathogen that the body recognizes as foreign or harmful to the body, which can result in an immune response. When innate immunity barriers are not enough and pathogens are able to breach innate defences, a more targeted response is needed i.e., adaptive immunity. Acquired or adaptive immunity includes a third line of defence. Hence the correct option is c.

Question 3. Neenu and her friends were discussing their kid’s health conditions. They found that the kids who already had chicken pox once in their lifetime will not have the disease again. It is considered a magical ability of our immune system. Which of the following abilities of the immune system prevents such diseases after first exposure?

a. Differentiate between self and non self
b. Memory cells generated from the first exposure
c. Recognising MHC or receptor peptide on cell surface
d. Generating plasma B cells to fight against the pathogen

Solution: Memory lymphocytes (B and T cells) are specialised cells of the immune system that are formed during the first attack or primary infection. They are considered important in generating an accelerated and more accurate antibody-mediated immune response in the case of re-infections. It is also known as a secondary immune response. Therefore, if a kid has chicken pox once during their lifetime, the ability of the immune system to keep a memory of exposure prevents the infection for the next time. Hence the correct option is b.

Question 4. ______i______ : Non - Specific type of defence : ______ii_________.
______iii_____ : Pathogen specific defence mechanism : _____iv_______.

a. i - Innate immunity, ii - Obtained once encounters the pathogen, iii - Acquired immunity, iv - Present from birth
b. i - Acquired immunity, ii - Present from birth, iii - Innate immunity, iv - Obtained once encounters the pathogen
c. i - Innate immunity , ii - Present from birth, iii - Acquired immunity, iv - Obtained once encounters the pathogen
d. i - Acquired immunity, ii - Obtained once encounters the pathogen, iii - Innate immunity, iv - Present from birth

Solution: Innate immunity is a kind of non-specific immunity present at the time of birth and is activated immediately after the pathogen attack. It includes defence mechanisms and barriers to keep the body away from foreign particles or antigens. The acquired immunity or adaptive immunity, on the other hand, is the one that the body gains over time. It is not present at birth. When the immune system encounters a pathogen it starts to adapt itself and generate pathogen specific immunity. Hence the correct option is c.

FAQs of Acquired Immunity

Question 1. Explain the purpose of cell-mediated immunity?

Answer: Cell-mediated immunity is mainly directed for the microbes that survive in the phagocytes and the microbes that infect nonphagocytic cells. It can be most effective in destroying cells that are infected by viruses, intracellular bacteria and cancers. Cell-mediated immunity is also playing a major role in the rejection of organ transplants.

Question 2. What do you mean by APCs?

Answer: APCs are the antigen presenting cells. Mediators of cell mediated immunity are dendritic cells, lymphocytes and macrophages. Out of the cells of the immune response, macrophages, dendritic cells and B cells are together called the antigen presenting cells (APCs). 

They have the ability to present antigen on their surface. 

Question 3. What is MHC?

Answer: Macrophages, dendritic cells and B cells are together called the antigen presenting cells (APCs). They have the ability to present antigen on their surface with the help of a protein complex called major histocompatibility complex or MHC. MHC is a protein complex present on all cells of our body. When an antigen enters, T cells use MHC to differentiate self and non-self.

Question 4. How is cell-mediated immunity gained by one?

Answer: Cell mediated immunity is mediated by T lymphocytes. The antigens of pathogens are expressed on the cell surface or on an antigen-presenting cell. Thus one person attains cell-mediated immunity by retaining the memory.

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