The following points highlight the top three types of immunity found in humans. The types are: 1. Immunity in Bacterial Infections 2. Immunity in Viral Infections 3. Immunity to Protozoa and Helminthes.
Immunity: Type # 1. Immunity in Bacterial Infections:
(a) Antibody Mediated Immunity:
Because of the production of the exotoxin, some microorganisms (i.e. Corynebacterium diphtheriae, Clostridium tetani, CI. welchii, CI. botulinum) are pathogenic. Antibodies acquired by immunization or previous infection or given passively as antiserum are able to neutralize the bacterial toxins.
Bacterial toxins are enzymatic in nature; most probably the antibody interacts with the active site of the enzyme and neutralizes effectively the toxin.
When bacteria do not produce exotoxins, antibodies get attached to the surface of the bacteria. The most important effect of this attachment is to encourage phagocytosis by blood macrophages or polymorphs (opsonisation).
The mechanism is that the antibody alters the surface change of organisms and makes them more susceptible to phagocytosis. The phagocytes can also destroy the organisms by digesting by various enzymes produced in the intracellular lysosomes; but Streptococcus pyogenes, Typhoid bacillus, Mycobacterium tuberculosis are able to resist the digestion by enzymes.
If immunity exists due to previous infection or artificial immunization, streptococcus is susceptible to intracellular digestion, whilsst the smooth strains are able to resist the digestion.
In case of enteric infections (typhoid and paratyphoid due to salmonella) antibodies are secreted into the intestinal lumen and attack these organisms before they invade the intestinal mucosa. These antibodies are known as “coproantibodies” and they are of IgA type which is selectively produced in mucous membrane of respiratory and intestinal tract.
Other effects of antibody attachment to the surface of the organism are:
(a) The lysis of Gram-negative bacteria brought about by the activation of complement system. The effect of the complement is to digest the cell wall of lipopolysaccharide so that the structural mucopeptide is exposed to the attack by lysozyme.
(b) Attachment of bacteria to red blood cells in presence of antibody and complement. This phenomenon, known as “Immune adherence,” may encourage the phagocytosis.
(c) Antibody precipitated on the surface of certain parasitic nematodes can block the excretory orifices of the nematodes.
(b) Cell Mediated Immunity in Bacterial Infections:
Macrophages from animal immune to tubercle bacilli are more actively phagocytic than those taken from a normal animal. Similarly, macrophages from brucella infected animals show enhanced cellular immunity. The same was observed in Listeria monocytogenes.
Immunity: Type # 2. Immunity in Viral Infections:
(a) Antibody Mediated Immunity:
In virus infection, the antibody is mostly efficacious, if the virus passes through the blood stream to reach its target organ. For example, polio virus initially crosses the intestinal route, invades the blood stream and passes on rare occasions to the spinal cord and brain where it proliferates. Antibody present in the blood stream can neutralize the virus before it reaches its target cells in the nervous system.
A number of virus behave in the same way (e.g. virus of measles, small pox, mumps, rubella and chicken pox). In case of influenza and common cold, the viruses do not pass through the blood stream as their target organ is the respiratory mucous membrane. In this type of infection, a high level of antibody will be less effective against these viruses in comparison with its effect on blood borne viruses.
Most of the antibody should pass through the mucous membrane into the respiratory secretion to act on such respiratory viruses. There is very little IgG and often no IgM in the content of mucous secretion, as this mucous membrane is not, most probably, very permeable to these classes of antibody.
But IgA is manufactured by the plasma cells in the lamina propria of the mucous membrane and secreted in the nasal secretion. IgA has neutralizing activity against common cold viruses. Intranasal administration of live attenuated Influenza virus can stimulate local production of IgA antibody in the mucous membrane of the nose.
The high degree of immunity was provided by live attenuated oral polio virus may be due to locally produced antibody in the gut by neutralizing the virus even before it reaches the blood stream. After oral vaccination, IgA against polio virus has been demonstrated in the faeces, in duodenal fluid and in saliva, whereas no antibody was demonstrated after injection of inactivated polio virus vaccine.
(b) Cell Mediated Immunity:
The children with congenital hypogammaglobulinemia can recover from virus infection without producing any demonstrable virus neutralizing antibody, because of cell mediated immunity. The patients with Swiss type agammaglobulinaemia and an additional cell mediated deficiency are very much susceptible to virus infection and, ultimately, they may die.
Immunity: Type # 3. Immunity to Protozoa and Helminthes:
The life cycle of parasite is complicated and the immune response can be effective, if the life cycle is interrupted at a stage when the parasite is accessible to immune process. The antibody may, most probably, have the access to the parasite if the permeability of the red blood cell membrane is altered so that the immunoglobulin can enter and attack the parasite.
In protozoal infection, immunoglobulin production is increased and all classes of immunoglobulin’s are involved.
Life cycle of helminths is also complex as in protozoa protective immuno-responses may act only at an early stage in the life cycle. IgE (reaginic) antibody with pulmonary eosinophilia will appear in the parasitic infections and the immediate hypersensitivity reactions of anaphylactic type (type 1) is involved in the pathogenesis of helminth infections.