Topic 5: Immunity

Immune system

There are two types of WBCs: phagocytes and lymphocytes.

Lymphocytes that fit exactly those of the body’s own cells are either suppressed or die- cell death(= apoptosis). Only lymphocytes that respond to foreign material will be left.

Phagocytosis

Phagocytes ingest and destroy pathogens before they cause harm in a process of phagocytosis. This is a non-specific immune response.

1. chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen.

2. Receptors on phagocytes’ cell-surface membrane recognise and attach to chemicals of the pathogen.

3. Lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the bacterium through invagination of the cell membrane.

4. Lysosomes release their contents into the phagosome. The lysozymes hydrolyze bacterial cell walls therefore destroying the bacteria as it becomes permeable to all substances.

5. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

T lymphocytes in cell mediated immunity/ the cellular response

This is a specific immune response to antigens that are presented on a body cell (rather than those found within the body fluids).

1. After phagocytosis, the cells become antigen-presenting cells. Receptor sites on the helper T cell (TH cell) fit exactly to the antigens.

2. The attachment of TH cells to the antigen stimulate Tcells to divide rapidly by mitosis and form a clone of genetically identical cells.

3. The cloned T cells:

   1. Develop into memory cells that enable a rapid response to future infections by the same pathogen

   2. Stimulate phagocytes to engulf pathogens by phagocytosis

   3. Stimulate B cells to divide and secrete their antibody

   4. Activate cytotoxic T cells.

Cytotoxic T cells kill infected cells by producing a protein called perforin. This makes holes in the cell-surface membrane so it becomes freely permeable to all substances. The cell dies as a result. This action is most effective against viruses because they rely on other living cells for their replication so will have no mechanism by which new particles are made.

B lymphocytes in humoral immunity

This is another specific immune response to antigens in the blood plasma.

1. When an antigen enters the blood, antibodies on the surface of B cells which are complementary to the antigen will attach.

2. The antigen enters the B cell by endocytosis and gets presented on its surface.

3. TH cells (activated in the cell mediated response) attach to the processed antigens on the B cell thereby activating the B cell to divide by mitosis to give a clone of plasma cells.

4. Cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen  on the pathogen’s surface.

5. The antibody attached to the antigens on the pathogen and destroys them. (The primary immune response.) This is usually short-lived- a few days.

6. Some B cells develop into memory cells which can respond to future infections by the same pathogen by mitotically dividing and developing into plasma cells that produce antibodies. This is the secondary immune response. Memory…