The human body has got several mechanisms to combat infections due to pathogenic organisms. It should be realised that many harmless organisms live in symbiosis in our body in the upper respiratory tract, in the lower bowels and in the skin. These may occasionally infect the tissues when the resistance is extremely low or viral infections have devitalised mucosal membrane.
The chief factors which are utilised to fight external infections are as follows:
Immunity is associated with the formation of antibodies, which acquire specific power to inactivate or kill the invading pathogens. Immunity can be defined as active immunity or passive immunity. Active immunity is developed in the tissues due to an infection, e.g., after an attack of smallpox or enteric fever or poliomyelitis.
Polio virus sometimes may affect without any clinical manifestation and thus human immunity may be acquired. Similarly, active immunity may be bacterial proteins or inactivated or killed organisms. This is the basis of protective inoculation and vaccination and it is by this method that infections like smallpox, diphtheria, tetanus, polio, whooping cough, etc., are prevented in modern times.
2. Susceptibility to Infection:
There are some individuals who are inherently susceptible to infection as they have got certain genetic and acquired weaknesses in their ability to form antibodies. There may be deficiency of gamma globulin which usually is connected to antibody formation.
3. Cellular Defence:
During the process of infection in the tissues, there is always a local inflammatory response to fight microbic infection. There is vasodilatation with increased permeability of capillary beds and polymorph white cells accumulate in large numbers, to engulf the attacking organisms and destroy them by their own enzymes.
Subsequently, lymphocytes, plasma cells, monocytes and macrophages also take part in clearing local inflammation, but if the invasion is too severe, pus formation may take place. These tissue changes may ultimately clear up or, in case of suppuration, abscess may have to be drained. Sometimes good healing takes place with or without local fibrosis.
4. Humoral Defence:
Humoral defence is really the basis of chemical immunity based on specific globulin fraction of the plasma proteins. During the course of viral or bacterial infection, antibodies form which fight the infection.
In modern times great advances have been made in the field of active immunisation against various preventable diseases. Vaccination against smallpox is well known. Active immunisation against measles has also been prepared. This can be given to susceptibles by S.C. or I.M. injection in single dose in the second year of life. Cholera vaccine is not routinely given but should be injected during an epidemic.
In endemic areas vaccines for typhoids, and paratyphoid organism should be given from childhood and in adults these injections are to be repeated annually.
5. Mechanical Defence:
Mechanical barriers also help to prevent infection as follows:
(i) Secretion of mucus entangles organisms.
(ii) Ciliary movements brush out invaders.
(iii) Cough reflex itself is a mechanical defence.
(iv) Fibrinous exudate is another example, which limits infection.
(v) Local fibrosis and calcification actively localises infection.
(vi) Skin and mucous membranes are great mechanical barriers.