T-lymphocytes & cell-mediated immunity

Define ‘immunity’

Ability of organisms to resist infection by protecting against disease-causing microorganisms or toxins & involves recognition of antigens

Define an antigen

An antigen is a molecule, usually a protein or glycoprotein, that is recognised by the immune system. Non-self antigens can trigger an immune response.

What does the prescence of an antigen initiate?

Production of antibody as part of body’s defence system

Define non-specific immune responses

Defence mechanisms that protect the body against a wide range of pathogens, rather than targeting a specific antigen, e.g phagocytosis. Does not provide long-term immunity.

Define specific immune responses

Involve antigens & can provide long-term immunity

Describe B-lymphocytes

Mature in bone marrow, associated with humoral immunity - immunity involving antibodies present in body fluids/humours e.g blood plasma

Describe T-lymphocytes

Mature in thymus gland & associated with cell-mediated immunity - immunity involving body cells

Give 4 reasons why T-lymphocytes can distinguish invader cells from normal cells

• Phagocytes that engulfed & hydrolysed a pathogen present some of pathogen’s antigens on their own cell-surface membrane

• Body cells invaded by a virus present some of viral antigens on their own cell-surface membrane

• Transplanted cells have different antigens on cell-surface membrane

• Cancer cells present different antigens on cell-surface membrane

Define cell-mediated immunity

When T-lymphocytes respond to antigens present on a body fluid

Describe response of T-lymphocytes to a pathogen

1. Pathogens invade body cells or taken by phagocytes

2. Phagocyte places antigens from pathogen onto its cell-surface membrane

3. Receptors on T-cell fit exactly onto these antigens

4. Attachment activates T-cell to divide rapidly by mitosis & form a clone of genetically identical cells

5. The cloned t-cells:

• Develop into memory cells that enable a rapid response to future infections by same pathogen

• Stimulate phagocytes to perform phagocytosis

• Stimulate B-cells to divide & secrete their antibody

• Activate cytoxic T-cells

Describe how cytoxic T-cells kill infected cells

• Kill abnormal cells and infected body cells

‣ Release perforin ➔ makes holes in the cell membrane.

‣ Cell membrane becomes freely permeable ➔ cell dies.