Phagocytes and their Relatives

Phagocytes and their Relatives

Neutrophils – kill bacteria 

  • 60% of WBCs
  • ‘Patrol tissues’ as they squeeze out of the capillaries
  • Large numbers are released during infections
  • Short lived – die after digesting bacteria
  • Dead neutrophils make up a large proportion of puss

Monocytesare chief phagocytes found in the blood 

  • Made in bone marrow as monocytes and the circulate in the blood for 1-2 days before being called macrophages once they reach organs.

Macrophages – Found in the organs, not the blood

  • Larger than neutrophils and long lived – involved in phagocytosis, release interferon and interleukin (which stimulates  production of cells of the Specific Defense System)  
  • Macrophages also act as scavengers, ridding the body of worn-out cells and other debris by ingesting cellular debris, foreign material, bacteria and fungi 
  • Versatile cells that reside within tissues and produce a wide array of chemicals including enzymes, complement proteins, and regulatory factors such as interleukin
  • Antigen-presenting cells that activate the adaptive immune system they display antigens from the pathogens to the lymphocytes. 

Basophils – are capable of ingesting foreign particles and produce heparin and histamine and which induce inflammation, are often associated with asthma and allergies  

Mast cells reside in connective tissues and mucous membranes, and regulate the inflammatory response.  They are most often associated with allergy and anaphylaxis: for example, they release histamine – this is why anti-histamines help allergic reactions

Dendritic cells are phagocytes in tissues that are in contact with the external environment

  • Located mainly in the skin, nose, lungs, stomach, and intestines (are in no way connected to the nervous system)
  • Dendritic cells serve as a link between the innate and adaptive immune systems, as they present antigens to T cells, one of the key cell types of the adaptive immune system

Eosinophils – weakly phagocytic of pathogens kill parasitic worms   

NK cells (natural killer) – used to combat tumor cells or virus-infected cells

  • A class of lymphocytes which attack and induce cells to kill themselves (self-induced apoptosis)  They complement both specific and nonspecific defenses
  • May also attack some tumor cells 
  • Also secrete interferons, proteins produced by virus infected cells which binds to receptors of non-infected cells, causing these cells to produce a substance that will interfere with viral reproduction and activate macrophages and other immune cells


Most infections never make it past the first and second level of defense.

Specific (Adaptive) Responsework on specific types of invaders which it identifies and targets for destruction – takes longer to react

  • The response is directed at specific targets and is not restricted to initial site of invasion/infection
  • Lag time occurs between exposure and maximal response
  • The adaptive immune system allows for a stronger immune response as well as immunological memory, where each pathogen is “remembered” by its signature antigen
  • Antigens are proteins or carbohydrate chain of a glycoprotein within a plasma membrane which the body recognizes as “nonself” 
  • The specific immune response is antigen-specific and requires the recognition of specific “non-self” antigens during a process called antigen presentation
  • Antigen specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells
  • The ability to mount these tailored responses is maintained in the body by “memory cells
  • Should a pathogen infect the body more than once, these specific memory cells are used to quickly eliminate

Third line of defense – mounts attack against particular foreign substances antigens throughout the body