Adaptive Immune System

Describe the process to activate the adaptive immune system.

When there are pathogens, its antigens pass through the first line of defence (barriers) or during inflammation, dendritic cells at the site of injury phagocytose and presents non-self antigens on MHC-Class II.
The dendritic cells migrate to the lymph nodes through the lymphatic system to activate the adaptive immune response (third line of defence).

State which cells MHC Class 1 is found on.

MHC class I markers are found on all body cells that have a nucleus.
Not found in red blood cells because they don’t have a nucleus.
MH1 molecules present antigens that are found in the cytoplasm.

State which cells MHC Class 2 is found.

MHC class II markers are found only on antigen-presenting cells (APCs): macrophages dendritic cells and B lymphocytes.
MHC class II on APCs presents antigens to the adaptive immune cells such as T cells.

How does antigen-presenting cells present antigens on their MHC - Class 2?

MHC 2 molecules present antigens from pathogens that have been phagocytosed.

Describe the nature/characteristic of the adaptive immunity.

Specific -
- The immune system is able to recognise specific antigens to trigger multiple defense mechanisms to destroy the pathogen.
- Adaptive immune system is capable of distinguishing millions of different antigens.
-This means that each lymphocyte (naïve or mature) is specific to one antigen's shape because it has different receptors.

Immunological memory
-Cells of the adaptive immune system can 'remember' antigens after initial exposure and respond more efficiently if exposed to same antigen again.

Describe how an adaptive immune response is initiated during a bacterial infection.

DC at the first line of defence phagocytose any antigens with pathogens,

DC presents that antigen from what it phagocytosed on its MHC -2.

DC migrate to the lymph node through the lympatic system.

DC presents antigens to to the helper T cells that have complementary receptors to the antigen.

Define Extracellular pathogen.

Describe the immune response when an extracellular pathogen is detected.

Extracellular (EC)
Pathogens can exist and reproduce outside of the cells.
For example; Vibrio cholera adhere to epithelial surfaces and cause disease by secreting potent toxins.

The immune response must:
Target and destroy the extracellular pathogen
Prevent the effect of the toxin
Prevent the spread of the pathogen

Define Intracellular pathogen.
Outline the body's response to an intacellular pathogen.

Pathogens which exist and reproduce inside the cell.
For example; Influenza virus infects epithelial cells in the lungs by inserting itself to the host cells DNA and hijacking transcription and translation to make more viral progenies.
The immune response must:
Target and destroy the infected cells
Prevent the spread of the pathogen

Lymphocytes

Leukocytes that are specialised for adaptive immune responses.
Each lymphocyte has a different receptor for a particular antigen

Describe the function of lymphocytes when it is activated.

Once activated, the lymphocyte is able to create clones of itself (proliferation) that are dedicated to eliminating the antigen that triggered it.

Describe the two mechanisms of the adaptive immunity

Humoral immunity
- B lymphocytes have their clones, plasma cell produce specific antibodies and release them into the blood and lymph.
Cell-mediated immunity
- Involves the action of T lymphocytes and involves destroying or eliminating virally infected cells.

Describe the role of Helper T lymphocytes (TH)

•Secrete cytokines that activates B cells and cytotoxic T cells (promote proliferation)
•Activate macrophages and B lymphocytes

Describe the role of Cytotoxic T lymphocytes (Tc)

Release toxic compounds that kill foreign, infected or abnormal host cells

Describe the role of Memory T lymphocytes.

•Antigen-specific
•Remembers the antigen that activated it and allows for faster response if reencountered.

What happens when an Antigen-presenting cells encounter a pathogen?

Antigen-presenting cells (APC) break down the antigens in the pathogens they engulf into small peptides and present them as antigens on their MHC-II , they then present this to helper T lymphocytes

1.APC (antigen presenting cell) presents foreign antigen on
MHCII to Th (helper T cell.)
1.Th cell that recognises the antigen presented by the APC, undergo clonal selection and proliferation.
2.Th secretes cytokines which activates Tc with receptors that is specific for the foreign antigen to undergo clonal selection.
3.Tc travel to infected site where they interact with the same antigen on MHCI molecule on an infected cell.
4.Tc release cytotoxic granules/perforins which cause lyses and death of cell.
5.Memory T cells remain for subsequent infections with the same antigen enabling a faster and more rigorous response.

Define, describe the role and features and state the location of B lymphocytes

-B lymphocytes are a type of white blood cell that circulate around the body.
-Their surfaces are covered in B cell receptors, also known as antibodies.
-They travel around the body in the bloodstream and reside in high numbers in lymph nodes.
-B lymphocytes are formed in bone marrow, hence they are known as 'B' cells!!!

What are naive B cells?

B lymphocytes that have not been activated by an antigen.

What happens when naive B cells bind to a specific antigen? jjjjjjj

They divide and further differentiate into plasma cells and memory B lymphocytes

Purpose of humoral immunity

Humoral immunity involves the neutralisation and/or destruction of a pathogen via the production and secretion of antibodies.
- The response is generated by interactions between T helper cells and B lymphocytes.

Activation of humoral response

1- Naive b cell with complementary receptor binds to antigen on pathogen. This B cell is 'selected.'
2-The helper T cell (Th) with complementary receptor binds to the same antigens and secrete cytokines.
3-The cytokines causes B cell to undergo proliferation/clonal expansion and differentiation into memory B cells or plasma cells.
4-Plasma cells secrete antibodies into the blood in order to defend against the selected pathogen.
Memory B cells provides immunological memory.

