Human Immunity and Blood

A- Allograft or allotransplant is the transplant of cells, tissues, or organs to a recipient from a genetically non-identical donor of same species. Most of the organ or tissue transplants in human are allografts.  

Graft rejection is primarily a cell-mediated immune response where T cells recognize foreign MHC molecules on the transplanted tissue. While other cells participate, T cells are the primary effectors of the rejection process.

• The drugs that lower the immune system' are called immunosuppressants. 
• These drugs are given to prevent graft rejection by lowering the immune system's ability to reject the transplanted organ. 
• If immunity is not lowered the T lymphocytes which are the components of cell-mediated immunity destroy the transplanted tissue causing graft rejection. Thus the correct answer is option C.

Organ transplantation is a medical procedure in which an organ is removed from one body and placed in the body of a recipient, to replace a damaged or missing organ. One of the major problem associated with organ transplantation is the transplantation rejection, during which the body has an immune response to the transplanted organ leading to transplant failure and the need to immediately remove the organ from the recipient. In order to prevent the transplant rejection, the immunosuppressant drugs are used which suppresses the immune system responses.

The human leukocyte antigen (HLA) system or complex is a gene complex encoding the major histocompatibility complex (MHC) proteins in humans. These cell-surface proteins are responsible for the regulation of the immune system in humans.HLA is the major cause of organ transplant rejections.  Mutations in HLA may be linked to autoimmune disease. Hence, HLA is a group of genes.

Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Killer T cells, also called cytotoxic T lymphocytes (CTLs), have CD8 receptors that recognize the transplanted tissue's MHC (major histocompatibility complex) class I molecules, which display the donor's self-antigens. Thereupon, the T cell receptors (TCRs) of the killer T cells recognize their matching epitope and trigger the target cell's programmed cell death by apoptosis.

• The grafted kidney may be rejected in a patient due to the cell-mediated immune response that is mediated by T-lymphocytes.
•  The body is able to differentiate 'self' and 'non-self'. 
• Therefore, tissue matching, blood group matching are essential before undertaking any graft/transplant and even after this the patient has to take immunosuppressants all his/her life.
•  Hence, the Grafted kidney may be rejected in a patient due to Cell-mediated immune response.
• So, the correct answer is 'Cell-mediated immune response'.

Azathioprine is a purine analogue that interferes with DNA synthesis, thereby inhibiting mitosis and the proliferation of T and B cells. Since both A and B describe the mechanism of action, D is the correct choice.

• In humans, four parathyroid glands are present on the back side of the thyroid gland, one pair each in the two lobes of the thyroid gland.
• The parathyroid glands secrete a peptide hormone called parathyroid hormone (PTH) or parathormone.
• Parathormone increases the $Ca^{2+}$ levels in the blood.
  
• Increase in bleeding time and delay in blood coagulation is due to the deficiency of parathormone because its deficiency causes a decrease in blood calcium levels and calcium ion is essential for blood coagulation.
  
• So, the correct answer is 'Parathormone'.

Immunoglobulins are the proteinaceous substances produced by the immune cells (B cell) that function as antibodies. The terms antibody and immunoglobulin are often used interchangeably. Thus, the correct answer is option A.

• Antibodies are produced by specialized white blood cells called B lymphocytes (or B cells).
• B lymphocytes are activated on their encounter with foreign agents. These foreign particles act as foreign markers.
• The B-cells immediately differentiate into plasma cells and memory cells. The plasma cells produce antibodies specific to that foreign particle or so-called antigen.
  
• These antibodies attach to the surface of the antigen/foreign agent.
  
• These antibodies detect any antigen in the body and destroy it.
  
• Hence Antibodies are formed by Plasma cells.
• So, the correct answer is 'Plasma cells'.

Immunoglobulin E (IgE) is a type of antibody that has been found only in mammals. IgE is synthesised by plasma cells. IgE also has an essential role in type I hypersensitivity which includes allergic diseases, such as allergic asthma, sinusitis, allergic rhinitis, food allergies, and specific types of chronic urticaria and atopic dermatitis.

A lymphocyte is one of the subtypes of white blood cell in a vertebrate's immune system. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic adaptive immunity), and B cells (for humoral, antibody-driven adaptive immunity). They are the main type of cell found in lymph. Lymphocytes are involved in the activation of the immune system. Hence, B-lymphocytes are involved in the production of an antibody.

Platelets are parts of the blood that help the blood clot. They are smaller than red or white blood cells. Platelets are a natural source of growth factors. They circulate in the blood of mammals and are involved in hemostasis, leading to the formation of blood clots. Platelets release thread-like fibers to form these clots.