An introduction to immunology and immunopathology

Phagocytic/endocytic barriers

• Various cells internalize (endocytosis) and break down foreign macromolecules

• Specialized cells (blood monocytes, neutrophils, tissue macrophages) internalize (phagocytose), kill and digest 

whole organisms

Inflammatory barriers

• Tissue damage and infection induce leakage of vascular fluid containing serum protein with antibacterial activity, 

leading to influx of phagocytic cells into the affected area

---

Page 7 of 14

Marshall et al. Allergy Asthma Clin Immunol 2018, 14(Suppl 2):49

cells that not only play a role in phagocytosis, but are also 

involved in antigen presentation to T cells (see Fig. 

1

) [

1

].

Dendritic cells also phagocytose and function as APCs, 

initiating the acquired immune response and acting 

as important messengers between innate and adaptive 

immunity. Mast cells and basophils share many salient 

features with each other, and both are instrumental in 

the initiation of acute inflammatory responses, such as 

those seen in allergy and asthma. Mast cells also have 

important functions as immune “sentinel cells” and are 

early producers of cytokines in response to infection 

or injury. Unlike mast cells, which generally reside in 

the connective tissue surrounding blood vessels and 

are particularly common at mucosal surfaces, basophils 

reside in the circulation. Eosinophils are granulocytes 

that possess phagocytic properties and play an important 

role in the destruction of parasites that are often too large 

to be phagocytosed. Along with mast cells and basophils, 

they also control mechanisms associated with allergy and 

asthma. Natural killer (NK) cells play a major role in the 

rejection of tumours and the destruction of cells infected 

by viruses. Destruction of infected cells is achieved 

through the release of perforins and granzymes (proteins 

that cause lysis of target cells) from NK-cell granules 

which induce apoptosis (programmed cell death) [

4

]. NK 

cells are also an important source of another cytokine, 

interferon-gamma (IFN-γ), which helps to mobilize 

APCs and promote the development of effective anti-

viral immunity. Innate lymphoid cells (ILCs) play a more 

regulatory role. Depending on their type (i.e., ILC-1, 

ILC-2, ILC-3), they selectively produce cytokines such as 

IL-4, IFN-γ and IL-17 that help to direct the appropriate