Cytokines released by the activation of Helper T cells causes clonal expansion (proliferation of the selection B cells) and differentiation (B cells becoming either plasma cells or B memory cells)

Describe the features of an antibody.

Antibodies are also known as immunoglobulins (Ig).

All antibodies have a constant region (most of the Y shape) and variable regions at the two tips of the Y, where there are two identical binding sites that are complementary to a specific antigen

Antibodies are proteins composed of a sequence of amino acid monomers, eventually forming a large complex structure. Explain what 'tertiary structure is' and what is responsible for its formation.

The antibodies consist of four polypeptide chains: two heavy chains and two light chains that are arranged in the shape of a Y.

Describe the four way in which antibodies can cause the destruction of pathogens and protection against pathogens.

1.agglutination - pathogens become trapped in a network of antibodies, making them susceptible to destruction through phagocytosis.
2.opsonisation - bound antibodies 'tag' pathogens for destruction, making it easier for phagocytes to locate them
3.neutralisation - bound antibodies block antigens from binding to other targets. In this case, the antibodies prevent toxins destroying a cell.
complement activation - bound antibodies activate a cascade of complement proteins.

One type of phagocytic cell is no involved in the activation of the adaptive immune response. Identify and justify why.

Explain how MHC Class 2 assists in performing the cells function.

Neutrophils - due to it not having MHC Class 2.

Two types of immunological memory

Primary immune response -
First time a T or B lymphocyte is exposed to a specific antigen.
Memory B and T lymphocytes are formed after initial exposure.
Secondary immune response
- Subsequent encounters with the same antigen.
Faster lymphocyte proliferation due to presence of memory lymphocytes. - understand immunological memory - measured by antibodies

Describe the trend observed in the graph below - 2 marks

1. First response - Primary response (first exposure to antigen) causes the production of antibodies.
   2.Secondary response (second exposure to the same antigen) results in a higher production of antibodies.

Describe the trend observed in the graph below - 3 marks

1) First exposure to antigen is called the primary immune response.
'Naïve Lymphocytes' are seeing the antigen for the first time and is 'immunologically inexperience'
2) Subsequent encounters with the same antigen usually result in a more rapid, larger response that is better to eliminate the antigen than the primary response.

Secondary response is the result of the activation of memory lymphocytes, which are long-lived cells that were in the primary immune response.

As memory lymphocytes have already experienced dealing with the antigen, it optimizes the ability of the immune system to combat infections.

'Long-lived plasma cells (memory cells) recognise the antigens faster which leads to a faster production of antibodies to combat the antigen.

The role of the lymphatic system in the adaptive immune response - 3 marks

•transportation of antigen-presenting cells to secondary lymphoid tissues for antigen
•recognition and initiation of the adaptive immune response
•production of leukocytes, including lymphocytes in primary lymphoid tissues

What occurs in the lymphatic system?

• storing memory cells or lymphocytes
• carrying antigens to the lymph nodes by the dendritic cells to present to Helper T cells.
• trapping antigens in the lymph nodes
• site of clonal selection and proliferation
• transporting fluid containing lymphocytes, antibodies or immune cells

Primary lymphoid organs
Bone marrow and the thymus - Both B and T lymphocytes originate in bone marrow but T lymphocytes mature in the thymus!
Secondary lymphoid organs and tissues
Lymph nodes, spleen, tonsils, adenoids, appendix and Peyer's patches.
Sites of the adaptive immune response.

Primary lymphoid organs
Bone marrow and the thymus - Both B and T lymphocytes originate in bone marrow but T lymphocytes mature in the thymus!


Secondary lymphoid organs and tissues
Lymph nodes, spleen, tonsils, adenoids, appendix and Peyer's patches.
Sites of the adaptive immune response.

Lymph nodes & Spleen.

In these tissues, mature lymphocytes are clustered together and 'scan' passing lymph for the presence of any pathogens or antigen-presenting cells.
If a foreign antigen matches the receptors of specific lymphocytes, these lymphocytes then undergo clonal selection and differentiation. This results in many B and T cells being created within these tissues, resulting in the characteristic swelling of lymph nodes when you're sick

Where does the cells go when there is inflammation?

Lymphatic capillaries are extremely small vessels that exist throughout the tissues of the body, collecting fluid in tissues as well as any pathogens that might be present. Once the fluid enters the lymphatic capillaries, it is known as lymph and is carried away into the lymphatic system, where it eventually arrives at a lymph node. The direction is only one way, towards the lymph node

The adaptive immune is composed of humoral immunity and cell-mediated immunity. Activation of either of these pathways begins with the presentation of a pathogenic antigen by an antigen-presenting cell to a complementary T help cell. After selection, a T helper cell secretes cytokine to activate the selected B or T cell.

Humoral immune response

- Extracellular pathogens are phagocytosed by antigen - presenting cells    I

- Antigen- presenting cells interact with T helper cell.  V

- T helper cells produce cytokines to stimulate selected B cells II

- B cells differentiate into plasma cells and B memory cells III

- Antibodies travel to the pathogen and bind to the antigen IV

APC presents foreign antigen on MHC class 2 to Th cell. The cell that recognises the antigen.