The body defence systemWatch
Just to add some meat to Kallisto and SmilingWombat's useful inputs:
The ways described above to prevent entry of organisms into our bodies are known as the innate immunity - when an infection does start, adaptive immunity takes over.
a) T lymphocytes mediate the cell-mediated arm of the response to infection - they include killer T cells that engulf [phagocytose (Greek phago = take in; cyto = cell)] small pathogens, in particular viruses [hence the classical presentation of AIDS in which the HIV virus very cleverly infects CD4 T-cells themselves thus disarming the very soldiers of our body [leaving the patient defenceless][same principle as the Americans bombing the barracks in Iraq] includes fulminant viral infections such as CMV [cytomegalovirus] and toxoplasmosis. Other phagocytes in our armoury of "pathogen-eaters" are macrophages [those in blood are called monocytes], and polymorphonuclear [Greek poly = many; morpho = shape/structure; nuclear = self-explanatory: i.e. they have nuclei with many branching protrusions - go to Google images] leucocytes [leuco = white as in leucoderma: a disease where skin pigment is lost; cyto = cell].
b) B lymphocytes mediate the humoral arm of adaptive immunity - the part that works by the production of antibodies [proteins shaped like a Y with two heavy chains (kappa or lambda ones) and two light chains [these chains are short polypeptides][light chains can be alpha, , delta, epsilon,
gamma or mu giving five main types of antibody [IgA, IgD, IgE, IgG and IgM, respectively - Ig stands for immunoglobulin, which is the gamma-globulin portion of blood proteins].
IgG is a small molecule so can cross the placenta during pregnancy, conferring immune cover for the newborn baby for 6 months after which immunization [vaccines] is necessary.
IgE is the antibody involved in protection against worms e.g. tapeworm in the intestine or Onchocerca volvulus in the eye [talking about eyes, tear flow helps drain away pathogens and tears have lysozyme, an enzyme that destroys bacteria and tears also have lactoferrin [google it!].
IgA is the antibody in secretions - so prevents infection again in the eye, nose [RE:nasal secretion], breast and other secretory organs.
IgM is a large molecule with five binding sites for antigen [IgM looks like a five-point star][the part of the pathogen that induces an immune response, usually a protein, but google "hapten"].
The function of IgD is debatable.
The antibody molecule has a constant end (Fc) and a variable end (Fv) - the latter is specific to the antigen concerned and there are several million different possibilities.
A B lymphocyte, when stimulated converts to a plasma cell (rich in RER - work out why!) and this produces monoclonal (identical one type) antibody in millions of molecules. (A plasma cell cancer is called myeloma).
The antibody can neutralize or precipitate or agglutinize an antigen. It can also make it easier for a T lymphocyte to phagocytose a pathogen (called opsonisation).
Bacteria are, however, not stupid haha (they have many ways of preventing our defence systems killing them e.g. they can surround themselves with a capsule that prevents identification of antigen specificity; they can produce toxins that attack T and B lymphocytes; they can produce penicillinase, an enzyme that destroys penicillin [a more generic version called beta-lactamase also attacks another group of antibiotics called cephalosporins, whose chemical structure is similar to the penicillins, i.e, both have a beta-lactam ring] affording resistance to this antibiotic [we have developed penicillinase-resistant penicillins like methicillin and cloxacillin, but bacteria are always one step ahead - a major world human health issue for which there is a current Longitude Prize of £10 million for anyone who can find an effective, quick solution].
I WOULD SUGGEST YOU GOOGLE ALL THE TECHNICAL WORDS IN MY ANSWER TO UNDERSTAND THE SUBJECT FURTHER AND TO FIND MORE DETAIL.
Best of luck!