Antigens and Phagocytosis

What is a pathogen? Name examples.

Any disease causing microorganism e.g. parasites, virus, prion, fungi, protozoa and prokaryote.

• What are the specific and non-specific responses of leukocytes?

• Phagocytes- non-specific response if immediate and the same for all pathogens.

• Lymphocytes- specific response is slower and specific to each pathogen.

How is the immune system able to distinguish between 'self' cells and 'non-self' cells?

Each type of cell has specific molecules on its surface that identify it. These molecules are usually proteins.

What is an antigen?

A molecule (often a protein) that the body recognises as 'foreign' and so triggers an immune response.

What is antigen variability?

A DNA mutation, base sequence change, which causes a conformational change of an antigen meaning any previous immunity to this pathogen is no longer effective as all the memory cells have a memory of the old antigen shape.

How do lymphocytes recognise cells?

Lymphocytes are made when you are a foetus and are unlikely to be exposed to any other cells than self cells so lymphocytes complementary to the antigens on self cells will die or be suppressed in production. The remaining lymphocytes are complementary to pathogenic and non-self cells. The same process occurs after birth in the bone marrow.

What happens when someone has an autoimmune disease?

It means the lymphocytes which attack self cells are produced.

What are the steps of phagocytosis?

1) Chemicals are released by the pathogen.

2) The phagocytes are attracted to these chemicals and move towards the pathogen.

3) The phagocyte engulfs the pathogen into a vesicle called a phagosome (by endocytosis).

4) Lysosomes found in the phagocyte move towards the vesicle and fuse with the phagosome.

5) Lysozymes found in the lysosomes digests the pathogen.

6) The phagocyte displays the important antigens in its cell surface membrane in which it can now be called an antigen presenting cell.

What are the two specific immune system responses?

1. Cell mediated response- involves T lymphocytes

2. Humoral response- involves B lymphocytes

What are the stages of The Specific Immune System?

- T cells are produced in the bone marrow and mature in the thymus and of those, some are complementary to the antigens on the surface of a specific pathogen.

- That T cell (now helper T cell) finds the antigen, which has undergone phagocytosis and is presented on a phagocyte, and binds to it.

- This activates the T cell to divide by mitosis.

- Of those new T cells, some will stimulate phagocytosis, some will become cytotoxic/killer T cells (which destroy infected cells), some will become memory T cells and some will stimulate B cells to divide by mitosis.

- B cells are produced and mature in the bone marrow and of those, some have antibodies complementary to the antigens.

- The B cells produced binds to the antigen and becomes an antigen presenting cell so that the helper T cells can bind to it and stimulate the B cell to divide by mitosis.

- Of those B cells, some will become memory B cells, some will be plasma cells which secrete the complementary antibodies.