Over 200 different cell types in the human are known. The most numerous are the red blood cells, then platelets/thrombocytes and 3) white blood cells or leukocytes. These cells form a variety of different tissues. Blood can be separated in a centrifuge into a fluid and a cellular fraction. The fluid fraction is the plasma which contains all of the soluble small molecules and macromolecules of blood like fibrin and other proteins required for the formation of blood clots. If blood or plasma is allowed to clot, the fluid phase that remains is called serum (the liquid, noncellular component of coagulated blood).

Human blood serum is a very complex fluid containing many thousands of proteins representing nearly all human protein families. Most serum proteins are present in concentrations at least 8-10 orders of magnitude less than the hihgly abudnant proteins such as albumin, immunoglobulines, transferrin and haptoglobin which compose more than 90% of the total serum proteome.  Antibodies reside in the serum. About half of the serum proteins have a molecular mass below 45kDA (i.e., the kidney size cutoff). Such molecules can only exist in the circulation for a limited time. However, if a small protein is associated with a larger protein molecule (such as immunoglobulins), it can persist for a relatively long time. For example, IgG molecules have a long half-life of about 23 days in serum. Complexes of IgG molecules involved in interactions with the C domains of the IgG molecules are oftein designated as “non-immune” complexes (non-IC), as distinct from “immune” complexes (IC) formed by the antigen-combining sites located in V domains (Nezlin, Immun. Lett., 122, 2009, 141-144)

The cellular fraction of blood contains 1) red blood cells or erythrocytes, 2) white blood cells or leucocytes, 3)platelets and 4) plasma.

Peripheral blood mononuclear cells (PBMCs) are a mixed population consisting of several cell types: CD4+ and CD8+ T-lymphocytes (70%), B-lympohocytes (15%), natural killer cells (10%), monocytes (5%), and dendritic cells (<1%), each expressing a unique set of genes. 

Hematopoiesis

Cells of the body are formed and developed in a process referred to as hematopoiesis. Every mature blood cell is derived from a common stem cell which is pluripotent or able to differentiate along a number of pathways. Early in hematopoiesis, a pluripotent stem cell differentiates along 1 of 2 pathways, giving rise to either a common lymphoid progenitor cell or a common myeloid progenitor cell. Myeloid stem cells generate progenitors of red blood cells (erythrocytes), many of the various white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells) and platelets. A lympohoid progenitor becomes a lymphoblast and then one of the 3 types of lymphocytes (white blood cells), a B lymphocyte, a T lymphocyte or a natural killer cell (NTK) 

In the adult, hemopoietic stem cells are found mainly in bone marrow and they grow and mature on a meshwork of stromal cells which include fat cells, endothelial cells, fibroblasts, and macropages. Progenitor committment depends on various growth factors called “cytokines”. In the absence of infection, bone marrow stromal cells are the major source of hematopoietic cytokines. In the presence of infection, cytokines produced by activated macrophages and induce additional hematopoietic activity resulting the the rapid expansion of the WBC population that is necessary for fighting infection.

Various growth factors are required for the survival, and maturation of hematopoietic cells in culture. These hematopoietic cytokines include the colony-stimulating factors (CSFs) which induce the formation of distinct hematopoietic cell lines. Another important cytokine is erythropoietin (EPO) which is produced by the kidney and induces the terminal development of erythrocytes and regulates the production of red blood cells. The ability of a given  to signal growth and differentiation is dependent upon the presence of a receptor for that cytokine on the surface of the target cell.

The regulation of hematopoiesis is quite complex. For example, the binding CSF to its receptor causes some of the receptors to be internalized by the cells which serves to down modulate receptor expression by the cell. Steady state hematopoisis is also regulated by cell apoptosis or death. Abnormalities in the expression of hematopoietic cytokines or their receptors may result in some leukemias.

Plant Cells:

Vacuoles: Plant cells have specialized membrane bounded structures called vacuoles. The most conspicuous example is the alrge centrl vacuole seein in most plant cells. The membrane surrounding this vacuole is called the tonoplast because it contains channels for water that are used to help the cell maintain its tonicity, or osmotic balance. 

The centrol vacuol is important for a number of roles. The water channels in the tonoplast maintain the tonicity of the cell, allowing the cell to expand and contract, depending on conditions. The central vacuoles are also invovled in cell growth by occupying most of the volume of the cell. Plant cells grwo by expanding the vacuole, rather than by increasing cytoplasmic volume. 

Vacuoles with a vareity of functions are also fuound in some fungi and protists. 

Chloroplasts: Plant cells and cells of other eukaryotic organisms taht carry out photosynthesis typically contain one to several hundred chloroplasts. Chloroplasts contain the photocsynthetic pigment chlorophyll that gives most plants their green color. 

Cloroplasts, liek their coutnerpart in animal cells for energy metabolism, are surrounded by two membranes. However, choroplasts are larger and more complex than mitochondria. Chloroplasts also have closed compartments of stacked membranes called grana. Each granum may contain from a few to several dozen disk shaped thylakoids. On teh surface of the thylakoids are the light capturing photosynthetic pigments. Surroudning the thylakoid is a fluid matrix called the stroma. The enzyems used to synthesize glucose during photosynthesis are found in teh stroma. 

Like mitochondria, chloroplasts contain DNA but many of the genes that specify chloroplast components are also located in the nucleus. Some of the elemtns used in photosynthesis, including the specific protein components necessary to accomplish the reaciton, are synthesized entirely within the chloroplast. 

Both mitochondria and chloroplasts are thought to ahve arison by endosymbiosis when a free living cell was taken up byt not digested. According to one endosymbiont theory, the engulfed prokaryotes provdied their hosts with certain advantages assocaited with their specific metabolic abilities. Thus, mitochondria are thought to ahve originated as bacterai capable of carrying out oxidative metabolism and chloroplasts are thought to have arose form photsynthetic bacteria.