Question

1. Which cells "costimulate" cytotoxic T cells and B cells by secreting cytokines?

2. What are the functions of lymph nodes? What are germinal centers?

3. What is meant by the terms "memory" and "specificity" as they pertain to immunity?

Answer 1

1) T- helper cells costimualte the cytotoxic T cells and B cells by secreting cytokines.

Explanation -

Cytokines are chemical substances which serve as messengers, mediating interaction and communication between various cells of the immune system.

Interleukins, a class of cytokines produced by white blood cells.

In humoral immunity ( antibody mediated), when T-cells recognise the antigenic fragments, it releases interleukins which stimulate the B-cells to proliferate and differentiate into memory cells and plasma cells.

In cell mediated immunity, T - helper cells secretes various types of interleukin which transform activated T-cytotoxic cells into effector cytotoxic T - cells.

2) Functions of lymph node -

a) Lymph nodes are centers of lymphocyte production. Both B- lymphocytes and T- lymphocytes are produced here by multiplication of preexisting lymphocytes. These lymphocytes pass into lymph and thus reach the blood stream.

b) Bacteria and other particulate matter are removed from lymph through phagocytosis by macrophages. Antigens thus carried into these cells are presented to lymphocytes stimulating their proliferation. In this way lymph nodes play an important role in the immune response to antigens.

c) Plasma cells (representing fully mature B-lymphocytes) produce antibodies against invading antigens, while T-lymphocytes attach cells that are foreign to the host body.

Germinal center - It is present in the cortex of lymph node and spleen.

It contains dividing B-cells of various stages.

It is the site where activation of B- cells takes place.

3) Memory - It is due to memory cells, which are present in the humoral (antibody mediated) immunity.

Acquired immunity have a memory component. A proportion of B-cells and T-cells become memory cells following primary contact of antigen.

They have longer life span (months to years).

They are in resting stage, but following subsequent antigenic stimulus, they get activated and plays an important role in secondary immune response.

Specificity - Acquired immunity is highly specific, directed against specific antigen that are unique to the microbes.

It is specific interaction of epitope of antigen with the corresponding paratope of homologous antibody.