Immunity

- Receptors on T helper cells are complimentary to the antigens on an antigen-presenting cell.

- This stimulates other T cells to divide rapidly by mitosis to form clones.

- The cloned T cells:

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

b. Stimulate phagocytes to engulf pathogens by phagocytosis,

c. Stimulate B cells to divide,

d. Kill infected cells by producing a protein that makes holes in the cell surface membrane, making it freely permeable.

- Each B cell produces a different antibody complimentary to a specific antigen.

- When an antigen-antibody complex is formed, the B cells divide by mitosis to form clones.

- These cloned B cells then develop into:

1. Plasma cells, which secrete antibodies and are responsible for the immediate defence of the body; this is the primary immune response.

2. Memory cells, which provide long-term immunity against the original infection; this is the secondary immune response. This response is more rapid and intense than the primary immune response, and individuals are often unaware that they were ever infected.