Bacteria
There are 4 type of bacterial cells:

• Gram negative bacteria
• Gram positive bacteria
• Mycobacteria
• Spirochetes
  

The gram negative organism can be killed by lyzing membranes (complement). Most of the bacteria must be killed by the uptake of phagocytes.

Parasites

Parasites are large and may have resistant body walls. They may avoid phagocytosis because of their ability to migrate away from an inflamed area. This sets them apart from bacterial and viral infections. Macrophages, neutrophils, and eosinophils may demonstrate direct toxicity orphagocytosis toward parasites. Parasites are still among the major causes of disease and death around the world, particularly in areas of poor hygienic practices and sanitation.  Citizens of developed nations often acquire microorganisms as they travel. The protozoan and intestinal helminths are two of the most common parasites acquired.

Intestinal Protozoa

Intestinal Helminths

Fungi
Fungi are formally found superficially, but a few fungi can cause serious disease. The fungal infections are divided into 4 classes:

• Superficial mycoses – caused by dermatophyte and are restricted to the nonliving heratinized components of skin, hair and nail.
• Subcutaneous mycoses – saprophytic fungi can cause ulcers in subcutaneous tissue following trauma.
• Respiratory mycoses – soil saprophytes produce subclinical or acute lung infections such as histoplasmosis.
• Candidiasis – causes superficial infections of skin and mucous membranes

Viral, Rickettsial, and Mycoplasmal Diseases
The characteristic process associated with viral infections is cellular replication, which may or may not lead to cell death. Interferon plays a major role in body defenses against viral infections. Antibodies are valuable in preventing entry and blood-borne spread of some viruses, but the ability of another viruses to spread from cell to cell places the burden of adaptive immunity on the T-cell system. Macrophages may also play a role in immunity. Some of the most virulent viruses to humans are zoonoses (e.g., rabies). Other viruses, however, can persist for years without symptoms and then be reactivated to cause serious disease, possibly including tumors (carcinomas).

Organisms intermediate between viruses and bacteria are those obligatory intracellular organisms with cell walls (Rickettsiae) and those without cell walls but capable of extracellular replication (Mycoplasma). Immunlogically, the former are closer to viruses, the latter to bacteria.

Allergens
The extent of an allergic response is partly influenced by the port of entry of the allergen. Allergens are the antigens that are able to elicit IgE antibody responses in certain individuals. Various clinical conditions, including asthma, eczema, and hay fever, are clinical symptoms, are clinical symptoms associated with atopy. Atopy is a genetic predisposition to type I hypersensitivity and is associated with increased production of IgE antibody. If both parents are atopic for hay fever or asthma, there is a 75% chance that the child will be atopic. If one parent is affected, there is a 50% chance that the child will develop symptoms. Approximately 5% to 10% of the population exposed to airborne allergens becomes sensitized.

Allergic reactions are of major concerns. These reactions are characterized by an immediate response on exposure to an offending antigen and the release of mediators (e.g. histamine, leukotrienes, and prostaglandins) that are capable of initiating signs and symptoms. Although allergic reactions are associated with IgE, not all allergic reaction are IgE-mediated. Complement activation by immune complexes or via the alternative complement pathway has been shown to release complements C3a and C5a anaphylatoxins capable of producing similar reactions.

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