Question

Adaptive Immunity Worksheet in the body. a. What 1. Adaptive immune cells will interact mainly at the about this organ makes it a good place for the cells to interact? 2. The adaptive immune system is divided into two responses: The response, which uses B cells, and the response, which uses T cells. 3. What is one difference between antigen that B cells respond to vs. antigen that T cells respond to? 4. Fill in the following information about 3 different types of T cells. a. First type: i Abbreviated: ii. Surface marker: iii. Function: b. Second type: Abbreviated: ii. Surface marker: i. Function: c. Third type: Abbreviated: 1. Surface marker: Il Function: United States

Answer 1

Answered and justified as many questions as I could in the time alotted.

1. Adaptive immune cells interact mainly at the secondary lymphoid organs in the body.

Adaptive immune cells are basically B- and T-cells. These immune cells mature in the primary lymphoid organs (bone marrow or thymus) and migrate to the secondary lymphoid organs (spleen, lymph nodes and Mucosa Associated Lymphoid Tissue-MALT). This is the site where they encounter antigens and act on them.

1. What about this organ makes it a good place for cells to interact?

The secondary lymphoid organs are highly specialized in trapping and concentrating antigens and foreign organisms. Since, B-cells differentiate in secondary lymphoid organs on contact with antigens and start producing antibodies, these organs serve as a hub for antibody production. They serve as a one-point centre for all the immune cells to accumulate without disturbing the normal functioning of the body.

2. The adaptive immune system is divided into two responses: The humoral response, which uses B cells, and the cell mediated response, which uses T cells.

Humoral response is mediated via antibodies produced exclusively by B-cells. The B-cells come in contact with antigen whereby they are stimulated and they produce antigen-specific antibodies. Humoral means relating to body fluids and the response is called so because the B-cells produce soluble antibodies outside the cells which diffuse into the body fluids and act thereafter.

Cell-mediated response is carried by T-cells. They are stimulated by antigens in many ways and they in turn initiate a cascade of reactions which leads to stimulate many other immune cells. Conclusively, they too help eliminate the antigen or pathogen from the body by phagocytosis, cytotoxic t cells, generating memory cells etc.

3. What is one difference between antigen that B-cells respond to and antigen that T-cells respond to?

T-cells mainly require MHC-antigen complex to recognise the antigen presented by the antigen presenting cell. If it is a viral antigen, the virus will infect the cell, the cell will process the antigen and present it on its surface using an MHC molecule. This will then be recognised by the T-cell. In contrast, the B-cell can recognise antigens directly from the bacterial or viral cell surface due to membrane-bound antibodies on the B-cell surface. Thus, T-cells can only recognise processed antigens, whereas B-cells can recognise both processed and unprocessed antigens.

4. Fill the following about 3 different types of T cells:
   1. First type: Cytotoxic T cells
      1. Abbreviated: T_C Cells (C for cytotoxic and T for T-cell or thymus cell)
      2. Surface marker: CD8 (Cluster of differentiation 8 marker is a T-cell coreceptor found only on cytotoxic T-cells)
      3. Function: These cells bind exclusively to MHC class 1 molecules presented by all nucleated cells of the body. Thus, if a cell is infected by a virus, the cell will process its antigen and present it on its surface using a MHC-1 molecule. The cytotoxic cells will bind to this with their T-cell receptor and CD8 will help recognise MHC-1 molecule by acting as a coreceptor. CD8 has a long cytoplasmic tail which helps initiate a signal transduction pathway in the T cell which leads to various effector functions. It binds to alpha 3 subunit of MHC-1 molecule.
   2. Second type: Helper T cells
      1. Abbreviated: T_H Cells (H for Helper and T for T-cell)
      2. Surface marker: CD4 (Cluster of differentiation 4 marker is a T-cell coreceptor found on helper T-cells)
      3. Function: These cells bind exclusively to MHC class 2 molecules presented by antigen presenting cells (APCs) of the body. Thus, if an APC detects a pathogen, the cell will process its antigen and present it on its surface using an MHC-2 molecule. The helper cells will bind to this with their T-cell receptor and CD4 will help recognise MHC-2 molecule by acting as a coreceptor. CD4 has a long cytoplasmic tail which helps initiate a signal transduction pathway in the T cell which leads to various effector functions. It binds to beta 2 subunit of MHC-2 molecule.
   3. Third type: Regulatory T cells
      1. Abbreviated: T_reg cells (reg for regulatory and T for T-cells)
      2. Surface marker: CD4
      3. Function: Although they also possess the CD4 marker, they do not act like helper T-cells. Instead, the shut down the immune response when not require to prevent hypersensitivity. The pathways that get activated in these cells are different than those activated in helper T-cells. Thus, they transcribe different genes in response to CD4 signal transduction and release different cytokines which are anti-inflammatory thus suppressing the immune response. They also release substances that kill the activated immune cells. They modify professional APCs like dendritic cells in such a way that they can no longer activate T-cells.
5. Purpose of MHC.

The majorhistocompatibility complex (MHC) has two major functions. First being the presentation of processed antigens on their surface in order to initiate an immune response. Which MHC molecule activates which cell has been elaborated in the previous answer. The second function of the MHC molecules is to help the immune cells recognise self and non-self cells. The T-cells, during their maturation, are trained and conditioned so that they know which cells are self and which are foreign. This differentiation is majorly done based on the MHC molecules presented by the body cells of the individual. MHC genes are polymorphic, thus no two individuals except twins will possess the same set of MHC markers on their cells. This makes sure that the immune system is activated only when a foreign antigen is present and not otherwise.

6. Fill in information about MHC classes below:
   1. Type of MHC: Class 1
      1. Cell type: all nucleated cells
      2. It shows its antigen to CD8⁺ T_CCells. Because they can attach to the alpha 3 domain of the MHC 1 molecule.
      3. It basically helps the immune system raise a flag for invasive infections i.e. those infections involving pathogens that enter the cells. The nucleated cells process the antigen and present it on MHC 1 molecules on their surface. This activates the Tc cells and leads to the death of the infected cell via perforin-granzyme pathway.
   2. Type of MHC: class 2
      1. Cell type: antigen presenting cells (B-cells, dendritic cells)
      2. It shows antigen to CD4⁺ TH cells. Because they can attach to beta 2 subunit of MHC2 molecule.
      3. It bridges the gap between cell mediated and humoral immunity. B-cells are part of humoral immunity as well as APCs. Thus, they bind to a surafec bacterial or viral antigen and secrete antibodies. They also process some of the antigen and present it to helper t cells to initiate cell mediated immunity. Dendritic cells are professional APCs which constitutively express MHC 2 molecules and perform functions of activating CMI and licensing of other APCs.