Leukocytes
As soon as pathogens are past the physical barriers in the body, they are confronted by non-specific defence cells, all referred to as Leukocytes, or white blood cells, even though they are found not only in the bloodstream, but also in the lymphatic system and in interstitial fluid. These white blood cells include neutrophils, monocytes, eosinophils, basophils, and lymphocytes. See below for a detailed look at the functions of these leukocytes.
White blood cells are usually not in actuality white, but are so called because after the centrifugation of a blood sample, they are found in the Buffy Coat, a thin layer between the blood plasma and erythrocytes (non-nucleated red blood cells). Leukocytes are nucleated, unlike mature red blood cells (immature red blood cells are called erythroblasts).
When fighting an infection, the white blood cell count in blood, normally at about 1% of total blood, will increase drastically, because specific lymphocytes will be relied upon to increase their numbers based on the type of antigen they respond to. Incidentally, white blood cells are found in much higher concentration outside of the bloodstream than are red blood cells, because they can squeeze through holes in blood vessel walls, unlike red blood cells. Leukocytes, in particular lymphocytes, are quite often found in lymphatic system rather than in the bloodstream, but can also be found in interstitial fluid or in tissue.
Not all leukocytes are part of the Non-Specific, or Innate Immune Response. Lymphocytes and macrophages take part in the Adaptive, or true Immune Response.
Neutrophils
Which Response?: Neutrophils are the first responders to bacterial infections. Hence, they are part of the Innate Immune Response only.
About Neutrophils: Neutrophils form the majority of leukocytes (about 70%), and their death in large numbers results in pus. They deal with primarily with bacterial infections.
Figure 1.A - An example of a neutrophil
Eosinophils
Which Response?: Eosinophils are part of the Innate Immune Response.
About Eosinophils: Not much is known about Eosinophils other than the fact that they deal with only with parasitic infections, and form about 2.3% of overall leukocytes.
Figure 1.B - An example of an eosinophil
Basophils
Which Response?: Basophils are part of the Innate Immune Response.
About Basophils: Basophils deal mainly with allergic and antigen response and release histamine to cause inflammation. They form under 1% of all leukocytes.
Figure 1.C - An example of a basophil
Lymphocytes
Which Response?: Lymphocytes are part of the Adaptive Immune Response.
About Lymphocytes: Lymphocytes execute the Adaptive Immune Response, and are more common in the lymphatic system. They form 25% of leukocytes.
Figure 1.D - An example of a lymphocyte
Monocytes
Which Response?: Monocytes are part of both the Innate Immune Response and the Adaptive Immune Response.
About Monocytes: Monocytes act as neutrophils with extra functions. They present pathogens to T-cells so that antibodies may be formed. They form 6% of all leukocytes.
Figure 1.E - An example of a monocyte
Macrophages
Which Response?: Macrophages are part mostly of the Adaptive Immune Response, although they also play a part in the Innate Immune Response.
About Macrophages: Macrophages are a special form of monocyte. A monocyte is termed a macrophage when it migrates from the blood to enter tissue.
Figure 1.F - An example of a macrophage
