Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

114 Self and the Phenomenon of Life

b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”

Table 6.2. Cells Participating in Animal Immunity

A. Cells involved in innate immunity:

These cells mount an immediate, general response to invaders without reference to

specific antigens.

1. Macrophages – Large phagocytic cells present in all tissues. Main function is
   “garbage disposal” – engulfing and digesting foreign bodies and cellular debris.
   Recruit neutrophils to the site of infection.


2. Neutrophils – Major phagocytes in the blood. “Foot soldiers” against invading
   bacteria. Arrive rapidly at the scene of infection and destroy pathogens on the spot.


3. Dendritic cells – Main function is to present foreign antigens to the T
   lymphocytes to trigger adaptive immunity. Serve as immune “sentinels” by
   patrolling the whole body in search of pathogens. Digest pathogen proteins and
   display the peptide fragments on cell surface for presentation to T cells.


4. Natural killer cells (NK cells) – Innate immune cells circulating in the blood.
   Eliminate virus-infected cells and certain tumor cells.
   B. Cells involved in adaptive (acquired) immunity:
   These cells mount a delayed response directed to specific foreign antigens following
   provocation.


5. B lymphocytes –
   Main function is to produce antibodies. Activated upon exposure to circulating foreign
   antigens. Fully mature antibody-secreting B cells are called plasma cells.


6. T lymphocytes –
   a. Cytotoxic T cells: Activated by dendritic cells. Seek out virus-infected cells and
   destroy them, thus the nickname “killer T cells.”
   b. Helper T cells: Activated by dendritic cells. Main function is to enhance immune
   response by stimulating B cells and macrophages.
   c. Regulatory T cells: Activated by dendritic cells. Main function is to suppress
   immune response when immunity is no longer needed.


7. Memory cells –
   These are activated B and T cells that do not participate in attacking the invading
   pathogens but are reserved for future activity when invaded again by the same
   pathogens (re-infection). Immune response to re-infection is much faster and robust
   than first infection.
   Note: Immune cells circulating in the blood make up most of the white blood cells
   (leukocytes).

Except for NK cells, they recognize pathogens through their pattern
recognition receptors, though each plays a different role in fighting the
invaders.
Macrophages and neutrophils are both phagocytes — cells capa-
ble of engulfing pathogens. Mature macrophages patrol all tissues in